Experimental parasitism treatment
To assess whether parasitic eggs induce a stress response in American
robin females, we experimentally parasitized their nests during
incubation with either a mimetic or non-mimetic model egg (Fig. 1). We
used 3D printed white nylon eggs, sourced from Shapeways.com (model ID
“cow bird”), which were modeled after a natural cowbird egg and are
similar in shape, size, and weight to it (Igic et al. 2015).
These eggs were painted either immaculate light blue (mimetic treatment,
resembling natural robin eggshell’s color) or immaculate beige
(non-mimetic treatment, resembling the cowbird eggshell’s color)
following published protocols (Canniff et al. 2018; Hauberet al. 2019). We placed one model egg in the nest of the
incubating female, without removing any host eggs: prior studies showed
that the removal of own eggs in Turdus thrushes did not affect
their responses to experimental parasitism (Grim et al. 2011).
Variation in natural or artificial olfactory cues also does not induce
egg rejection in American robins (Hauber 2020).