Results
Heart rate showed a significant decrease with time after arrival (LMM, estimate = -0.020, df = 208, t = -10.530, p < 0.001), but variation in heart rate was not explained by the treatment date (LMM, estimate = 0.002, df = 11, t = 0.485, p=0.637) or treatment order (LMM, estimate = 0.051, df = 11, t = 0.682, p =0.509). Model egg color type had no statistical effect on incubating females’ (n=14) heart rate immediately after return to the nest (LMM, estimate = 0.009, df = 208, t = 0.824, p = 0.411, Fig. 2). Furthermore, initial heart rate did not differ between rejecter and acceptor females during the non-mimetic egg trials (LMM, estimate = -0.053, df = 9, t = -0.875, p = 0.404). Eight out of 14 females rejected the non-mimetic egg within 2 hrs.
Subjects exposed to mimetic or non-mimetic model eggs did not show significant differences in circulating corticosterone concentrations 2 hours after experimental parasitism (LM, estimate: -0.118, t = -0.913, df = 40, p = 0.367; Fig. 2). Plasma corticosterone concentrations decreased across the season (LM, estimate = -0.012, t = -4.301, df = 40, p < 0.001). None of the mimetic blue eggs were rejected (n=22) whereas 7 out of 21 (33%) of robins rejected the non-mimetic egg within 2 hours of its addition (Fisher’s exact test, p=0.0036). However, plasma corticosterone did not differ between females who rejected or accepted the non-mimetic egg (LM, estimate: -0.008, df = 18, t = -0.044, p = 0.966; Fig. 3).
Prior to false discovery rate (FDR) correction, 312 pituitary-expressed genes were statistically significantly differentially expressed between birds exposed to the mimetic and non-mimetic egg treatments (Supplementary Table 1) two hours following the experimental parasitism. Among these, POMC showed higher expression in pituitary in birds exposed to non-mimetic model eggs (n=6) compared to birds exposed to mimetic eggs (n=7, fold-change = 0.719, p = 0.042, Fig. 4). One of the most significantly differentially expressed genes prior to FDR correction was ATF3, a transcription factor specifically associated with the stress response (Hai et al. 1999) , which was upregulated in the birds exposed to non-mimetic eggs (fold change=0.323, p<0.001). No genes were significantly differentially expressed between the model egg treatments following FDR correction. POMC expression levels and corticosterone concentrations were not correlated with each other (Spearman’s rho = 0.055, df = 11, p = 0.863, Fig. 5).