Flock feeding and time budgets in the house sparrow (Passer domesticus L.)

Anita. Behav., 1980, 28, 295-309

FLOCK FEEDING AND TIME BUDGETS IN THE H O U S E SPARROW (PASSER DOMESTICUS L.) BY C. J. BARNARD*

Animal Behaviour Research Group, Department of Zoology, South Parks Road, Oxford OX1 3PS Abstract. A winter population of house sparrows at a farm fed on barley seed in two distinct types of habitat: cattlesheds and open fields. The risk of predation was apparently higher in the fields where birds scanned more frequently than in the cattlesheds and where scanning was negatively influenced by flock size but positively influenced by distance from cover. Individual time budgets were more influenced by flock size than by seed density in the fields but more influenced by seed density than by flock size in the cattlesheds. Higher rates of scanning resulted in greater flock vigilance and longer flight distances in the fields but flight distance was negatively influenced by the density of seeds on which birds were feeding. comparable with those in the cattlesheds (mean density, fields: 870.87 • 205.39 seeds/m2; sheds: 9 0 1 . 4 8 • 121.47 seeds/m2; 0.25 m 2 quadrat samples, n = 80 and 73, respectively). However, although the two habitats were broadly similar in terms of barley seed density, they apparently differed in the risk of predation they imposed on feeding birds. In the cattlesheds the only predators were feral cats of which the farm supported a population of six at the time of the study. However, the cats mainly patrolled either along the side of the farm buildings or around the outside and in debris scattered about the yard. They seldom ventured into the cattlesheds where there was little cover for concealment and they were constantly disturbed by the movements of cattle. They therefore presented little threat to birds feeding within the sheds. This supposition is also based on the fact that, during the entire period of field observation (approx. 900 h), I saw only 14 attempts (all unsuccessful) by cats to catch birds outside the sheds, but none inside. Apart from two brief visits by a male kestrel (Falco tinnunculus), I did not see any aerial predators in or around the buildings. The only other 'predators' were the cattle, which constantly walked about between feeding birds and presumably constituted a threat from trampling, and the farm staff whose activities appeared to create the greatest disturbance to feeding birds (see Barnard 1978). In the open fields, however, birds were completely exposed to attacks by aerial predators. Two pairs of kestrels and an adult female and juvenile sparrowhawk (Accipiter nisus) were resident near the farm and regularly patrolled the hedges and fields immediately surrounding it.

Recent theoretical and empirical investigations into the organization of individual feeding behaviour in flock-feeding birds have shown that the size and composition of the flock may influence the amount of time a bird allocates to different behaviours (e.g. Pulliam 1973, 1975; Pulliam et al. 1974; Caraco, in press a and b). In this paper I compare the flocking behaviour and time budgets of sparrows feeding in two different habitats on a farm outside Oxford. Individual birds regularly used both habitats and they were not divided into two distinct field and shed populations (Barnard 1978). In one habitat, cattlesheds, the risk of predation was low, while in the other, open fields, there was apparently a higher risk of predation: I predicted that this difference in predation pressure between the habitats would result in flock size exerting a greater influence on behavioural time budgets in the open fields. The Cattleshed and Open Field Habitats Although the activity of the birds was more or less confined to the area in and around the farm buildings where they fed on various supplies of barley seed, the area could be divided into two distinct types of habitat: cattlesheds and open fields (see Fig. 1). Both habitats provided food in the form of barley seed although in the cattlesheds these were sometimes concealed in bedding straw. At the time of year when I made observations in the open fields (October) seed density there was much higher than during the mid-winter period, owing to harvesting activity and the sowing of the winter crop, and densities were *Present address: Department of Zoology, University of Nottingham, UniversityPark, Nottingham NG7 2RD. 295

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Two of the feral cats also patrolled along the hedges and concealed themselves in the foliage for considerable periods. During approximately 200 h of observation I recorded a total of 12 attacks by aerial predators on flocks of feeding sparrows m the fields: 9 by sparrowhawks and 3 by kestrels (see also Barnard in press). One of the sparrowhawk attacks was successful. Even though the number of observed attacks was low, one or more raptors was present in the vicinity of the farm for a large part of most days. Although I cannot conclusively show that the risk of death by predation for a sparrow was higher in the fields, it is reasonable to conclude that predators were a greater potential threat there than in the sheds.

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In this paper I discuss the results of some field observations which were designed to see whether these differences were associated with differences in the influence of flock size on individual feeding behaviour in the two habitats. The paper is divided into two sections in which I discuss separately how flock size affects individual feeding behaviour in each habitat. The study is discussed as a whole at the end.

Section A: The Organization of Feeding Behaviour in the Cattlesheds (i) Flock Size and Time Budgets During a feeding bout a bird performs several different behaviours. In addition to actually picking up and handling prey, it moves about the feeding site, scans for food, other birds or predators, and interacts with other foragers (Holling 1959; Smith & Dawkins 1971; Murton 1971; Lazarus 1972; Krebs et al. 1972). The relative amount of time spent in each behaviour or, under certain conditions, the frequency with which each is performed during a feeding bout, can be described as a bird's time budget (Kiester & Slatkin 1974; Caraco in press a, b). Recent models of individual time budgets in feeding flocks have reduced the budget to three mutually exclusive behaviours: feeding, scanning for predators and aggression (Kiester & Slatkin 1974; Pulliam et al. 1974; Pulliam 1975; Caraco in press a, b; Pulliam et al. MS). The essence of these models is a hierarchical organization of behaviour in the budget: scanning for predators takes priority over feeding and feeding takes priority over initiating fights (see Pulliam 1975 and also Boyd 1953). The models suggest that birds join flocks in order to be able to reduce their commitment to scanning for predators and devote more time to feeding. The models were devised mainly with respect to species exploiting food supplies which are virtually unlimited and predictable in time and space, but are unsatisfactory for the present study for three main reasons. Firstly, as Caraco (in press a) and Pulliam (personal communication) agree, where these feeding conditions do not hold (as on the farm), the hierarchical organization of the budget may break down. For example, in overwintering small passerines which depend on patchily distributed and ephemeral food supplies, fighting may occur when feeding priority is high. This appears to be the case in house sparrows (Barnard 1978). Secondly, when fighting occurred in sparrows it only accounted for about 6% of behaviours

BARNARD: FLOCK FEEDING AND TIME BUDGETS IN SPARROWS performed during feeding (determined as percentage of total acts recorded in a feeding sequence) and did not appear to compete seriously with other behaviours for time. Patterson (1975)and Feare & Inglis (in press) similarly found that the amount of fighting had no effect on feeding rate in rooks (Corvus Jrugilegus) (except during periods of snow cover), and starlings (Sturnus vulgaris) respectively. Thirdly, the models do not take into account the need to travel between local sources of food even though such movement is incompatible with scanning, pecking and fighting. For these reasons and as a result of preliminary field observations, I have defined the feeding budgets of house sparrows as being composed of pecking, hopping and upright looking. Pecking refers to behaviour involved in picking up and handling food items since handling was usually done in a crouching position and did not involve a radical change in posture after the food item had been secured. Since each peck I monitored was followed by characteristic mandibular movements, I have assumed pecking rate to reflect the rate of energy intake. Hopping refers to the movement of birds around the feeding site and was usually carried out in a semi-erect posture contrasting with that adopted during handling. The term 'upright looking' characterizes side-to-side head movement about the vertical axis. This was usually associated with a sleek, erect posture (see Summers-Smith 1963, page 19) and frequently appeared after a sudden disturbance or just prior to flight (Summers-Smith 1963). Because of its behavioural context, I have assumed upright looking to be an expression of vigilance for predators. The same interpretation has been placed on similar behaviour in various other species (e.g. Lazarus 1972; Davis 1975; Inglis 1977; Lazarus & Inglis 1979). From now on I shall refer to upright looking simply as 'looking'. Another type of head movement which occurs during feeding is head-cocking. This implies rotation of the head about the longitudinal axis of the body so that one eye looks up and the other down. Some authors have argued that head-cocking is associated with vigilance for predators (e.g. Pulliam 1973) but, in sparrows (Barnard 1978) and some other species (e.g. Krebs & Partridge 1973), it appears to be used to scan the environment for food. Furthermore, in sparrows, head-cocking can be performed simultaneously with hopping and so may not compete for time in the time budget. Pecking,

297

hopping and looking account for at least 94 of a feeding bout but, as Caraco (in press a) points out, it is not surprising that birds only perform behaviours which have direct bearing on immediate feeding efficiency during a feeding bout since an individual is unlikely to perform non-foraging, maintenance behaviours such as preening when those could be accomplished in the safety of cover. Having defined the feeding behaviours, I now want to consider how they are affected in the cattleshed habitat by variation in flock size. Methods. I recorded the time budgets of birds feeding on seeds concealed in bedding straw in cattleshed A (Fig. 1). This shed was chosen because it provided the largest feeding area within the farm buildings in which the widest range of flock sizes occurred and contained stacked bales of straw from which I could obtain an elevated view of feeding birds. This meant that birds were not obscured by bedding straw and I could record complete feeding sequences. I recorded feeding behaviour as sequences of pecking, hopping and looking on a cassette tape recorder and simultaneously recorded the length of a bout on a stopwatch. Birds were chosen at random from flocks of different sizes and sequences were recorded for as long as a bird was visible or until the size of the flock changed (because a bird arrived or departed). I had to make observations on an opportunistic basis because the occurrence of birds in the shed was governed by chance disturbances from farm staff and cattle. In all I recorded 279 sequences (mean sequence length = 14.12 i 1.81 s, range 5.6-43.8 s). Results. Figure 2 (a--c) shows the rate of performance of each of the three behaviours plotted separately against flock size. All three behaviours were performed as events of negligible duration. For this reason the use of rates of performance is appropriate as an estimate of time budgeting. Each behaviour was dependent on flock size over a limited range but pecking increased linearly with flock size over a range of 1-12 birds, and the rates of hopping and looking decreased over the ranges 1-10 and 1-17 birds respectively. The residual sum of squares was smaller when I fitted two intersecting straight lines (see Hudson 1966) to the data than with any of a range of curves (exponential, parabola and rectangular hyperbola). Although several o t h e r workers have found relationships between flock size and the rates of energy intake and scanning

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for predators in birds (e.g. Murton 1971; Lazarus 1972; Krebs 1974) these have described relatively smooth curves. The relationships described here appear to have a sharp transition. Before I can explain the transition, however, I must consider the organization of behaviour during feeding and the relationship of each behaviour with environmental variables. a} 1.5

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To tease out the relationship between hopping and looking I carried out a partial regression analysis relating two of the behaviours while holding the third constant. This showed that not only was the rate of pecking negatively correlated with the rates of hopping and looking (partial correl, coeff, hopping: --0.411, P < 0.01; looking: --0.601, P < 0.001) but that the rates of hopping and looking were also negatively correlated (partial correl, coeff.: --0.301, P < 0.05). There was no tendency for any behaviour to occur in runs within a feeding bout (one sample runs test on bouts containing two of the three behaviours) although only 16.67% of looks or sequences of looks interrupted or followed a sequence of pecks (mean percentage sequence comprising pecks: 64.7 :k 5.47, hops: 22.5 • 3.62, looks: 13.1 • 3.65). Feeding was therefore seldom interrupted to scan although it was often interrupted to move to a new area where it could be resumed (41.67% of hops interrupted or followed a sequence of pecks). To test whether food availability modified the effects of flock size on the organization of time budgets in sparrows, I again examined the relationship between time budgets and flock size but this time taking into account seed density in the immediate area in which an observed bird had been feeding.

50 flock size

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50 flock size

Fig. 2. The relationships between the rates of pecking (a), hopping (b) and looking (c) and the size of the feeding flock. The data in each graph are best described by two intersecting straight lines. Lines are fitted by

linear regression and the points of intersection determined by the 95 % confidencelimits to the slope of the regression for the dependent part of the data. Bars represent standard errors. N = 279 feeding bouts. Correlation coefficients from 0 to point of inflexion (a) = 0.901, P < 0.001 (1-12 birds) (b) = -- 0.844, P < 0.001 (1-10 birds) (c) = -- 0.849, P < 0.001 (1-17 birds)

(ii) Flock Size, Food Availability and Time Budgets The relationships between a predator's feeding rate and the density of its prey was first rigorously quantified by Holling (1959). He suggested that, depending on the choice of prey in the environment, predators should show one of three characteristic patterns of feeding rate with prey density which he termed Type 1, Type 2 and Type 3 'functional responses'. Where only one type of prey is available, or where encounter rate with one prey type is high (Hassell et al. 1977), predators usually show the Type 2 response. The Type 2 response is described accurately by the so-called Disc Equation (Holling 1959) which states that, all things being equal, the reason that predators take a smaller and smaller proportion of the available prey as prey density goes up is because, as the attack rate goes up, the predator spends an increasingly greater proportion of its time handling the prey. However, factors other than handling time may contribute towards the shape of the curve (see e.g. Krebs 1973).

BARNARD: FLOCK FEEDING AND TIME BUDGETS IN SPARROWS Since the sparrows in the shed appeared to feed mainly on one type of food, at relatively high densities, we might expect peck rate to have been related to seed density in a manner analogous to Holling's Type 2 response if no serious constraint was being placed on feeding rate by other factors. Several field studies have shown that flock-feeding birds do exhibit a Type 2 response to food density although under certain conditions of prey availability the response may be modified (e.g. Murton et al. 1963; Murton 1968; Goss-Custard 1977). M e t h o d s . I recorded 57 feeding sequences (mean sequence length: 12.57 i 0.07 s, range: 4.2-34.8 s) of randomly chosen birds in cattleshed A in exactly the same way as described in section (i) but, as well as recording flock size, I measured the density of barley seeds in the area in which an observed bird had been feeding immediately after making an observation using a 0.25 m2 quadrat. I used a quadrat of this size because the density of seeds varied considerably over very short distances and it was important to measure the density available to the bird at the time of observation. Because the seeds were obscured in bedding straw, I standardized my sampling technique by taking into account all the seeds that fell within the quadrat regardless of depth in the straw. Although it probably led to an over-estimat~.~n of the amount of seed actually available to a feeding bird, I decided on this standardization because birds frequently 'dug' down into the straw with their bills making it difficult to be certain as to the vertical limits of seed availability. However, owing to trampling by cattle, the depth of the straw rarely exceeded 5 cm except when new straw had just been laid so it is possible that, for a large part of the time, most of the seeds were available

to the birds. I also recorded the ambient air temperature at the time of observation and the minimum air temperature for the 24 h preceding observation since both factors have been shown to influence feeding and mobility in house sparrows and other species (e.g. Odum 1942; Kendeigh 1973; Kontogiannis 1968). Results. The results of partial regression analysis relating each of the behaviours to the recorded environmental variables are shown as partial correlation coefficients in Table I. Variables were entered in the order shown and each sequentially partialled out with other variables held constant. Taking other factors into account, none of the three behaviours was independently correlated with flock size per se. However, both pecking rate and hopping rate were significantly correlated with local seed density; birds pecked faster and moved more slowly the higher the density of seeds on which they were feeding (Fig. 3). When the rate of pecking is plotted against seed density (Fig. 4) a curve is produced which is not significantly different (predicted points fall within one standard error of observed) from that expected on the basis of Holling's Disc Equation. The relationship between pecking rate and seed density therefore approximated to Holling's Type 2 functional response which suggests that handling time may have been a major factor in limiting the rate of food intake. The rates of pecking and hopping were also significantly affected by variation in the minim u m air temperature over the 24 h preceding observation but not by variation in ambient temperature. Decreased feeding rate and increased mobility with low overnight temperature may have been associated with a decrease in food availability through icy ground (see

Table I. Partial Correlation Coefficient Matrix for the Relationship between each of the Feeding Behaviours and Environmental Variables in the Cattleshed Habitat. The Coefficients Were Produced by Partial Regression Analysis which Related Each of the Behaviours in Turn to One of the Environmental Variables while Others Were Held Constant (see also Fig. 3)

Dependent variable Flock size Independent variable

299

Seed density

Current temperature

Minimum temperature

Pecking rate

--0.100

0.715 ****

0.174

0.457 ****

Hopping rate

-- 0.164

- 0.544 ****

-- 0.001

0.454 ****

Looking rate

-- 0.053

-- 0.078

Date for 54 feeding bouts, each recorded from a different bird.

*P < 0.05, **P < 0.02, ***P < 0.01, ****P < 0.001.

0.043

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Barnard 1978). This would have tended to decrease pecking rate and thereby increase hopping (see also Lawrence 1958). In contrast to the rates of pecking and hopping, looking rate did not appear to be significantly correlated with any of the environmental variables recorded although, like hopping, it was negatively influenced by seed density and minimum air temperature. It may have been that looking was correlated with unidentified, local movement, noise or other disturbances and occurred mainly in response to 'danger' cues. Whether or not this was the reason, the low level of correlation with the variables I measured further suggests that looking was of little importance in the time budget. However, these results do not explain the relationships previously obtained between the rates of pecking, hopping and looking and flock size (Fig. 2 a-c). From Table I it seems that a possible explanation would be a correlation between flock size and the density of seeds on which birds were feeding. To test this I carried out a further series of observations which were designed to relate flock size to the amount of time birds spent feeding on various densities of seed.

(iii) The Role of Food Patchiness in Feeding Behaviour Most food items for any given predator are not scattered randomly throughout the environment (Taylor 1961; Southwood 1966) but commonly occur in aggregations which can be termed 'patches' (MacArthur & Pianka 1966).

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Patchiness refers to the qualitative and/or quantitative distribution of resources within a given environment and may be related to local or large scale changes within the environment. In recent years, several workers have shown that patchiness in the distribution of food is important both in the way animals make decisions about their foraging behaviour (e.g. Smith 1974a, b; Cowie 1977) and in determining their behaviour in a more incidental manner, for example by facilitating a higher feeding rate in areas of abundant prey (Murton 1971; Krebs 1974; Goss-Custard 1977). Birds which rely on patchily distributed or unpredictable food supplies may use the presence of individuals which are already feeding as indicators of the best place to land and begin searching. This appears to apply to a variety of bird species (e.g. Murton & Isaacson 1962; Krebs 1974; Barnard 1978) and if it applied to sparrows feeding in the cattleshed, it might explain the relationship between feeding behaviours and flock size shown in Fig. 2. The curves may result from birds tending to spend a greater proportion of their feeding time in patches of high seed density when more birds were present and new arrivals joining the groups which form in these places. Methods. To determine the distribution of seeds in the shed, I sampled the bedding straw with a 0.25 m2 quadrat, taking 80 samples in all. Although there was considerable variation in the density of seeds throughout the shed, sampling revealed a distinct patch of very high density 1.0

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Fig. 3. Three-dimensional graph of the relationship between pecking rate and both flock size and local seed density. Partial correlation coefficientsare shown in Table I. The peaks increase in height significantly with seed density but show little change with variation in flock size. N = 54 feeding bouts.

Fig. 4. The relationship between pecking rate and seed density in sparrows feeding in Cattleshed A. The distribution is not significantly different from that expected by Holling's Disc Equation (dotted curve) with an empirically determined handling time of 1.02 • 0.082 s and therefore approximates to a Type 2 functional response. N = 54 feeding bouts. Bars represent standard errors.

BARNARD" FLOCK FEEDING AND TIME BUDGETS IN SPARROWS

(mean seed density: 908 seeds/m 2, compared to 168 seeds/m2 in surrounding straw) occupying approximately 1/4 of the floor space in the shed. This varied in its precise location over periods of two to three days and was created by the manner in which farm staff replenished the bedding straw in the shed (the straw being replenished every two to three days). Instead of being spread evenly over the floor of the shed, the straw was left in bales with the string cut for cattle to trample down. Within 1-2 h, cattle activity rendered the new straw indistinguishable in outward appearance from the old. This meant that the area of new straw, and thus high seed density, could not be located visually. Presumably, therefore, birds that fed in the shed could only locate good feeding areas by sampling. For each period of observation I used naturally existing objects such as clumps of straw and dung heaps to act as markers for the boundary of the dense patch. This ensured that no novel, visual cues were provided by which birds could locate the patch other than by sampling. I then chose birds at random from flocks of different size and measured with two stopwatches the length of time they spent feeding in both the dense patch and in the surrounding old straw (mean length of recorded feeding sequences: 19.75 ~ 3.26 s, range: 4.5-49.7 s). Results. Figure 5 shows the proportion of their feeding time birds in different sized flocks spent in the dense patch. The proportion rose steeply with increasing flock size when more than two to three birds were present, but levelled off sharply at around 15 birds. The curve did not simply reflect birds having been at a site longer in larger flocks and therefore having located the best patch, because observations were not made sequentially as flocks built up, but instantly when flocks of a given size had been feeding for varying lengths of time. Although there was variation in seed density within the dense patch, the patch nevertheless represented the zone of maximum available seed density within the shed. At the time of observation, the mean density of seeds within the patch was lower than that at which the functional response reached an asymptote (see Fig. 4). The patch therefore represented an area within the shed environment in which birds reached a maximum pecking rate which, in this case, was lower than that imposed by the constraint of handling time. This may explain why the horizontal section of the curve in Fig. 2a (and

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consequently those in Fig. 2b and c) occurs at a lower pecking rate than the asymptote in Fig. 4. Sparrows newly arriving at a site are more likely to land near a feeding conspecific than elsewhere in the environment (Barnard 1978). However, the first few birds to arrive during a feeding bout at the cattleshed usually fed near the periphery of the shed, frequently flying up to perch on the surrounding railings. These birds therefore tended to feed initially in old, depleted straw. As they continued to feed, the birds gradually extended their range away from the periphery and, because of the negative relationship between hopping rate and seed density, accumulated in areas &higher density. Depending on the time interval between successively arriving birds, subsequent arrivals which landed near already feeding birds were more likely to begin feeding on relatively high densities of seed. The longer birds fed in the area, therefore, and the more birds that were present, the greater the likelihood that individuals were feeding in the most profitable area within the site. In the cattleshed considered here, birds appeared to be spending all their feeding time in the area of highest seed density by the time approximately 15 birds had accumulated. This may explain the inflexions in Fig. 2 (a-c) in the region of 10 to 17 birds. The relationship between the feeding behaviours and flock size may have been an artefact of the independent, individual responses of birds to the density and distribution of their food supply and the correlation between flock 100

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size and the proportion of its feeding time a given individual spent in the area of highest food density. The curves in Fig. 2 plateau sharply because birds reached a maximum pecking rate determined by the environment.

Section B: The Organization of Feeding in the Open Field Habitat (i) Flock Size and Time Budgets In the cattleshed habitat, the relationship between flock size and pecking and hopping rates appeared to be a consequence of the correlation between flock size and the amount of time an individual spent in the patch of highest food density in the environment. Flock size itself did not appear to play a direct role in influencing behaviour and flocks were correspondingly loosely organized with the pecking rate of individuals being largely governed by their functional response to seed density. It should be re-emphasized here that when talking about the correlation between flock size and seed density, I refer to a relationship between the effect of flock size and a bird's response to the local seed density immediately surrounding it and not to a broad correlation between flock size and overall seed density at a site. However, since the organization of flocks in open fields suggested that flocking was a means by which individuals reduced their risk of predation, it is reasonable to suppose that, if time budgets were adaptive (Caraco, in press a) (i) flock size, assumed to be an important variable in determining an individual's probability of escape from a predator, might influence the time allocated to each of pecking, hopping and looking and (ii) the importance of flock size in determining the amount of time allocated to each behaviour would increase with distance from cover. To test these two hypotheses I examined the relationships between flock size, seed density and time budgets in the same way as for the cattleshed habitat. Methods. I recorded the feeding behaviour of individual birds, the size of feeding flocks, local seed density and current and minimum air temperatures in exactly the same way as described in section A (ii) Methods. The only difference was that, since seeds in the fields were not covered by bedding straw, sampling was simply a matter of counting seeds on the surface of the ground. At the same time I also measured the distance from the point at which

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an observed bird had been feeding to the foot of the hedge which was being used as a vantage point during that particular feeding bout. I recorded a total of 101 feeding sequences (mean sequence length: 11.85 4- 3.12 s, range: 3.223.7 s.). Results. Partial regression analysis showed the relationships between the three behaviours to be the same as for the shed environment; pecking rate was negatively correlated with the rates of hopping and looking (partial correl, coeff. hopping: -- 0.507, P < 0.001; looking: -0.532, P < 0.001) and the rates of hopping and looking were negatively correlated (partial correl, coeff.: -- 0.299, P < 0.05). There was a striking difference between habitats, however, in the rate of looking. Table II shows the mean rates of performance of each of the three behaviours in both habitats. Although there was no significant difference between the rates of pecking and hopping, the rate of looking was much higher in the open fields than in the shed. Looking also interrupted or followed sequences of pecks more frequently than in the shed (44.4% of looks occurred between pecks (cf. 16.67% in the sheds); mean percentage sequence comprising pecks: 32.2 ~ 6.22, hops: 37.3 4- 6.22, looks: 30.9 4-5.12). These data suggest that scanning for predators was of much greater importance to birds when they were feeding in the fields than when in the cattlesheds and consequently occurred more frequently during feeding. Since looking rate was negatively correlated with the rate of pecking and looks were likely to interrupt pecking sequences, the rate of looking may have constrained pecking. To test this I looked to see whether feeding behaviour was still infuenced mainly by seed density as in the sheds. Table III shows that pecking rate and looking rate were significantly correlated with flock size when seed density was corrected for. This contrasts with the results of analyses on birds feeding in the cattleshed where correlations of these behaviours with flock size were minimal. Moreover, when the rate of performance of each behaviour was plotted separately against flock size (Fig. 6), there was no evidence of the sudden transitions seen in Fig. 2. However, rates of all three behaviours were significantly correlated with local seed density and the rate of pecking still approximated to a Type 2 response (Fig. 7) although it was not significantly affected by ambient or minimum air temperature. This suggests that flock size may have had a modi-

BARNARD: FLOCK FEEDING AND TIME BUDGETS IN SPARROWS fying influence on time budgets in the fields in comparison with the sheds and that this may have been brought about by an increased rate of looking in the fields and the correlation between looking rate and flock size. Since both flock size and distance from cover are likely to affect an individual's probability of being captured by a predator, we might expect the rate of scanning to have been affected independently by variation in these two parameters. Partial regression analysis (see Table III) showed that looking rate was indeed significantly correlated with distance from cover as well as with flock size; birds therefore scanned less frequently in larger flocks at a given distance and more frequently in a given flock size the further out they were feeding. The relationship between the rate of looking and both flock size and distance from cover is shown in Fig. 8. The reduction in looking rate with increasing flock size would only be advantageous if the overall rate of scanning by the flock (flock vigilance) did not decrease. If it did, then any possible advantage gained by individuals in terms of energy intake may be lost through increased risk of predation. To check this I examined data from feeding bouts within a restricted range of distance from cover (between 2 and 3 m out from a hedge) to minimize changes in looking rate with varying distance. Overall flock vigilance was defined as the product of the

303

rate of looking for an observed individual and the number of birds in the flock in which they were feeding. The results obtained for a range of flock sizes are plotted in Fig. 9. Although there is a great deal of fluctuation (probably owing to the small sample size), linear regression produced a line whose slope was not significantly different from zero. There is, therefore, no reason to suppose that overall flock vigilance decreased with individual rate of looking. This suggests that birds adjusted their rate of scanning to the minimum required to maintain flock vigilance and is comparable with the models and observations of Pulliam (1973) and Caraco (in press a, b). According to Pulliam (1973), flock vigilance is maintained as individuals reduce their scanning rate with increasing flock size because each individual scans independently and there is little overlap between individuals for scanning at any given time. Pulliam et al. (MS) have recently devised a model which suggests how birds might regulate their scanning rate with reference to the rest of the flock. This analysis of the organization of feeding behaviour suggests that the rate of food intake may have been restricted by the need to ensure adequate protection from predators in an open habitat. Although pecking rate was enhanced by increased seed density, it was mainly influenced by the size of the feeding flock. In contrast to expected results, however, it did

Table H. Comparison of the Mean Rates (Nos./s) of Pecking, Hopping and Looking between the two Habitats

Cattleshed Open fields t

Pecking rate

Hopping rate

Looking rate

n

0.623 :i: 0.048 0.643J: 0.057 0.268 Ns

0.406=k 0.061 0.4944- 0.058 1.05 NS

0.232:k 0.020 0.480=t=0.054 4.31, P < 0.001

57 101

Table IH. Partial Correlation Coefficient Matrix for the Relationship between Each of the Feeding Behaviours and Environmental Variables in the Open Field Habitat (see Table I and Fig. 6)

Dependent variable Flock size Independent Pecking rate variable Hopping rate Looking rate

0.624 **** -- 0.140 -- 0.479 ****

Seed density 0.260 *** -- 0.241 ** 0.362 ****

Data for 98 feeding bouts. P < 0.05 *, P < 0.02 **, P <: 0.01 ***, P < 0.001 ****

Distance

from cover 0.092 -- 0.184 0.216 *

Current t ~ --0.066 0.286 *** -- 0.187

Minimumt o 0.143 -- 0.180 -- 0.166

304

ANIMAL

BEHAVIOUR,

not appear to be influenced by distance from cover. Another surprising result was the significantly positive relationship between looking rate and seed density. These two anomalous results suggest a positive correlation between

a) 2.0

28,

1

distance from cover and seed density. An analysis of the relationship between these two variables showed a significantly positive correlation (correl. coeff.: 0.205, P < 0.05) which may have been sufficient to enhance the rate of pecking so that it masked any distance effect, but not sufficient to reduce the rate of looking when birds were feeding far from cover. The increase in seed density with distance from the hedges was caused by farm staff leaving an unsown border around the fields. Although seed fell into this area during harvesting and sowing, and in the form of spillages from farm vehicles, densities were in general not as high as those in the sown field.

(ii) Flock Vigilance and Flight Distance e-

0

50 flock size

b) 1.0

g 0

In the last section I introduced the concept of flock vigilance to describe the overall rate of scanning by a flock. The method of estimation is of course, a fairly crude one because it assumes that all birds in a flock are behaving similarly at a given time, but it nevertheless provides a means of relating the behaviour of an individual to the flock as a whole. Also in the last section, I showed that the mean rate of looking was significantly higher in the open fields than in the cattlesheds, suggesting that there may also have been differences in the levels of flock vigilance in the two habitats. If so, then, on average, we might expect birds to have reacted to approaching predators sooner in habitats where flock vigilance was high be-

2.

C

0

2.0

50 flock size

c) 1.0

T

T

T,;

7 t

e~

u e~

6

e-

e-

0

.......

50 flock size

~ ........

Fig. 6. The relationship between the rates of pecking (a), hopping (b) and looking (c), and the size of the feeding flock in the open field habitat. N = 101 feeding bouts. Bars represent standard errors. Curves fitted by regression (cf. Fig. 2).

>8000 seed density perm 2 Fig. 7. The relationship between mean peck rate and seed density in the open field habitat. The data do not differ significantly from a Type 2 functional response based on Holling's Disc Equation (solid curve). N = 101 feeding bouts. Bars represent standard errors (see also Fig. 4).

BARNARD: FLOCK FEEDING AND TIME BUDGETS IN SPARROWS cause, at any given time, one or more individuals would have been more likely to scan in the direction of the approaching predator if birds were scanning independently in terms of both time and direction o f orientation. One possible way o f quantifying the effect of variation in flock vigilance is to compare what has been termed the 'flight distance' (Hediger 1950) o f birds at different levels of vigilance. The flight distance is the minimum distance from itself to which an animal will tolerate the approach of a predator before moving away (assuming that it spots the predator before this distance). Since most animals regard man as a predator, some workers have used the flight distance between themselves and an observed animal as a measure o f the limit to which animals tolerate tile approach o f a terrestrial predator (e.g. A l t m a n n 1958; Owens 1977). For solitary animals, this is fairly straightforward but, with flocks o f sparrows it is comm o n for several individuals to move away at one distance, with the remainder leaving only when the 'predator' has approached substantially closer. In the flocks I observed it was c o m m o n for all except one or two birds to leave together in the event of a disturbance and the others either to leave later or remain until the rest o f the flock returned. In my measurement of flight distance I t o o k the distance at which the first group (always the great majority) of birds departed since this clearly indicated that the birds had been alarmed. Methods. To measure the flight distance for feeding flocks I effectively acted as an approach-

305

ing terrestrial predator by walking at a constant pace (1.5 paces/s) towards groups of feeding birds. ! stopped as soon as the first birds t o o k flight and measured the distance between myself and the periphery o f the flock. ! measured distance to the periphery firstly because m y approach was more likely to have been spotted by a peripheral bird (Vine 1971, 1973; Pulliam 1973) and secondly, because measuring to the centre of the flock may have resulted in bias towards longer distances in larger flocks. I sampled flocks feeding in both the cattlesheds and the open fields but only recorded distances where unobscured approaches of 50 m or more were possible. I n addition to measuring flight distance, I recorded the size o f the feeding flock. In 18 cases I also sampled the density of seed on which the birds had been feeding (using a 0.25 m2 quadrat) and recorded current temperature and the minimum air temperature for the preceding 24 h. 1 recorded seed density and temperature to test the notion that the enhancement of pecking rate by increasing seed density may have reduced the level o f flock vigilance and thereby decreased flight distance. Results. As expected on the basis o f the differences in the rates o f looking, flock vigilance was significantly higher in the open fields than in the sheds (mean flock viligance (looks/s) for sheds: 3.20 -~ 0.50, n = 54 flocks; for fields: 5.28 _-lz0.87, n -- 98 flocks; t = 2.07, P < 0.05). Correspondingly, flight distances in the open fields were much longer (mean flight distance (m) for sheds: 17.27 4- 0.96, n -- 31 flocks; for fields: 34.55 :~ 2.34, n = 24 flocks; t = 6.83,

.9

Iooks/s.

(/)

u

u

_o

E 30

Fig. 8. Three-dimensional graph of the relationship between looking rate and both flock size and distance from cover. The height of the peaks shows a significantly negative trend with flock size and a significantly positive trend with distance from cover. Partial correlation coefficients shown in Table III. Data for 98 feeding bouts.

flock size

Fig. 9. Flock vigilance as a function of flock size for birds feeding at a distance of between 2 and 3 metres from cover. N = 46 feeding bouts.

ANIMAL

306

BEHAVIOUR,

28,

1

P < 0.001). This effect was not due to approaches starting at greater distances from the flock or to predators being more easily spotted at greater distances in the open fields, since all samples were started at least 50 m from a feeding flock and with an unobscured view of the flock. The results suggest that the increased rate of scanning in the open fields was adaptive in that it facilitated earlier perception of approaching predators. Table IV shows the results of partial regression analysis of flight distance in relation to the environmental variables measured. In keeping with tile results discussed in section B(i) and shown in Fig. 9, flock size did not significantly influence the length of the flight distance. This is as expected if birds regulated their rate of scanning to the size of the feeding flock in order to maintain a relatively constant level of overall vigilance, but contrast with the findings of other workers (e.g. Powell 1974; Siegfried & Underhill 1975) that large flocks spot predators sooner than small flocks. Flight distance was affected, however, by the density of seeds on which birds were feeding at the time of alarm. The negative relationship between flight distance and seed density may have been attributable to the greater proportion of feeding time taken up by pecking or to the greater benefit of staying at the site per given risk of predation. The flight distance was similarly affected by variation in the minim u m air temperature for the previous 24 h and may reflect the earlier findings that pecking rate was reduced after low minimum temperatures. The response of birds to the availability of their food may therefore have imposed a substantial restriction on their ability to avoid predation. It must also be emphasized that these observations were carried out with a relatively slow moving terrestrial 'predator'; the effect of avoiding aerial predators is likely to be more acute since aerial predators in general approach more rapidly and from a greater range of directions.

In the cattleshed environment where birds were feeding on barley seed concealed in bedding straw, the time budgets of three behaviours (pecking, hopping and looking) appeared initially to be related to variation in flock size although relationships were better described by two intersecting straight lines than by smooth curves. In the past such relationships have led workers to conclude that flocking may enhance the rate at which individuals acquire energy by decreasing their commitment to scanning for predators (Lazarus 1972; Pulliam 1973; Powell 1974); i.e. the need for vigilance constrains feeding rate. The results of this study, however, show that for sparrows feeding in the sheds time budgets were almost completely governed by the functional response of individuals to variations in seed density. When the relationship of feeding behaviours with flock size were analysed in conjunction with environmental variables which affected feeding rate, there was no significant correlation with flock size. In addition to conforming to a more or less Type 2 response to seed density, the rates of pecking and hopping were influenced by the minimum air temperature for the preceding 24 h. Such adjustment would only be expected if these behaviours were not seriously constrained by variables other than the availability of food. The sequential organization o f behaviour in the sheds suggested that pecking and hopping took priority over scanning for predators, and the low levels of correlation of looking with the recorded environmental variables further supports the notion that it played little part in the organization of feeding. In a species which is a dietary opportunist (see Summers-Smith 1963) and which depends in many cases on a highly unpredictable food supply this is perhaps not surprising. In the cattleshed the risk of aerial predation was negligible and upright looking more likely to have been an expression of vigilance for feral cats, cattle or approaching farm staff. Since these are all terrestrial 'pre-

Discussion

Table IV. Partial Correlation Coefficient Matrix for the Relationship between the Flight Distance of Feeding Flocks and Environmental Variables in both Habitats

The results of this study suggest that differences in risk of predation between habitats within a population's environment may have several consequences for the relationship between flock size and behavioural time budgets. The population of sparrows I studied fed in two habitats which differed in the apparent risk of predation to feeding birds.

Flock size Flight distance

0.327

Data for 18 flocks. **P < 0.02.

Seed density --0.584**

Current t~

Minimum t~

0.376 --0.602**

BARNARD: FLOCK FEEDING AND TIME BUDGETS IN SPARROWS dators', their approach could probably have been spotted incidentally during feeding behaviour. This would make the need for special vigilance low except at times of unexpected disturbance and the frequency of looking may have been correlated with the frequency of sudden disturbance in the environment. Several authors have described how unidentifiable disturbances played an important role in the organization of flock feeding in various small passerines (Summers-Smith 1963; Newton 1972; Davis 1973) although it is not clear how this might have resulted in the relationship between looking rate and flock size in Fig. 2c. The curves in Fig. 2a and b, however, may have arisen because flock size was related to the proportion of their feeding time that birds spent in the best feeding area in the environment. As birds spent longer feeding in the shed, they gravitated to areas of higher seed density by hopping more slowly on higher densities. The tendency for birds to land and feed near birds already at a feeding site meant that, as numbers increased with time, birds were more likely to be landing in areas of high density in which they stayed for the duration of their feeding bout. Several features of feeding behaviour in the open fields, however, suggest that it was geared to minimizing the risk to individual birds of being captured by a predator. Flock size was and important influence on the time budgets, the rate of looking was much higher in the fields than in the sheds and the sequential organization of behaviour showed that looking frequently interrupted sequences of pecks. Looking may, therefore, have placed a constraint on the rate of feeding. By associating in flocks, birds appeared to be able to reduce their rate of scanning without significantly altering the overall effectiveness of the flock in spotting predators. This may have complemented the reduction of an individual's statistical probability of capture by joining a flock and facilitated feeding at greater distances from cover. However, increasing distance still had an independent and positive effect on the rate of looking so that, for any given flock size, birds scanned more frequently the further out they were feeding. By reducing their rate of scanning to the level required to maintain flock vigilance, birds were able to allocate more time to feeding than might have been possible if they had fed singly. Birds in the field responded to increasing seed density by pecking more frequently although pecking was not further enhanced by low ram1-

307

mum temperatures preceding observation. Since increased pecking rate negatively influenced the rate of looking, birds feeding on relatively high densities of food, both in the open fields and in the sheds, had a lower level of flock vigilance and shorter flight distances. Because of their higher rate of looking, however, birds which were feeding in the fields had much longer flight distances than when feeding in the sheds, although flight distance was not related to flock size. These results support recent models of avian flock-feeding in which flocking is seen as a mechanism by which birds can reduce their commitment to scanning for predators and increase the time available for feeding; they do not, however, support the findings of other workers that increasing flock size enables birds to spot predators sooner. The main conclusion that can be drawn from the study is that the organization of both flocking and the time budgeting of feeding behaviour reflected the selective pressures of the habitat in which birds were feeding. When birds were feeding in a habitat relatively protected from predators they behaved independently and time budgets were dominated by the functional response to food density. In a habitat where the risk of predation was relatively high, the overall organization of flocking and of feeding budgets reflected the necessity for individuals to minimize their risk of capture. In the population of house sparrows I studied, therefore, birds changed the rules of their feeding behaviour when they moved from one type of habitat to another and these changes appeared to be adaptive in view of the selective pressures they faced.

Acknowledgments I would like to thank Dr J. R. Krebs and Jane Brockmann for reading and criticizing the manuscript, Dr N. B. Davies and Dr J. D. GossCustard for further helpful criticism, Alasdair Houston, Jon Erichsen and Brian Partridge for advice on statistics and computing and Mr P. Gresswell of Hinksey Hill farm for permission to work on his land. I would also like to thank my wife Sifin for criticizing the manuscript and for doing much to enhance its style and presentation. Valerie Goodwin and Carole Parr typed the manuscript and the work was supported by a grant from the Science Research Council.

ANIMAL

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BEHAVIOUR,

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(Received 1 November 1978; revised 5 March 1979; MS. number: 1829)