Alternative nesting behaviours following colonisation of a novel environment by a passerine bird

Abstract

When birds settle in new environments, they may encounter different suites of nest‐predators, resulting in strong selection to nest in novel locations. An important axis of variation is in height of nest placement, because off‐ground nests are likely to be subject to different predation pressures than on‐ground nests. In their native habitats, i.e. temperate montane forest, dark‐eyed juncosJunco hyemalispredominantly nest on the ground. A population of juncos became established on the urban campus of the Univ. of California at San Diego in the early 1980s, and now has a relatively high frequency (∼20%) of off‐ground nests. Off‐ground nesting is associated with strong fitness benefits: based on returns from 579 nests, an egg laid off the ground has ∼80% higher chance of producing a recruit to the next generation than one laid on the ground. In addition, some nests were occasionally re‐used by ∼10% of females and off‐ground nests were more likely to be re‐used. Many females that bred off‐ground also built an on‐ground nests and we did not detect an association in nest site positions of mothers and daughters, implying low heritability of this novel nest placement trait. By contrast, certain territories consistently had on‐ground nests irrespective of the identity of the breeding female. We suggest that a limitation of suitable off‐ground nest sites for a species predominantly adapted to on‐ground nesting greatly slows the rate of evolution of this trait.