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).