Abstract
Secondary cavity-nesting birds depend on tree cavities for nesting and roosting, but many studies of these birds are conducted using nest boxes. Implementation of effective conservation strategies for cavity-nesting species such as nest-site supplementation requires careful comparisons of fecundity and other vital rates for birds using both natural and artificial nest site types. We compared breeding phenology, clutch and brood sizes, and fledging success of Tree Swallows (Tachycineta bicolor) nesting in tree cavities and nest boxes during 2001–2003 in British Columbia, Canada. Swallows using nest boxes initiated egg-laying and hatched young at approximately the same time as those in tree cavities (2 June, 23 June, respectively). Female Tree Swallows in boxes laid larger clutches (5.9 ± 0.9 eggs, N = 76) than those in tree cavities (4.2 ± 1.6 eggs, N = 67). The mean number of nestlings hatched was greater in nest boxes (5.2 ± 1.1 nestlings, N = 67) than in tree cavities (2.6 ± 2.0 nestlings, N = 58). Pairs in boxes were over twice as successful in producing fledglings (93.4%; 57 of 61 pairs fledged > 1 young) than those in tree cavities (35.8%; 19 of 53 pairs). Of those successful nests, pairs nesting in boxes fledged 5.1 ± 1.1 young (N = 57), whereas those in tree cavities fledged 3.5 ± 1.2 young (N = 18). Because cavities in nest boxes averaged 60% larger in volume and 1.8 cm wider internally than tree cavities, we suggest that increased reproductive output was correlated with boxes enabling a larger clutch size. In previous research, we found that Tree Swallows were a poor competitor with other cavity-nesting passerines for tree cavities. The addition of nest boxes may serve as an effective way to supplement local reproduction for secondary cavity-nesting bird populations by reducing competition for limited nest sites. This is especially true in regions where the availability of natural nesting sites is highly variable, and where species compete with many other cavity-nesting passerines using a similar ecological niche and nesting cavities.
Highlights
Tree Swallow (Tachycineta bicolor) populations, like many other aerial insectivores in North America, have shown continent-wide declines since the 1980s [1,2]
Tree Swallows in North America depend on acquiring nesting cavities in ecological conditions that generally involve a limited supply of suitable tree cavities and a suite of strong nest site competitors
Our results that Tree Swallows laid larger, and more successful clutches in nest boxes suggests that nest boxes provide a reproductive advantage to Tree Swallow populations, where competition for nest-sites is high
Summary
Tree Swallow (Tachycineta bicolor) populations, like many other aerial insectivores in North America, have shown continent-wide declines since the 1980s [1,2]. Comparisons of the breeding biology of birds nesting in tree cavities and nest boxes has revealed that pairs in nest boxes may initiate clutches earlier [11]and have larger clutches [11,12,13], experience lower predation rates [11,12,14,15], and produce more fledglings [6,7,8,9,11,15,16,17,18]. Other studies have revealed that birds nesting in tree cavities and nest boxes did not differ in clutch initiation dates [11,13,17,19], or clutch sizes [11,17,19], and that nest predation rates were greater in boxes than in natural cavities in some years [11,13,20]. The risk that nest-boxes might be ineffective at improving reproductive success, or worse, further exacerbate population declines must be explored in order to test the effectiveness of nest-site supplementation for the implementation of meaningful conservation programs
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