Plumage variability functions for status signalling in least auklets

Anim. Behav., 1990,39, 967-975

Plumage variability functions for status signalling in least auklets I A N L. J O N E S Department of Biology, Queen's University, Kingston, Ontario K7L 3N6, Canada

Abstract. This study investigated whether the variable plumage of the least auklet, Aethia pusilla, a small

seabird of the Bering Sea, functions for status signalling. Adult least auklets varied continuously in underpart colour from unmarked white to nearly black. Although there was variability within age groups, adults tended to lighten in plumage as they aged. There were no differences in plumage between the sexes. Observations of natural interactions among adult and immature birds of different plumage indicated that lighter birds usually defeated darker birds in agonistic interactions within each age group. Model presentation experiments were performed to test whether plumage colour alone signals social status. Auklet models presented in white plumage were approached less closely and induced more responding adult and immature birds to flee compared to the same models presented with dark plumage. Furthermore, birds with lighter plumage approached the white model closer than did birds with darker plumage. The results provide evidence for status signalling by plumage in a non-passerine bird and document another species where status signalling works within age (adult versus immature) categories.

To explain the conspicuous plumage variability of some passerine birds that overwinter in flocks, Rohwer (1975) proposed the status-signalling hypothesis. This hypothesis suggests that variable plumage traits have evolved to signal individual dominance status. The hypothesis assumes that in avian flocks there is variability in individuals' fighting ability, that social status correlates with this fighting ability, that individuals gain advantage if they signal their status/fighting ability to others, and that cheating (displaying a signal inappropriate to an individual's fighting ability) is costly (Rohwer & Ewald 1981; Rohwer 1982). Evidence for the hypothesis has been mixed, and its interpretation has been controversial. Studies of great tits, Parus major (J~irvi & Bakken 1984), and house sparrows, Passer domesticus (Moller 1987a), have shown that status signalling may occur within age and sex categories, apparently providing full support for Rohwer's (1975) hypothesis. Balph & Balph (unpublished data) have claimed to have evidence for individual status signalling in pine siskins, Carduelis spinus, but their study did not involve experimental manipulations to confirm signal function and their results have never been presented in detail. Studies of several other species (white-crowned sparrows, Zonotrichia leucophrys, Fugle et al. 1984; Harris' sparrows, Z. querula, Rohwer et al. 1981; Rohwer 1985; Watt 1986; Jackson et al. 1988; and dark-eyed juncos, Junco hyemalis, Balph et al. 1979) have shown plumage 0003-3472/90/050967 + 09 $03.00/0

variability signalling status only between, and not within, age and sex categories. All tests of the status-signalling hypothesis have concerned small passerine birds, except Whitfield's (1986), which provided evidence that plumage variability functions for individual recognition, but not for status signalling, in ruddy turnstones, Arenaria interpres. A controversial aspect of the status-signalling hypothesis concerns the degree to which individual status and plumage are related to age and sex. Whitfield (1987) suggested that when plumage badges serve only to signal status differences between age and sex categories, they should not be included under the term 'status signalling'. The logic of this point is that plumage variability related mainly to age and sex is likely to have evolved for reasons other than selection pressure for individual status signalling. This would contradict Rohwer's (1975) hypothesis, which proposed that plumage variability has evolved specifically for signal function. However, it may be appropriate to classify plumage badges as status signals even if they do relate to age and sexual differences, since the term 'status signalling' does not itself imply evolutionary origin. Including the mode of evolutionary origin in the broad definition of status signalling may confuse the issue, since it is seldom easy to investigate directly the origin of traits like plumage badges. Two important questions regarding status signalling have emerged. (1) Does Rohwer's (1975) original status-signalling hypothesis provide a

9 1990The Association for the Study of Animal Behaviour 967

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general explanation for the evolution of avian plumage variability, or does status signalling normally relate to pre-existing plumage traits related to age and sex (Whitfield 1987)? (2) Is the status-signalling hypothesis applicable to species other than small passerines that overwinter in flocks? To help answer these questions I investigated whether least auklet, Aethia pusilla (Alcidae), plumage variability has a status signalling function. Least auklets are small (75-100 g) seabirds of the Bering Sea region. During the breeding season (May-August), auklets congregate at dense colonies situated on talus slopes and boulder strewn shorelines. In April, least auklets moult into a conspicuously polymorphic summer plumage; the breast and belly feathering varies continuously among birds from unmarked white to nearly black (Brdard & Sealy 1984). Agonistic interactions are frequent at the colony and may involve contests over access to mates or crevice nest sites. To assess whether plumage variability functions for individual status signalling, I first looked for a relationship between plumage and dominance by scoring the outcome of interactions between birds of different plumage. Later, I conducted a model presentation experiment to test whether plumage alone could function as a signal of dominance and I measured the change in plumage of adult individuals between two breeding seasons.

METHODS Fieldwork was conducted at a colony of more than 10000 least auklets near Tolstoi Point, St Paul Island, Pribilof Islands, Alaska (57~ 170~ during May-July of 1987 and 1988. Auklet nesting habitat at this site consisted of sparsely vegetated boulders and rubble along several kilometres of shoreline at the base of a 50-m basalt cliff. To study plumage variability, auklets were captured with noose carpets at two study plots at Tolstoi. At an observation plot, 248 birds (234 adults, 14 immatures) were captured and colourmarked in 1987. In 1988, 369 birds (306 adults, 63 immatures) were captured and colour-marked at the same site. Immatures (2-year-olds) were distinguished by their worn forehead and flight feathers and spotted throats (Brdard & Seaty 1984). Each auklet was given a numbered USFWS stainless steel band and a unique combination of three

colour bands. Upon capture, birds were weighed to the nearest 1 g using a Pesola spring scale, and measurements of their culmen, wing length (flattened) and tarsus length were taken. I obtained repeated weighings of marked birds using three electronic balances (Ohaus D 1000 LA) set on display rocks on the study plot. Each bird was assigned to a plumage category according to my estimate of the density of dark spotting on the underparts: O, immaculate (0% dark); 1, lightly spotted (1-20% dark); 2, intermediate (21-50% dark); 3, heavily spotted (51-80% dark); or 4, dark (more than 80% dark). Since categories were estimated in the field, some error in the identification of birds intermediate between categories was inevitable, but estimates had a high degree of repeatability (more than 90%, 1987). Since sexual dimorphism is extremely slight in least auklets (Brdard & Sealy 1984), the captured birds could not be sexed from external measurements. However, I was able to identify a large sample of males by watching for their distinctive vocal advertising display (restricted to males, Jones, unpublished data). I was able to identify female members of mated pairs by observing their repeated associations with known males. To quantify plumage change between years, I banded and photographed the underparts of least auklets captured in 1987 at a separate plot at Tolstoi. In 1988, 35 of these birds were recaptured and rephotographed. Underpart plumage spotting was quantified using a grid marked on transparent Plexiglas. The grid was placed over the photograph of each bird's underparts and the number of grid intersections over dark versus white plumage was counted (out of 25 possible intersections). The plumage score was expressed as the count of intersections over dark feathering and thus represents the proportion of each bird's breast with dark plumage. To quantify further plumage variability in the population, I again used the grid to measure the density of dark spotting of an additional 83 colour-marked birds photographed in 1988. The relationship of plumage darkness to dominance was investigated by scoring the outcome of 899 agonistic interactions between least auklets. Least auklets are highly social and daytime densities of birds on the surface of the colony frequently exceeded 5 per m - 2 (Jones, unpublished data). I defined an agonistic interaction as occurring when two birds lunged or pecked at one another, or engaged in a short fight. The 'winner' of

Jones: Status signalling in least auklets

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Table I. Summary of the model presentation experimental design June 21 Model 1 Model2 Model3 Model4 Model5

24

28

July 29

30

14W 38W 141 271

1 51W

6

8

9

10

II

12

14

15

16D 69D 26W 49W 25W 25D 58D 37W 74W 32D 59D

511 521 631

7

441

621 28I 611 241

Totals 103W:85D 100W:83D lllW:91D 2071 2191

Timing and number of presentations of each model with white plumage (W), darkened plumage (D) or natural intermediate plumage (I).

an interaction was considered to be the bird that maintained its position, the 'loser' the bird that retreated or departed. Most birds observed in interactions were not colour-marked, so I identified the age (adult versus immature) and plumage category of the winner and loser of each interaction as it occurred. Observations were made at 10 different locations throughout the colony at Tolstoi, including the study plots, 1o preclude repeated observations of the same individual birds. To test whether plumage darkness functions as a signal by itself, I performed a model presentation experiment. Three adult male least auklets with immaculate white underparts were collected 6 km from the Tolstoi colony and mounted in similar lifelike alert postures. I quantified the response of auklets to the three models presented first with unaltered white underparts, then later with the underparts dyed to resemble intermediate plumaged birds. I collected and mounted two additional least auklets of intermediate plumage and presented these throughout the experimental time period to control for seasonal effects. All presentations were made on 10 different large display rocks at Tolstoi, each frequented daily by hundreds of least auklets. I divided the presentations of each model among different rocks to minimize the chance of recording the response of individual least auklets more than once. Models were actively approached by birds frequenting the display rocks. For each auklet that responded to the model, I measured the distance of closest approach (0 cm (touching), 1-10 cm, 1120 cm, 21-30 cm or greater than 30 cm, determined from a marked string placed beside the model) and the response time (time in seconds from when the responding bird first noticed the model until it lost

interest). I also noted whether the responding birds fled from, pecked or adopted a hunched display posture towards the model, I recorded the behaviour of the approaching birds for only the first 15 s ,of response, since approachers often reacted unnaturally to the immobility of the model after that time. The model presentations were made between 21 June and 15 July 1988 (Table I), when most of the breeding birds in the colony were incubating. At this stage of the season, the auklets on the surface of the colony at the time of day when the model experiments were conducted consisted of about 30 % nonbreeding adults, 30% breeding adults and 40% immature (2-year-old) birds (Jones, unpublished data). I used the model experiment to evaluate several predictions of the status-signalling hypothesis. If white plumage signals high social status, the models would appear more threatening when presented white compared to when presented with plumage dyed to resemble intermediate birds. Thus, I predicted that least auklets responding to the models would approach the white models less closely, and remain near them for a shorter time compared to the same models presented with darkened plumage. I also predicted that the white models would be less likely to be touched or pecked, and more likely to be fled from. A further prediction of the statussignalling hypothesis is that individuals with high social status should approach and engage in escalated interactions with other signalling high status. I predicted that if white plumage functions as a signal of high status, light-plumaged auklets responding to the white models should approach closer and for a longer time period than dark birds, and be more likely to exhibit aggressive behaviour. All statistics were performed with Statview 512 +

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Animal Behaviour, 39, 5

(Feldman & Gagnon 1986), except the log-linear analysis of the model experiment results, which were performed using SYSTAT (Wilkinson 1987).

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Plumage Variability Least auklet plumage was highly variable, with about 10% of adult birds having unmarked underparts and the remaining birds varying from lightly spotted to nearly black underparts (very rare; Figs 1, 2). Category 2 birds (21-50% of underparts covered with clark spotting) were the most common, comprising over 60% of adults (Fig. 1). Immature birds (2-year-olds, most returning to the colony for the first time after fledging) tended to be 'darker than adults (Mann-Whitney U-test, U= 9704.5, P < 0.0001, N = 235 adults, 63 immatures). I never noticed a white (category 0) immature auklet during my work at St Paul, lightly spotted (category 1) immature auklets were rare, and a greater proportion of immature auklets were classified as heavily spotted (category 3) than adults (Fig. 1). Based on the scoring of photographs of adult birds, the plumage variability is nearly continuous (Fig. 2). Males and females did not differ in plumage; the proportion of colour-marked breeding males (sex identified by advertising display behaviour) in the four plumage categories did not differ from proportions in the breeding population (Z2= 1.73, dr=3, P=0.630, N = 2 9 males, N = 1 6 females). Furthermore, there was no significant difference in the plumage of known males and females of breeding pairs (Mann-Whitney U-test, U=269, P = 0.3315, N = 2 9 males, N = 1 6 females). Breeding adult auklets were lighter in plumage than nonbreeding adults (Mann-Whitney U-test, U = 7758, P = 0.0056, N = 91 breeders, N = 141 non-breeders). Breeding and non-breeding auklets were distinguished by the breeding birds' regular deliveries of food to a nesting crevice on the study plot. In June and early July of 1988, I recaptured and rephotographed the underparts of 35 auklets that had been captured at the same stage of the nesting season in 1987. Based on the quantification of plumage darkness with the scoring grid, least auklets tended to get lighter with age (Wilcoxon signed-ranks, z = - 3 " 9 4 1 , P<0'001; Fig. 3). Auklets with darker than average plumage underwent the greatest plumage change between 1987 and 1988.

[0 0

I Plumage

2 categories

3

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Figure l. Plumage variability as indicated by the fourcategory classification, based on 298 auklets (235 adults: 9 63 immatures: []) captured in 1988.Immatures tended to be darker in plumage than adults (Za =32.94, df= I, P<0.0001). Plumage categories: 0, immaculate (0% dark); 1, lightly spotted (1 20% dark); 2, intermediate (21-50% dark); 3, heavily spotted (51-80% dark); or 4, dark (more than 80% dark feathering). Category 4 (dark >80% dark feathering) birds were not observed in this sample of banded birds.

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12 14 16 Dark Plumage score ( / 2 5 )

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Figure 2. Adult plumage variability as indicated by plumage scores (number of grid intersections over dark breast feathering out of 25 possible intersections) measured from photographs of 83 birds in 1988.

Relationship of Plumage to Body Measurements Based on measures of birds weighed after capture and with the remote electronic balance system, the overall means ( _ sz) mass of adults was 85.7 +__0.3 g and the mean weight ofimmatures was 79"8 ___0'4 g. Adults were significantly heavier than immatures (Table II) and adult mass differed among the plumage categories; both white and lightly flecked auklets were significantly heavier than auklets with intermediate coloured plumage (Table II). There were no significant differences among the plumage categories in other body size measurements of adults although adults did have longer wings than

Jones." Status signalling in least auklets t~

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Figure 3. Plumage change between 1987 and 1988, based on plumage scores from 34 birds photographed in both years. Auklets tended to get lighter with age (27 lighter versus 5 darker, binomial (two-tailed) z=-4.116, P < 0.001). Plumage change between the years was correlated with plumage score in 1987 (r=-0.387, P = 0.0236). immatures (Table II). Similarly, culmen length did not differ significantly among adult plumage categories, but adults had longer bills than immatures. There were no significant differences in weight, wing, culmen or tarsus between immature auklets of different plumage categories. Plumage Variability and Social Status Birds with lighter plumage usually won agonistic interactions between auklets of different plumage categories. For all plumage category combinations where more than 20 agonistic interactions were scored, the lighter birds won significantly more interactions than did the darker birds (Table III). For interactions between white and lightly flecked birds, and between lightly flecked and heavily flecked birds, the auklet with lighter plumage won over 75% of the time, but the result was not significant for the samples of observations obtained. Immature auklets frequently engaged in agonistic interactions, but nearly always lost (365 of 396 interactions (92%), binomial P < 0.0001). Among immature birds, birds with lighter plumage were also most likely to win interactions with birds with darker plumage: the intermediate birds won 36 (67%) of 54 interactions between immatures of intermediate and heavily flecked plumage categories (binomial, P = 0.0142, two-tailed). Model Experiments Least auklets responded to models in the manner consistent with the hypothesis that white plumage

971

signals high status. Responses did not differ significantly among the three models (chi-squared tests, P=0.299-0.9368), so the data were combined for analysis. Effects of model colour and the plumage of approaching birds on closest approach distance and response time were determined from log-linear analysis performed using SYSTAT (Wilkinson 1987). To fulfil the requirements of this analysis, categories were merged in cases where cell frequencies were small; for adults, responses of white birds (three scored) were merged with lightly flecked birds, and for immatures, one lightly flecked bird's response was merged with intermediate birds and one dark bird's response was merged with heavily flecked birds. Higher order interactions were insignificant and therefore excluded from the analysis. Each model was approached less closely when it was presented in the white form compared to the darkened form (Fig. 4). This held for both adult (partial X2 = 24.98, df= 3, P < 0'001) and immature auklets (partial X2=50.05, df=3, P<0'001). Adults responding to the models spent less time near white models than darkened models (partial Z2= 8.28, df= 2, P = 0.01; Fig. 4), but there was no difference in response time of immatures between white and darkened models (partial Z2=3.86, df= 2, P = 0.25; Fig. 4). Although the number of auklets fleeing the models was low, immature birds were more likely to flee from the white models than from the dark models (Z 2 = 12.00, df= 3, P=0.001). More adults fled from the white model (five) than from the darkened model (one), but the difference was not significant. Few responding birds pecked or directed the hunched display posture towards the models and the differences in these behaviours between white and darkened models were not significant. Light-plumaged adult birds approached the white model significantly closer than darker birds (partial X2= 15.56, df= 3, P=0.05). There was a similar difference in the response of immature birds related to their plumage (partial Z 2 = 15"56, df= 3, P=0"005). Light and dark birds did not respond differently to the dark models. The plumage of approaching birds had no significant effect on the duration of their response, and the sample of fleeing birds was insufficient for analysis of the effect of plumage. These results were not confounded by changes in response to the models related to date. Response of adult birds to all models was unrelated to date

Animal Behaviour, 39, 5

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Table II. The relationship among age, plumage, body mass and size measures

Age and plumage

Adult 0

Mean mass(g)* N Mean wing(mm)t N Meanculmen(mm)~ N Meantarsus(mm)w N

87.1 0.8 21 97.5 0.7 22 9.2 0.1 21 19.5 0.I 22

Adult 1 87.5 0.8 42 97.7 0.4 " 45 9.4 0.1 44 19.6 0.1 45

Adult 2

Adult 3

Immature 2, 3

84.9 0.4 144 96.9 0.2 147 9.3 0.1 145 19.4 0.1 149

85.1 1.1 19 96.9 0.4 19 9.4 0-1 19 19.2 0'2 19

76.8 0.6 62 95.4 0.3 62 9.2 0'1 63 19.5 0.1 62

*Adult mass differed among categories (ANOVA, F3.222 = 4.004, P = 0.0084) and mass differed significantly between categories 0 and 2, and between 1 and 2. (Fisher protected least significant difference test, P < 0.05). Adults were heavier than immatures (two-factor ANOVA, F1,286 = 33,933, P < 0.0001). tWing length did not differ among plumage categories (ANOVA, F3,229= 1.549, P=0.2026), but adults had longer wings than immatures (two-factor ANOVA, F1,293 = 11.511, P = 0.0008). ~Culmen length did not differ among plumage categories (ANOVA,/73,225 =0.374, P = 0.772), but adults had longer bills than immatures (two-factor ANOVA, F 1,29o= 5.075, P = 0.0253). w length did not differ among plumage categories (ANOVA, F 3,231 = 1.327, P = 0.2662), nor were there differences between adults and immatures (two-factor ANOVA, F1,295 = 1"136, P = 0'2877). Table IlL Results of interactions of adult least auklets of different plumage categories

Interactions 0 versus 1 0 versus 2 1 versus 2 1 versus 3 2 versus 3 All

Observations

Lighter bird wins

P (binomial)

12 22 105 7 76 222

9 (75%) 18 (82%) 69 (66%) 6 (86%) 61 (80%) 163 (73%)

0.084 0.002 <0.001 0.058 <0.001 <0.001

0: White plumage; 1: fight plumage; 2: intermediate; 3: heavily flecked plumage. Binomial, two-tailed tests. (Spearman rs=0"0224).456, P>0-05). Response of immatures tended to decrease slightly (closest approach distance decreased (Spearman r s = 0.1644)-593, P < 0 . 0 5 ) for the models during the 25-day period of the experiments. This may have related to the decline in numbers of immatures at the colony towards the end of the experimental period. The change in response by immatures over the experiment does not affect the interpretation of

the results since all immatures showed closer approaches to the darkened models late in the experiment compared to the white models presented earlier, opposite to the seasonal trend. DISCUSSION The results of this study provide support for the status-signalling hypothesis. Status signalling through plumage coloration occurred within the adult and immature age groups of least auklets. The presence of a status-signalling system was initially suggested by plumage-related differences in success in agonistic interactions (Table III). However, correlation of a plumage trait with dominance is not by itself evidence for signalling, since individuals could assert their dominance by other means, and plumage could simply be a correlated trait and not a signal. The model experiments provide evidence for signalling, since the birds' responses changed when the models were changed from white (high status) to intermediate (lowered status). I interpret the hesitancy of adults to approach closely the white model, compared to the same model

Jones: Status signalling in least auklets

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Figure4. Results of the model presentation experiment: (a) closest approach distances of adults; (b) response durations for adults; (c) closestapproach distances of immatures; and (d) response durations for immatures. The figuresshow the combined data for intermediate approachers (plumagecategory 2) for the three models. Both adults and immatures (2year-aids) approached closer when the models were presented dark than when presented white. []: Approaching white model; 9 approaching dark model.

darkened, as confirmation of the suggestion that white plumage signals high status. Furthermore, more responding immature birds fled from the white models than from the same models darkened. The observation that birds with light plumage approached the white models closer than birds with darker plumage was also consistent with the statussignalling interpretation. Since least auklet plumage tended to lighten with age, it is tempting to conclude that the signalling system may be based mainly on age, and thus does not support Rohwer's (1975) original formulation of the hypothesis. However, as in house sparrows (Moller 1987a) and great tits (J/irvi & Bakken 1984), where the study birds were identified as either adults or immatures, least auklet plumage variability functions for status signalling within the adult age group. Furthermore, lighter plumaged immatures were also the most successful in winning interactions within the immature age group (all 2 years old). The difficulty of obtaining sufficient numbers of known age birds of most species may hamper investigation of the most refined question about age: does plumage variability signal status differences within groups of birds of exactly the same age?

Since I was unable to determine the sex of most birds observed in natural interactions or approaching the models, the results do not permit a direct investigation of the relationship of sex and dominance. However, both sexes engaged in interactions, actively responded to the models, and were included in the analyses. I have assumed that the tendency to approach each model did not differ between the sexes. I detected no differences in plumage betweeen the sexes, and known females were observed defeating males in agonistic encounters. However, these results give few clues about the importance of sex in determining the outcome of interactions between birds of different plumage, and this question certainly deserves further study. Immatures changed their response behaviour when the plumage of models of adult birds was altered experimentally, suggesting that status signallingmay also occur between adults and immatures in auklets. Immatures differed from adults in plumage (B6dard & Sealy 1984) but I did not test whether the difference (spotted throat and reduced head ornamentation of most immatures) functions as a signal to adults of their low status. In addition to its relation to status, plumage variability was also correlated with body mass.

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Animal Behaviour, 39, 5

Among adults, lighter plumaged auklets tended to be heavier than darker plumaged auklets, although there were no significant differences in body size, as measured by eulmen, wing and tarsus lengths. This suggests that lighter plumaged birds were either in better condition or had greater gonadal development than darker birds. Either of these possibilities could explain why lighter plumaged birds were more likely to be breeders than birds with darker plumage. These results have several implications for the status-signalling system in least auklets. First, the ability of lighter plumaged birds to win interactions may be based on their greater mass, as well as their greater experience. Second, since birds with light plumage were more likely to breed, plumage lightness could be a signal of potential value in mate choice. Choosing a light-plumaged member of the opposite sex as a mate could have benefits, since least auklet pairs share demanding periods of incubation (32 days) and chick provisioning (28 days; Roby & Brink 1986a). A question about status signalling that is still wide open concerns the heritability of plumage badges of status. Moller (1989) has reported that badge size in house sparrows had significant heritability and is subject to sexual selection, but this is the only statussignalling species for which this information is available. In least auklets, plumage lightens with age, but there is variability within groups of knownage birds (e.g. 2-year-olds), so there is the possibility that some of the plumage variability could be heritable. The status-signalling hypothesis (Rohwer 1975; Rohwer & Ewald 1981) concerns phenotypic variability and does not propose that variable plumage status signals should be heritable. These findings suggest that we need more tests of whether status signalling involves either: (1) nonheritable phenotypic traits that vary within individuals and reflect dominance status; or (2) heritable traits that correlate with individual quality, are subject to sexual selection, and reflect dominance status. An interesting difference between the least auklet plumage signal and other status-signalling systems is that in auklets, light plumage (reduced pigmentation) signals high status, while in sparrows (e.g. Harris' sparrow, Rohwer et al. 1981; house sparrow, Moller 1987a), it is increased deposition of pigment that signals high status. If there is a direct physiological cost to deposition of black pigment in breast feathers, it is the low-status auklets that make the investment. The inverse relationship

between age and pigmentation in least auklets may relate to phylogenetic influences, since alcids are in the same order as gulls (Charadriiformes) in which immatures are also darker in plumage than adults and lighten with age. Over all species, the form of status signals (dark versus light plumage) may be somewhat arbitrary, but the resolution of this question awaits further study. Much attention has been given to the question of how 'cheating', or having a plumage badge that signals a status higher than an individual's true fighting ability, is controlled in natural populations (Rohwer & Ewald 1981; Rohwer 1982; Fugle & Rothstein 1987; Moller 1987b; Whitfield 1987). Although I did not directly investigate this, true status or fighting ability may be difficult for individual least auktets to conceal because of the constant testing inherent in a social system where agonistic encounters are so frequent. In this situation, there may be a high cost to cheaters, since they would lose escalated agonistic interactions with auklets showing a similar but honest signal. As suggested for white-crowned sparrows (Fugle & Rothstein 1987), another possible control on cheating is predation. Low-status auklets with dark plumage are more cryptically coloured than white high-status auklets when viewed against the dark lichen-covered boulders of their nesting habitat. Since least auklet colonies may be attended by both avian and terrestrial predators (Knudson & Byrd 1982, personal observation), signalling high status with lighter and more conspicuous plumage may carry a predation cost that less experienced immatures and low-status adults cannot afford. The results of this study do not preclude the possibility that least auklet plumage variability also functions for individual recognition. Individual variation in the pattern of underpart spotting was as striking as the variation in density (darkness) of the spotting (Jones, unpublished data). Furthermore, I was able to identify individually several marked birds at the study plot by their underpart pattern alone. These observations suggest that plumage variability may also function for individual recognition, at least among individuals that encounter one another regularly (e.gl mates). The least auklet is the only alcid species with continuous variation in adult plumage. The presence of a status-signalling system based on variable plumage in this species may be expected, since least auklets congregate in very high densities at their colonies, there is a high turnover of birds at any site

Jones: Status signalling in least auklets on the surface of the colony, and there is a high frequency of agonistic encounters which may involve territoriality, nest site acquisition and competition for mates (personal observations). The availability of suitable nest sites may limit auklet populations (Roby & Brink 1986b). These factors may give rise to a situation where conspicuous visual status signals are favoured by selection (Rohwer 1975; Whitfield 1987), since plumage signals would help individuals avoid costly escalated interactions with each unfamiliar individual they encounter. One other species of diurnal crevice nesting alcid of the Bering Sea, the crested auklet, Aethia cristatella, has similar colony structure to the least auklet. Crested auklets have a variable and conspicuous forehead crest, but otherwise there is little variability in plumage characteristics. Further study may reveal whether seabird ornaments, such as the crested auklet crest, could also function as status signals. ACKNOWLEDGMENTS Robert Montgomerie provided ideas, advice and support throughout this study. Thanks to Anne Harfenist and Simon Gawn for help with the fieldwork; Vernon Byrd and Art Sowls for helping with the logistical support provided by the Alaska Maritime National Wildlife Refuge, United States Fish and Wildlife Service; Henrik Smith for providing invaluable advice on statistical procedures; James V. Briskie, Monica Mather, Henrik Smith, Jean-Pierre Savard and Dan Roby for commenting on an earlier version of this manuscript; and Vern McCorkle and the City of St Paul for generously providing logistic and other aid. This work is dedicated to the cause of environmental preservation for the Pribilof Islands. Financial assistance was provided by grants from the National Geographic Society Committee for Research and Exploration, the Natural Sciences and Engineering Research Council (to R. D. Montgomerie) and the Frank M. Chapman Fund of the American Museum of Natural History. REFERENCES Balph, M. H., Balph, D. F. & Romesburg, H. C. 1979. Social status signalling in winter flocking birds: an examination of a current hypothesis. Auk, 96, 78-93. Brdard, J. & Sealy, S. G. 1984. Moults and feather generations in the least, crested and parakeet auklets. J. Zool., 202, 461-488.

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Feldman, D. S. & Gagnon, J. 1986. Statview 512+ The Professional Graphic Statistics Utility. Calabasa, California: Brainpower. Fugle, G. N. & Rothstein, S. I. 1987. Experimentson the control of deceptive signals of status in white-crowned sparrows. Auk, 104, 188 197. Fugle, G. N., Rothstein, S. I., Osenberg, C. W. & McGinley, M. A. 1984. Signals of status in wintering white-crowned sparrows, Zonotrichia leucophrys gambelli. Anita. Behav., 32, 86-93. Jackson, W. M., Rohwer, S. & Winnegrad, R. L. 1988. Status signallingis absent within age and sex classes of Harris' sparrows. Auk, 105, 424-427. J/irvi, T. & Bakken, M. 1984. The function of the variation in the breast stripe of the great tit (Parus major). Anita. Behav., 32, 590-596. Knudtson, E. P. & Byrd, G. V. 1982. Breedingbiology of crested, least and whiskered auklets at Buldir Island, Alaska. Condor, 84, 197-202. Moiler, A. P. 1987a.Variation in badge size in male house sparrows Passer domesticus: evidencefor status signalling. Anim. Behav., 35, 1637-1644. Moller, A. P. 1987b. Social control of deception among status signalling house sparrows Passer domesticus. Behav. Ecol. Sociobiol., 20, 307-311. Moller, A. P. 1989. Natural and sexual selection on a plumage signal of status and on morphology of house sparrows, Passer domesticus. J. Evol. Biol., 2, 125-140. Roby, D. D. & Brink, K. L. 1986a. Breeding biology of least auklets on the Pribilof Islands, Alaska. Condor, 88, 336-346. Roby, D. D. & Brink, K. L. 1986b. Decline of breeding least auklets on St George Island, Alaska. J. Fld OrnithoL, 57, 57-59. Rohwer, S. 1975. The social significanceof avian winter plumage variability. Evolution, 29, 593 610. Rohwer, S. 1982.The evolution of reliable and unreliable badges of fightingability. Am. Zool., 22, 531-546. Rohwer, S. 1985. Dyed birds achieve higher status than controls in Harris' sparrows. Anita. Behav., 33, 1325-1331. Rohwer, S. & Ewald, P. W. 1981. The cost of dominance and advantage of subordination in a badge signalling system. Evolution, 35, 441-454. Rohwer, S., Ewald, P. W. & Rohwer, F. C. 1981. Variation in size, appearance and dominance within and among the sex and age classes of Harris' sparrows. J. Fld Ornithol., 52, 291-303. Watt, D. J. 1986.A comparative study of status signalling in sparrows (genusZonotrichia). Anim. Behav., 34,1-15. Whitfield, D. P. 1986. Plumage variabiltiy and territoriality in breeding turnstone: status signallingor individual recognition?Anim. Behav., 34, 1471-1482. Whitfield, D. P. 1987. Plumage variability, status signalling and individual recognition in avian flocks. Trends Ecol. EvoL, 2, 13-18. Wilkinson, L. 1987. SYSTAT: the System for Statistics. Evanston, Illinois: Systat. (Received 3 May 1989; initial acceptance 26 June 1989; final acceptance 28 August 1989; MS. number: A5555)