Conditional Use of Nest Structures by White-Naped and White-Collared Swifts

Abstract

Since the nesting of the White-naped Swift (Streptoprocne semicollaris) was first described (Rowley and Orr 1962), this species has been thought to differ from other cypseloidine swifts in failing to build a nest structure. I studied nesting of White-naped Swifts and White-collared Swifts (Streptoprocne zonaris) at colonies where earlier nest descriptions were made. Based on a larger sample than that available to previous authors, I found the two swifts to differ little in nesting habits. Both species built or used existing nest structures on slanted substrates where eggs and young would otherwise have been at risk of rolling or falling, and both often laid eggs and reared young without a nest structure where the substrate was level or naturally enclosed. Frequency of use of risky vs. safe sites differed between colonies, reflecting apparent differences in the availability of safe sites. White-naped Swifts used nest structures on the same ledges where they laid eggs without nest structures 22 and 24 years earlier. Probably due to periodic flooding, these ledges apparently were less sandy in 1983 and 1985 than in 1961, and the swifts had responded to substrate changes by building nest structures in the absence of sandy oviposition sites. The supposedly unique nesting habits of S. semicollaris have been used as partial justification for a monotypic subgenus, Semicollum (Brooke 1970). Evidence presented here brings this rationale into question. Nests of both species, when built, were typical of the Cypseloidinae, except that semicollaris used more mud than is typical. Stereotyped behaviors used in nest building, when it does occur, are probably safer indicators of phylogeny than is mere frequency of nest building, which can differ between sites and change rapidly at a given site.