Effects of wintering under methylmercury exposure on spring reproductive onset in song sparrows ( <i>Melospiza melodia</i> )

Exposure to methylmercury (MeHg) on breeding grounds may have numerous deleterious effects on birds, including neurotoxicity, disruption of hormones, and impaired reproduction. But it is unknown if MeHg exposure on wintering grounds can carry over and produce negative effects on the following spring breeding seasonal transition. To evaluate this, we exposed male captive song sparrows (Melospiza melodia) to environmentally relevant levels of dietary MeHg for 3 months during winter. We then photostimulated the birds with a long-day photoperiod and observed them for 21 days post-exposure. Contrary to our predictions, we found no carry-over effects of MeHg on the timing of changes in spring reproductive physiology assessed by testes mass, syrinx mass, plasma androgen concentrations, number of GnRH neurosecretory cells, and body condition. However, following photostimulation, MeHg-exposed birds had smaller cloacal protuberances. Although we observed no obvious effects on the timing of reproductive onset, the results suggest that winter MeHg exposure could induce carry-over effects on secondary sexual traits that may affect birds' breeding performance. Overall, our findings indicate that songbirds can buffer against the main effects of prior winter MeHg exposure so as to not delay reproductive onset in spring, but more studies are required for long-term effects on breeding performance.