Settlement patterns and nest-site selection of Cliff Swallows, Hirundo pyrrhonota: males prefer to clump but females settle randomly Susan B. Meek and Robert M.R. Barclay Abstract: We assessed the settlement pattern of colonial Cliff Swallows (Hirundo pyrrhonota) to examine whether they avoid settling near to each other or prefer to nest in close proximity. Male Cliff Swallows occupied and defended nests before females paired with them, and males settled next to other males significantly more often than expected by chance. In contrast to that of males, female settlement was not significantly different from random: females neither preferred nor avoided settling next to other Cliff Swallows, nor did females select old nests on the basis of nest stability. Neither males nor females chose nests in the same order from year to year, suggesting that quality of old nests was not an important factor affecting settlement patterns. Male arrival date is related to quality in some species, but female Cliff Swallows did not select the earliest arriving males. There was also no correlation between the order in which females chose mates and male traits. All nest sites may have offered females an equal probability of success, leading to random settlement with respect to other females, nests, and males. RCsumC : Nous avons ktudik l'ktablissement d'Hirondelles a front blanc (Hirundo pyrrhonota) coloniales dans le but de determiner si ces oiseaux kvitent de s'installer les uns prks des autres ou s'ils choisissent plut8t de nicher a proximitk d'autres nids. Les miles de 1'Hirondelle a front blanc occupent et dkfendent leurs nids avant de se choisir une partenaire et ils ont tendance a s'installer prks d'autres miles significativement plus souvent qu'ils ne le feraient au hasard. Contrairement aux miles, les femelles s'ktablissent selon un mode qui ne differe pas significativement d'un choix alkatoire : elles n'kvitent pas les autres Hirondelles a front blanc et n'ont pas non plus tendance a s'ktablir prks d'elles; en outre, elles ne choisissent pas de vieux nids pour assurer leur stabilitk. Ni les miles, ni les femelles ne choisissent des nids dans le meme ordre d'une annke a l'autre, ce qui semble indiquer que la qualitk des vieux nids n'est pas un facteur important dans le choix d'un site d'ktablissement. La date d'arrivke des miles est relike a leur qualitk chez certaines espkces, mais les femelles de I'Hirondelle a front blanc ne choisissent pas nkcessairement les miles arrives les premiers. I1 n'y a pas de corrklation entre l'ordre que suivent les femelles pour choisir un partenaire et les caractkristiques des miles. Tous les sites de nidification offrent aux femelles des chances kgales de succks, ce qui explique leur ktablissement au hasard par rapport aux autres femelles, aux nids et aux miles. [Traduit par la Rkdaction] Introduction competition for resources by nesting asynchronously (Leonard The way in which animals are dispersed in the environment ranges from widely spaced solitary individuals or breeding pairs to tightly clumped colonies of numerous individuals. These spacing patterns are influenced by such factors as resource distribution and abundance, competition for resources and mates, predation pressure, and the tactics of mate and resource acquisition. It is commonly assumed that individuals choose where to settle according to the relative qualities of territories and partners. But in some species other factors are more important. For example, California ground squirrels (Spermophilus beecheyi) choose their territories so as to avoid inbreeding (Dobson 1979), returning male Painted Buntings (Passerina ciris) show site fidelity even when higher quality territories are available (Lanyon and Thompson 1986), and polygynous female Marsh Wrens (Cistothorus palustris) reduce and Picman 1987). Animals may breed in colonies because a lack of suitable breeding sites forces them into aggregations or because eco- logical factors such as patchily distributed food or predation pressure promote social living (Alexander 1974; Wittenberger and Hunt 1985). Whatever the origin, living in groups has both costs and benefits (Alexander 1974; Wittenberger and Hunt 1985). The benefits of coloniality can include enhanced foraging efficiency, protection from predators, and increased reproductive options (e.g., extra-pair matings). Costs of colo- niality can include increased rates of parasitism and disease, enhanced visibility to predators, and reduced reproductive success due to intraspecific brood parasitism (egg dumping), destruction of eggs or young, and extra-pair fertilizations. Colonial nesting occurs in about 13% of all bird species (Welty 1982). A number of studies have examined the rela- tive costs and benefits of coloniality in birds and assessed Received August 8, 1995. Accepted February 16, 1996. whether individuals should prefer to breed in aggregations or S.B. Meek (e-mail: sbmeek@acs.ucalgary .ca) and at solitary nests (e.g . , Hoogland and Sherman 1976; Snapp R.M.R. Barclay (e-mail : barclay@acs.ucalgary .ca). Ecology 1976; Moller 1987). However, little is known about how Division, Department of Biological Sciences, University of birds prefer to space themselves within a colony. We might Calgary, Calgary, AB T2N 1N4, Canada. expect that within a colony the relative costs and benefits Can. J. 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