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Influence of nestling experience on nest-type selection in captive kestrels
1028
Animal Behaviour, 33, 3
order in which cats were tested was varied ran- (Table I: compare S t, S~ and $3). This suggests that, domly between sessions. while perceptual processes may allow the formaThe mean numbers of trials before a preference tion of a map of a simple environment in a single was shown were 4.25 ( + 1.53 sE), 4.12 ( + 1.53 sE) exposure (Poucet 1984), only locomotor experience and 1"87 (+__0-74 SE) trials for days 1, 2 and 3 during exploratory activity can provide a basis for respectively. A Wilcoxon matched-pairs signed- the setting up of more complex maps. ranks test (one-tailed) revealed statistically significant differences between days 1 and 3 ( T = 3, df= 8, B. Pouc~x P<0.025) and between days 2 and 3 ( T = 4 , df=8, Dkpartement de Psychologie Animale, P < 0.025), but no difference between days 1 and 2. C.N.R.S.-INP9, 31 ehemin Joseph-Aiguier, Table I gives the frequencies of choices and 13402 Marseille cedex 9, France. numbers of subjects showing a preference for each route from each starting-point. Although the cats References were rewarded regardless of path chosen, they all Blancheteau, M. & Le Lorec, A. 1972. Raccourci et d&our chez le rat: durre, vitesse et longueur des showed a preference. In 21 cases out of 24, the parcours. Ann. Psyehol., 72, 7-16. preferred routes were those assumed to be optimal according to one parameter. Furthermore, prefer- Chapuis, N., Thinus-Blanc, C. & Poucet, B. 1983. Dissociation of mechanisms involved in dogs' oriented ences appear to have been established more rapidly displacements. Q. J. exp. Psychal., 35B, 213-219. when starting from $2 than from $1 and from $l Dashiell, J, F. 1931. Direction orientation in maze than from $3 (Friedman two-way analysis of running by the white rat. Comp. Psychol. Monogr., 7, variance: X2r= 6.08; dr= 3,8; P < 0-07). 1-72. Despite the spatial complexity of the environ- Poucet, B. 1984. Evaluation of connectedness by cats in path-selection problems. Percept. mot. Skills, 58, ment, cats accurately and spontaneously selected 51-54. routes according to their length and/or angular characteristics, thus showing further evidence of Poucet, B., Thinus-Blanc, C. & Chapuis, N. 1983. Route planning in cats, in relation to the visibility of the goal. route planning (Poucet et al. 1983). More specifiAnita. Behav., 31, 594-599. cally, length appears to be the most important path Tolman, E.C. 1948. Cognitive maps in rats and men. parameter (seven cats chose the shorter path from Psychot. Rev., 55, 189-208. $1 and $3), followed by angular deviation and complexity (St and $2 trials: see Table I). Concern(Received 13 November 1984; revised 14 January 1985; MS. number: sc-226) ing the last two parameters, their effect is restricted almost exclusively to situations in which length is not relevant. Moreover, complexity seems to play a minor part as compared with angular deviation (Sa trials). This result concerning angular deviation Influence of Nestling Experience on Nest-type confirms previous results showing that the localizaSelection in Captive Kestrels tion of the goal on the basis of external visual cues is probably the first step involved in route planning Many management programmes have been insti(Poucet et al. 1983; Poucet 1984). However, that tuted in which young birds are exposed to and length is taken into account, even if the shorter released in a specific environment with the hope path is more divergent, indicates that animals also that they will return to breed. F o r example, have a precise knowledge of more distant features peregrine falcons (Fatco peregrinus) have been related to goal location. This knowledge is gained released from buildings in Canada, and recently through experience, as shown by the improvement several have returned and nested successfully on in performance between days: during the first two city skyscrapers (Iola Price, personal communicadays the cats' various displacements allowed them tion). Peregrines released from wooden towers in to store information concerning route features salt marshes used those same structures to establish related to the third starting-point, although this breeding territories in later years (Barclay & Cade had no particular value at that time. This latent 1983). In three attempted nestings by peregrine learning phenomenon can be related to the building falcons on bridges in the eastern United States in up of a map of the different possibilities of 1983, at least three of the adults involved have been travelling through space (Tolman 1948). It is worth released from historic cliff sites (Cade & Dague noting that the formation of such a map seems to 1983). follow incremental rules, since (1) the improvement Is the subsequent choice of a nest structure did not appear before the third day and (2) route derived innately through genotype, or is it learned preference was more rapidly established when a during the nestling and/or fledging stages? Circumsmaller number of possible routes was provided stantial evidence (Temple 1977; Newton 1979)
Short Communications indicates that raptors will often choose nest situations similar to those they were raised in. There have been, to our knowledge, no published studies of the subsequent nest-type choice of raptors from a particular nest environment. Such a study would increase our understanding of the mechanisms involved in selection o f nest structures, as well as aiding in captive management and release work. Although it has a widespread range over differing environments, the American kestrel nests predominantly in tree cavities, and virtually always selects some form of enclosed nest site (Balgooyen 1976). However, it has been known to nest occasionally in alternative nest sites such as cliffcavities (Ganier 1946; Fyfe 1958), stream banks (Williams 1893), and under r o o f eaves (Skaggs 1974). In this last rare case, the kestrels did not nest in a true cavity, but established their nest on a rain gutter directly below the edge of the roof. Because the American kestrel breeds readily in captivity (Porter & Wiemeyer 1970) it is an ideal model species for nest-choice studies. Our overall objective was to expose nestling kestrels to two types of nests, open ledges versus nestboxes, and to determine their subsequent nest-type choice as breeding adults. The study was conducted between April 1982 and June 1983 on the pedigreed kestrel colony at the M a c d o n a l d Raptor Research Centre of McGill University (Bird 1982). Six pairs of kestrels were randomly assigned to each of six identical cages on April 15, 1982. Each enclosure measured
1029
2.4 x 1-2 • 2-4 m high and was equipped with two rope perches, a solid feeding and copulation perch, a nestbox and ledge. The nestbox measured 30"5 • 30"5 x 37 cm, with a 10-cm-diameter nest hole. The nest ledge was positioned in an opposing corner of each pen at the same height as the nestbox, about 1.3 m above the cage floor. The ledge was triangular in shape, 64 cm along the base and 46 em along each side. A 4-cm-high perch/ border ran along its outer edge. The usable floor areas of both nest-types were of similar size and were covered in wood shavings. Both nest sites were protected from rain by a plastic panel on the roof of each pen. Each pair laid a first clutch of eggs in the nestbox. This clutch was removed for other purposes, and all six pairs re-laid in the nestbox. O f the six second clutches, one was infertile, one female destroyed her eggs, and two nestlings from one clutch died shortly after hatching. Ten young birds from four pairs remained for study. The nestlings were moved from nestbox to ledge when the last-hatched bird was 2 days old. The entrance to the nestbox was subsequently blocked. All four pairs of adults raised their young on ledges, and all ten birds fledged, seemingly without incident. The family units remained in these pens until October 1982. On April 22, 1983, the 10 ledge-raised kestrels were paired as follows. Three ledge-raised females were paired with nestbox-raised males (two with prior experience of box-nesting); one ledge-raised
Table I. Clutch size and fertility of captive pairs of American kestrels raised as
nestlings on either nest ledges or nestboxes Nest-type selection Age Nestling Clutch size Pen no. Bird no. Sex (years) experience* Nest ledge Nestbox and fertility C1 C2 C3 C4 C5 C6 C7
1075
M
1
NB
1154
F
1
NL
1155 1054 758 1161 750 I161 1153 1157 1156 1158 1159 1160
M F M F M F M F M F M F
1 1 3~ 1 3:I: 1 1 1 1 1 1 1
NL NB NB NL NB NL NL NL NL NL NL NL
--
+
4/5t
-
+
3/5
--
+
2/5
--
+
2/5
-
+
1/5
-
+
3/5
-
+
2/3
* NB = nestbox; NL=nest ledge. t Four out of five eggs were fertile. $ These birds had previous experience with nestboxes.
1030
Animal Behaviour, 33, 3
male was paired with a nextbox female; in the remaining three pairs, both sexes had been raised on ledges (Table I). Pairs were chosen to minimize genetic relatedness between mates. The same cages and facilities, plus one breeding cage, were used in 1983. Test birds could choose a nestbox or ledge as a nest site. During pair formation and subsequent nest-site selection, behavioural observations through one-way glass windows were conducted on each pair for a 10-min period, daily, in the morning. To serve as controls, 15 pairs of kestrels raised in nestboxes were assigned to breeding cages of similar design containing both ledges and nestboxes. The seven experimental pairs all chose nestboxes, not ledges. Of the control birds, all 15 pairs selected nestboxes and subsequently laid eggs there. The test birds exhibited no sustained interest in nesting on ledges. In two pens, the ledge was used for caching food, while all pairs used it as a perch and occasionally for feeding. Each female laid three to five eggs in the nestbox (Table I). During behavioural observations, five of seven males were observed in the nestbox, trilling and scraping. In pen C4 no such activity was observed, the birds being largely inactive. In pen C2, the female who had nestbox experience was frequently seen entering the cavity, whereas the male, who was raised on a ledge, was never seen there. In pens C5-C7, both sexes were observed in nestboxes. All seven pairs produced fertile eggs, although in three instances the fertility rate was quite low (Table I). It is difficult to relate the fertility of eggs with the nestling experience of the adult pair, since, with the small sample size, observed differences may have resulted from mate incompatibility, regardless of prior nesting experience. Our data imply that there is an innate selection of nest types which overrides any post-hatching imprinting processes experienced by nestlings. Willoughby & Cade (1984) noted that the nest site (a nestbox) was an important stimulus in the production of fertile eggs. Hence, several qualities inherent in the nature of a cavity may be important in the successful completion of the breeding cycle. These may include the nest entrance itself, which could serve as a valuable stimulus for the female to lay. Female kestrels raised naturally can be induced to lay eggs in a nestbox without a male present (Bird & Buckland 1977). Also, Brockway (1962) found that lower light intensities, as found in a nest cavity, stimulated egg-laying behaviour in budgerigars (Melopsittacus undulatus). The kestrel has developed specific nesting requirements for the successful hatching and rearing of its young. Since it is a cavity-nesting species,
these needs can be accommodated in only a limited number of nest designs. Other members of the genus Falco have more general nesting requirements, needing only an open or partially-enclosed scrape or nest. This adaptability makes it relatively easy, for example, to manipulate peregrine falcons to nest on buildings, platforms and other atypical sites. The kestrel, in its adoption of cavities as nest sites, has lost this elasticity with the result that it is less able to use alternative nest types when preferred ones are lacking. We thank Nancy Shackell and Eliot Terry for conducting behavioural observations, Ian Ritchie for technical advice, le Ministdre du Loisir, de la Chasse et de la Pdehe du Quhbec and the Natural Science and Engineering Research Council for funding assistance. Reviews from T.J, Cade and H. Mueller were helpful. This is Macdonald Raptor Research Centre Scientific Publication No. 28. LAIRD J. SHUTT DAVID M. BIRD Macdonald Raptor Research Centre, Macdonald College o f McGill University, 21 111 Lakeshore Road, St. Anne de Bellevue, Qubbec, Canada H9X 1C0. References Balgooyen, T. G. 1976. Behavior and ecology of the American kestrel (Falco sparverius L.) in the Sierra Nevada of California. Univ. Calif. Publ. ZooL, 103, 1-83. Barclay, J. H. & Cade, T. J 1983. Restoration of the Peregrine falcon in the eastern United States. In: Bird Conservation L (Ed. by S.A. Temple), pp. 3-40. Madison: University of Wisconsin Press. Bird, D. M. & Buckland, R. B. 1977. The onset and duration of fertility in the American kestrel. Can. J. Zool., 54, 1595 1597. Bird, D. M. 1982. The American kestrel as a laboratory research animal. Nature, Lond., 299, 300-301. Brockway, B. F. 1962. The effects of nest-entrance positions and male vocalizations on reproduction in budgerigars. Living Bird, 1, 93-101. Cade, T. J. & Dague, P. R. 1983. Peregrine Fund Newsl., 11, 1-12. Fyfe, R. W. 1958. Cliff-dwelling sparrow hawk in southern Saskatchewan. Blue Jay, 16, 155. Ganier, A. F. & Clebsch, A. 1946. Sparrow hawks in a cliff. Migrant, 17, 26. Newton, I. 1979. Population Ecology of Raptors. Vermillion, South Dakota: Buteo Books. Porter, R. D. & Wiemeyer, S. N. 1970. Propagation of captive American kestrels. J. Wildl. Mgmt., 34, 594-604. Skaggs, M. B. 1974. Two kestrel nestlings. Inland Bird Band. News, 46, 171-176. Temple, S. A. 1977. Reintroducing birds of prey to the wild. In: Endangered Birds: Management Techniques
Short Communications .for Preserving Threatened Species (Ed. by S.A. Temple), pp. 353-363. Madison: University of Wisconsin Press. Williams, L. P. 1983. Unusual nesting of the sparrow hawk. Oologist, 10, 306. Willoughby, E. J. & Cade, T. J. 1964. Breeding behavior of the American kestrel (sparrow hawk). Living Bird, 3, 75-96. (Received 27 March 1984; revised27 December 1984;MS. number:As-276)
Are Colonies Supraoptimal Groups? Two recent theoretical discussions have concluded that the stable, and hence observed, sizes of foraging groups will be larger than the optimum at which mean individual fitness within the group is maximized (Sibly 1983; Clark & Mangel 1984). Giraldeau & Gillis (1985) demonstrated that this conclusion depends upon the shape of the function relating mean fitness to group size. Here I show that (1) the predicted supraoptimal size of stable groups also depends on specific assumptions about patterns of departure from established groups and (2) that incorporation of departure patterns into the models results in interesting, new predictions. One scenario proposed by Sibly (1983) starts with several groups with sizes that vary about the optimum. Each individual is permitted to decide whether to remain in its present group or join one of the others. With the fitness function used by Sibly, groups less than the optimal size have lower fitness than groups with an equal nmnber of individuals above the optimum. This generates departures of individuals from suboptimal groups to join optimal or supraoptimal groups. Sibly cautions that his model is subject to the constraint that decisions are made by one individual at a time. If small subgroups can break off and join other such subgroups, the population easily reverts to optimal group sizes. This is by no means unlikely since the size and composition of foraging subgroups often change rapidly (e.g. Seghers 1981; Lott & Minta 1983; Lefebvre & Giraldeau, 1984). A second scenario suggested by both Sibly (1983) and Clark & Mangel (1984) involves individuals immigrating to a foraging area. This produces groups much larger than the optimum because the fitness of each immigrant is higher if it joins an optimal or supraoptimal group than if it remains solitary, until the groups become so large that mean fitness in the group equals the fitness of isolated individuals. As with the previous scenario, these very large groups are unstable if subgroup formation is permitted, since all individuals can gain by
1031
fission of a supraoptimal group. Even if individuals only move singly, supraoptimal groups are unstable when their mean fitness is near that of solitary individuals (Clark & Mangel 1984), As soon as one individual decides not to join established groups, members of large groups can increase their fitness by moving to that individual. Differences in the slope of the fitness function, above and below the optimum, generate movement from the larger to the smaller group until they are both the same size. This analysis suggests that the relationship between observed and optimal group sizes, as well as the relationship between observed and maximal fitness, will depend on the mobility of individuals which have already joined groups. If groups can readily divide and fuse, as in many foraging groups, group sizes should be near the optimum and mean fitness within groups near the maximum. If individuals can move only one at a time, groups will be larger and fitness less than the optimum. However, the most extreme deviations from the optimum will occur when individuals cannot leave a group once they have joined; such groups will be much larger than the optimum and mean fitness will be almost as low as the fitness of solitary individuals. This situation may apply to the reproductive colonies of many mammals, birds and fish where the cost of moving is very high once a territory is established and nest construction begun. It may be relevant also to night roosts when darkness greatly increases the cost of moving to another site. Differences between the sizes of foraging and breeding groups in colonial species may be due less to differences between the fitness functions associated with feeding and reproductive groups than to differences in the mobility of organisms engaged in these activities. I thank Darren Gillis, Luc-Alain Giraldeau and Wayne Hunte for helpful comments on earlier drafts of the manuscript. DONALD L. KRAMER
Department of Biology, McGill University, 1205 Avenue Dr Penfield Montreal, Qudbec, H3A 1B1, Canada.
References Clark, C. W. & Mangel, M. 1984. Foraging and flocking strategies: information in an uncertain environment. Am. Nat., 123, 626-641. Giraldeau, L.-A. & Gillis, D. M. 1985. Optimal group sizes can be stable: a reply to Sibly. Anita. Behav., 33, 666-667.
Animal Behaviour, 33, 3
order in which cats were tested was varied ran- (Table I: compare S t, S~ and $3). This suggests that, domly between sessions. while perceptual processes may allow the formaThe mean numbers of trials before a preference tion of a map of a simple environment in a single was shown were 4.25 ( + 1.53 sE), 4.12 ( + 1.53 sE) exposure (Poucet 1984), only locomotor experience and 1"87 (+__0-74 SE) trials for days 1, 2 and 3 during exploratory activity can provide a basis for respectively. A Wilcoxon matched-pairs signed- the setting up of more complex maps. ranks test (one-tailed) revealed statistically significant differences between days 1 and 3 ( T = 3, df= 8, B. Pouc~x P<0.025) and between days 2 and 3 ( T = 4 , df=8, Dkpartement de Psychologie Animale, P < 0.025), but no difference between days 1 and 2. C.N.R.S.-INP9, 31 ehemin Joseph-Aiguier, Table I gives the frequencies of choices and 13402 Marseille cedex 9, France. numbers of subjects showing a preference for each route from each starting-point. Although the cats References were rewarded regardless of path chosen, they all Blancheteau, M. & Le Lorec, A. 1972. Raccourci et d&our chez le rat: durre, vitesse et longueur des showed a preference. In 21 cases out of 24, the parcours. Ann. Psyehol., 72, 7-16. preferred routes were those assumed to be optimal according to one parameter. Furthermore, prefer- Chapuis, N., Thinus-Blanc, C. & Poucet, B. 1983. Dissociation of mechanisms involved in dogs' oriented ences appear to have been established more rapidly displacements. Q. J. exp. Psychal., 35B, 213-219. when starting from $2 than from $1 and from $l Dashiell, J, F. 1931. Direction orientation in maze than from $3 (Friedman two-way analysis of running by the white rat. Comp. Psychol. Monogr., 7, variance: X2r= 6.08; dr= 3,8; P < 0-07). 1-72. Despite the spatial complexity of the environ- Poucet, B. 1984. Evaluation of connectedness by cats in path-selection problems. Percept. mot. Skills, 58, ment, cats accurately and spontaneously selected 51-54. routes according to their length and/or angular characteristics, thus showing further evidence of Poucet, B., Thinus-Blanc, C. & Chapuis, N. 1983. Route planning in cats, in relation to the visibility of the goal. route planning (Poucet et al. 1983). More specifiAnita. Behav., 31, 594-599. cally, length appears to be the most important path Tolman, E.C. 1948. Cognitive maps in rats and men. parameter (seven cats chose the shorter path from Psychot. Rev., 55, 189-208. $1 and $3), followed by angular deviation and complexity (St and $2 trials: see Table I). Concern(Received 13 November 1984; revised 14 January 1985; MS. number: sc-226) ing the last two parameters, their effect is restricted almost exclusively to situations in which length is not relevant. Moreover, complexity seems to play a minor part as compared with angular deviation (Sa trials). This result concerning angular deviation Influence of Nestling Experience on Nest-type confirms previous results showing that the localizaSelection in Captive Kestrels tion of the goal on the basis of external visual cues is probably the first step involved in route planning Many management programmes have been insti(Poucet et al. 1983; Poucet 1984). However, that tuted in which young birds are exposed to and length is taken into account, even if the shorter released in a specific environment with the hope path is more divergent, indicates that animals also that they will return to breed. F o r example, have a precise knowledge of more distant features peregrine falcons (Fatco peregrinus) have been related to goal location. This knowledge is gained released from buildings in Canada, and recently through experience, as shown by the improvement several have returned and nested successfully on in performance between days: during the first two city skyscrapers (Iola Price, personal communicadays the cats' various displacements allowed them tion). Peregrines released from wooden towers in to store information concerning route features salt marshes used those same structures to establish related to the third starting-point, although this breeding territories in later years (Barclay & Cade had no particular value at that time. This latent 1983). In three attempted nestings by peregrine learning phenomenon can be related to the building falcons on bridges in the eastern United States in up of a map of the different possibilities of 1983, at least three of the adults involved have been travelling through space (Tolman 1948). It is worth released from historic cliff sites (Cade & Dague noting that the formation of such a map seems to 1983). follow incremental rules, since (1) the improvement Is the subsequent choice of a nest structure did not appear before the third day and (2) route derived innately through genotype, or is it learned preference was more rapidly established when a during the nestling and/or fledging stages? Circumsmaller number of possible routes was provided stantial evidence (Temple 1977; Newton 1979)
Short Communications indicates that raptors will often choose nest situations similar to those they were raised in. There have been, to our knowledge, no published studies of the subsequent nest-type choice of raptors from a particular nest environment. Such a study would increase our understanding of the mechanisms involved in selection o f nest structures, as well as aiding in captive management and release work. Although it has a widespread range over differing environments, the American kestrel nests predominantly in tree cavities, and virtually always selects some form of enclosed nest site (Balgooyen 1976). However, it has been known to nest occasionally in alternative nest sites such as cliffcavities (Ganier 1946; Fyfe 1958), stream banks (Williams 1893), and under r o o f eaves (Skaggs 1974). In this last rare case, the kestrels did not nest in a true cavity, but established their nest on a rain gutter directly below the edge of the roof. Because the American kestrel breeds readily in captivity (Porter & Wiemeyer 1970) it is an ideal model species for nest-choice studies. Our overall objective was to expose nestling kestrels to two types of nests, open ledges versus nestboxes, and to determine their subsequent nest-type choice as breeding adults. The study was conducted between April 1982 and June 1983 on the pedigreed kestrel colony at the M a c d o n a l d Raptor Research Centre of McGill University (Bird 1982). Six pairs of kestrels were randomly assigned to each of six identical cages on April 15, 1982. Each enclosure measured
1029
2.4 x 1-2 • 2-4 m high and was equipped with two rope perches, a solid feeding and copulation perch, a nestbox and ledge. The nestbox measured 30"5 • 30"5 x 37 cm, with a 10-cm-diameter nest hole. The nest ledge was positioned in an opposing corner of each pen at the same height as the nestbox, about 1.3 m above the cage floor. The ledge was triangular in shape, 64 cm along the base and 46 em along each side. A 4-cm-high perch/ border ran along its outer edge. The usable floor areas of both nest-types were of similar size and were covered in wood shavings. Both nest sites were protected from rain by a plastic panel on the roof of each pen. Each pair laid a first clutch of eggs in the nestbox. This clutch was removed for other purposes, and all six pairs re-laid in the nestbox. O f the six second clutches, one was infertile, one female destroyed her eggs, and two nestlings from one clutch died shortly after hatching. Ten young birds from four pairs remained for study. The nestlings were moved from nestbox to ledge when the last-hatched bird was 2 days old. The entrance to the nestbox was subsequently blocked. All four pairs of adults raised their young on ledges, and all ten birds fledged, seemingly without incident. The family units remained in these pens until October 1982. On April 22, 1983, the 10 ledge-raised kestrels were paired as follows. Three ledge-raised females were paired with nestbox-raised males (two with prior experience of box-nesting); one ledge-raised
Table I. Clutch size and fertility of captive pairs of American kestrels raised as
nestlings on either nest ledges or nestboxes Nest-type selection Age Nestling Clutch size Pen no. Bird no. Sex (years) experience* Nest ledge Nestbox and fertility C1 C2 C3 C4 C5 C6 C7
1075
M
1
NB
1154
F
1
NL
1155 1054 758 1161 750 I161 1153 1157 1156 1158 1159 1160
M F M F M F M F M F M F
1 1 3~ 1 3:I: 1 1 1 1 1 1 1
NL NB NB NL NB NL NL NL NL NL NL NL
--
+
4/5t
-
+
3/5
--
+
2/5
--
+
2/5
-
+
1/5
-
+
3/5
-
+
2/3
* NB = nestbox; NL=nest ledge. t Four out of five eggs were fertile. $ These birds had previous experience with nestboxes.
1030
Animal Behaviour, 33, 3
male was paired with a nextbox female; in the remaining three pairs, both sexes had been raised on ledges (Table I). Pairs were chosen to minimize genetic relatedness between mates. The same cages and facilities, plus one breeding cage, were used in 1983. Test birds could choose a nestbox or ledge as a nest site. During pair formation and subsequent nest-site selection, behavioural observations through one-way glass windows were conducted on each pair for a 10-min period, daily, in the morning. To serve as controls, 15 pairs of kestrels raised in nestboxes were assigned to breeding cages of similar design containing both ledges and nestboxes. The seven experimental pairs all chose nestboxes, not ledges. Of the control birds, all 15 pairs selected nestboxes and subsequently laid eggs there. The test birds exhibited no sustained interest in nesting on ledges. In two pens, the ledge was used for caching food, while all pairs used it as a perch and occasionally for feeding. Each female laid three to five eggs in the nestbox (Table I). During behavioural observations, five of seven males were observed in the nestbox, trilling and scraping. In pen C4 no such activity was observed, the birds being largely inactive. In pen C2, the female who had nestbox experience was frequently seen entering the cavity, whereas the male, who was raised on a ledge, was never seen there. In pens C5-C7, both sexes were observed in nestboxes. All seven pairs produced fertile eggs, although in three instances the fertility rate was quite low (Table I). It is difficult to relate the fertility of eggs with the nestling experience of the adult pair, since, with the small sample size, observed differences may have resulted from mate incompatibility, regardless of prior nesting experience. Our data imply that there is an innate selection of nest types which overrides any post-hatching imprinting processes experienced by nestlings. Willoughby & Cade (1984) noted that the nest site (a nestbox) was an important stimulus in the production of fertile eggs. Hence, several qualities inherent in the nature of a cavity may be important in the successful completion of the breeding cycle. These may include the nest entrance itself, which could serve as a valuable stimulus for the female to lay. Female kestrels raised naturally can be induced to lay eggs in a nestbox without a male present (Bird & Buckland 1977). Also, Brockway (1962) found that lower light intensities, as found in a nest cavity, stimulated egg-laying behaviour in budgerigars (Melopsittacus undulatus). The kestrel has developed specific nesting requirements for the successful hatching and rearing of its young. Since it is a cavity-nesting species,
these needs can be accommodated in only a limited number of nest designs. Other members of the genus Falco have more general nesting requirements, needing only an open or partially-enclosed scrape or nest. This adaptability makes it relatively easy, for example, to manipulate peregrine falcons to nest on buildings, platforms and other atypical sites. The kestrel, in its adoption of cavities as nest sites, has lost this elasticity with the result that it is less able to use alternative nest types when preferred ones are lacking. We thank Nancy Shackell and Eliot Terry for conducting behavioural observations, Ian Ritchie for technical advice, le Ministdre du Loisir, de la Chasse et de la Pdehe du Quhbec and the Natural Science and Engineering Research Council for funding assistance. Reviews from T.J, Cade and H. Mueller were helpful. This is Macdonald Raptor Research Centre Scientific Publication No. 28. LAIRD J. SHUTT DAVID M. BIRD Macdonald Raptor Research Centre, Macdonald College o f McGill University, 21 111 Lakeshore Road, St. Anne de Bellevue, Qubbec, Canada H9X 1C0. References Balgooyen, T. G. 1976. Behavior and ecology of the American kestrel (Falco sparverius L.) in the Sierra Nevada of California. Univ. Calif. Publ. ZooL, 103, 1-83. Barclay, J. H. & Cade, T. J 1983. Restoration of the Peregrine falcon in the eastern United States. In: Bird Conservation L (Ed. by S.A. Temple), pp. 3-40. Madison: University of Wisconsin Press. Bird, D. M. & Buckland, R. B. 1977. The onset and duration of fertility in the American kestrel. Can. J. Zool., 54, 1595 1597. Bird, D. M. 1982. The American kestrel as a laboratory research animal. Nature, Lond., 299, 300-301. Brockway, B. F. 1962. The effects of nest-entrance positions and male vocalizations on reproduction in budgerigars. Living Bird, 1, 93-101. Cade, T. J. & Dague, P. R. 1983. Peregrine Fund Newsl., 11, 1-12. Fyfe, R. W. 1958. Cliff-dwelling sparrow hawk in southern Saskatchewan. Blue Jay, 16, 155. Ganier, A. F. & Clebsch, A. 1946. Sparrow hawks in a cliff. Migrant, 17, 26. Newton, I. 1979. Population Ecology of Raptors. Vermillion, South Dakota: Buteo Books. Porter, R. D. & Wiemeyer, S. N. 1970. Propagation of captive American kestrels. J. Wildl. Mgmt., 34, 594-604. Skaggs, M. B. 1974. Two kestrel nestlings. Inland Bird Band. News, 46, 171-176. Temple, S. A. 1977. Reintroducing birds of prey to the wild. In: Endangered Birds: Management Techniques
Short Communications .for Preserving Threatened Species (Ed. by S.A. Temple), pp. 353-363. Madison: University of Wisconsin Press. Williams, L. P. 1983. Unusual nesting of the sparrow hawk. Oologist, 10, 306. Willoughby, E. J. & Cade, T. J. 1964. Breeding behavior of the American kestrel (sparrow hawk). Living Bird, 3, 75-96. (Received 27 March 1984; revised27 December 1984;MS. number:As-276)
Are Colonies Supraoptimal Groups? Two recent theoretical discussions have concluded that the stable, and hence observed, sizes of foraging groups will be larger than the optimum at which mean individual fitness within the group is maximized (Sibly 1983; Clark & Mangel 1984). Giraldeau & Gillis (1985) demonstrated that this conclusion depends upon the shape of the function relating mean fitness to group size. Here I show that (1) the predicted supraoptimal size of stable groups also depends on specific assumptions about patterns of departure from established groups and (2) that incorporation of departure patterns into the models results in interesting, new predictions. One scenario proposed by Sibly (1983) starts with several groups with sizes that vary about the optimum. Each individual is permitted to decide whether to remain in its present group or join one of the others. With the fitness function used by Sibly, groups less than the optimal size have lower fitness than groups with an equal nmnber of individuals above the optimum. This generates departures of individuals from suboptimal groups to join optimal or supraoptimal groups. Sibly cautions that his model is subject to the constraint that decisions are made by one individual at a time. If small subgroups can break off and join other such subgroups, the population easily reverts to optimal group sizes. This is by no means unlikely since the size and composition of foraging subgroups often change rapidly (e.g. Seghers 1981; Lott & Minta 1983; Lefebvre & Giraldeau, 1984). A second scenario suggested by both Sibly (1983) and Clark & Mangel (1984) involves individuals immigrating to a foraging area. This produces groups much larger than the optimum because the fitness of each immigrant is higher if it joins an optimal or supraoptimal group than if it remains solitary, until the groups become so large that mean fitness in the group equals the fitness of isolated individuals. As with the previous scenario, these very large groups are unstable if subgroup formation is permitted, since all individuals can gain by
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fission of a supraoptimal group. Even if individuals only move singly, supraoptimal groups are unstable when their mean fitness is near that of solitary individuals (Clark & Mangel 1984), As soon as one individual decides not to join established groups, members of large groups can increase their fitness by moving to that individual. Differences in the slope of the fitness function, above and below the optimum, generate movement from the larger to the smaller group until they are both the same size. This analysis suggests that the relationship between observed and optimal group sizes, as well as the relationship between observed and maximal fitness, will depend on the mobility of individuals which have already joined groups. If groups can readily divide and fuse, as in many foraging groups, group sizes should be near the optimum and mean fitness within groups near the maximum. If individuals can move only one at a time, groups will be larger and fitness less than the optimum. However, the most extreme deviations from the optimum will occur when individuals cannot leave a group once they have joined; such groups will be much larger than the optimum and mean fitness will be almost as low as the fitness of solitary individuals. This situation may apply to the reproductive colonies of many mammals, birds and fish where the cost of moving is very high once a territory is established and nest construction begun. It may be relevant also to night roosts when darkness greatly increases the cost of moving to another site. Differences between the sizes of foraging and breeding groups in colonial species may be due less to differences between the fitness functions associated with feeding and reproductive groups than to differences in the mobility of organisms engaged in these activities. I thank Darren Gillis, Luc-Alain Giraldeau and Wayne Hunte for helpful comments on earlier drafts of the manuscript. DONALD L. KRAMER
Department of Biology, McGill University, 1205 Avenue Dr Penfield Montreal, Qudbec, H3A 1B1, Canada.
References Clark, C. W. & Mangel, M. 1984. Foraging and flocking strategies: information in an uncertain environment. Am. Nat., 123, 626-641. Giraldeau, L.-A. & Gillis, D. M. 1985. Optimal group sizes can be stable: a reply to Sibly. Anita. Behav., 33, 666-667.