Materials & Methods
We studied American robin females breeding in a tree farm near Urbana,
IL, USA (lat: 40.128184; long: -88.105349) from April-July in 2018 and
2019. We focused only on female robins as it is the sex responsible for
egg rejections in this species (Hauber et al. 2019). We surveyed the
area every 3 days to detect incubating females. Incubation stage was
unknown for most subjects and is not included in the analyses as it does
not influence egg rejection rate in this species and is not associated
with variation in corticosterone levels at early incubation stages in
our study population (Abolins-Abols & Hauber 2020b).