Summary
The order in which females settle on territories likely reflects patterns of female choice of territory and/or mates. Yet in most studies of female choice, settlement dates are usually only inferred from dates of nest initiation or first egg date. To test whether there is a close correlation between the order of female settlement and the initiation of nest building or egg laying, we determined the settlement date of female Tree Swallows (Tachycineta bicolor) through direct observation. The order of female settlement on territories was not significantly correlated with either the initiation of nest building or first egg date. The interval between settlement and egg laying shortened as settlement date advanced, and females began egg laying during a short period in May, regardless of when they had settled. Some females that we experimentally prevented from settling until early May began egg laying at the same time as females that settled in early April. We suggest that female Tree Swallows are under pressure to settle early due to intense competition for limited nesting sites, but then time their egg laying for mid-May to benefit from synchronous breeding and favorable environmental conditions. We predict that the order of settlement is not closely correlated with initiation of nesting activities by females in other species in which females breed synchronously and have a long interval between settlement and egg laying.
Methodology
This study was conducted during the summers of 1985 and 1986 at the Queen's University Biological Station on Lake Opinicon, 50 km north of Kingston, Ontario. The population studied has been established for 10 years, and has remained at a relatively constant size (55 to 65 pairs) since 1982. Tree Swallows breed in nest boxes mounted on aluminum posts 1.5 m high, which are distributed throughout four hayfields that cover a total area of about 5 ha. In late March of both years, nest boxes were placed on every other post of a rectangular grid (20-m spacing), such that the interbox distance was 40 m along a row and 28 m across the diagonal. The four fields (NB, BG, SP, HU) contained 11, 22, 8, and 18 nest boxes respectively during the settlement period. Some nest sites were not settled early in the season because they were defended by neighboring Tree Swallows (Harris 1979, Robertson and Gibbs 1982, Muldal et al. 1985), or occupied by Eastern Bluebirds (Sialia sialis). Nest sites where there had been a definite change of female residency were excluded from the analysis.