Do Nest Building and First Egg Dates Reflect Settlement Patterns of Females?

Abstract

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.