Introduction
Locating a site suitable for successful reproduction is an essential
task of many mobile organisms. Breeding animals often need to find
denning, burrowing, or other nesting locations that simultaneously
provide access to nearby resources and limit resource competition while,
critically, also protect both the parents and developing offspring from
adverse weather/microclimates, parasites, competitors, and/or predators
(Mainwaring et al. 2017). These constraints have led to many species
evolving to use relatively consistent breeding sites and substrates
among individuals. Accordingly, most songbird species can be readily
classified into categories such as tree, ground, or cavity nesters (Nagy
et al. 2019), although some species show plasticity in nest site
preference depending on the shifting habitat structure (e.g. song
sparrows (Melospiza melodia ) raise their nest height from the
ground to shrubs as the breeding season progresses: Nice 1931). Other
species may shift their nest placement to avoid predators (e.g.
orange-crowned warblers (Vermivora celata ) shift nests off the
ground and into shrubs on islands where ground predators are more
prevalent and avian predator pressure is reduced: Peluc et al. 2008).
Observing such instances of nest-site switching allows us to address
both the drivers of this behavioral plasticity and their implications
for reproductive success.
American robins (Turdus migratorius , hereafter “robin”) are an
iconic backyard songbird species in North America that build a bulky
mud-lined nest off the ground in trees, shrubs, and human-made
structures (Vanderhoff et al. 2020). Robins are classified as arboreal
nesters (Vanderhoff et al. 2020); there are only two published examples
of ground-nesting robins and these are both from “extreme”
environments amongst what is representative for this species, e.g.,
where trees and shrubs are altogether absent (e.g., in tilled soyfields
in Central Illinois, VanBeek et al. 2014) or on islands with no
mammalian predators (Collias 1964). However, we observed robins nesting
on the ground at a tree-dense and predator-rich study site over
multiple study years.