Drought disrupts year-round breeding readiness in a tropical songbird

Climate change is expected to increase the prevalence and severity of extreme weather events like drought. For many opportunistically breeding tropical bird species, precipitation serves as a primary cue for onset of breeding, and sustained drought can have major impacts on reproductive fitness. The physiological effects of drought are poorly understood, but understanding physiological responses can help resolve the mechanisms underlying effects of drought on population demography. We used physiological data collected on a year-round breeding New Guinea endemic passerine, the White-shouldered Fairywren (Malurus alboscapulatus), during years with persistent rainfall and during the 2015-2016 El Nino drought event to assess patterns of breeding readiness, body condition, and molt. Many males lost their cloacal protuberances, a sperm storage organ they typically maintain year-round, during the drought period. Body molt, which also occurs year-round in this species, was higher in males during the drought period. Using a sliding-window statistical model approach, we identified a critical precipitation window of 0-40 days prior to capture for cloacal protuberance volume and a window of 8-53 days for molt, with lower precipitation associated with smaller cloacal protuberances and greater molt. Plasma androgens were predictive of variation in male cloacal protuberance volume and molt, thus potentially mediating transitions between life-history stages in response to environmental conditions. Female molt, androgens, and male mass varied with photoperiod, with longer but decreasing day lengths characteristic of the austral summer associated with increasing molt and decreasing androgens. Collectively, our results indicate the potential for photoperiod to cue some life-history stages, whereas reduced rainfall impacts male but apparently not female reproductive physiology and readiness to breed. Improving our understanding of environmental regulation of life-history transitions in tropical taxa is essential for identifying which taxa are most at risk under a changing climate.

Automated summary

Climate change is expected to increase the prevalence and severity of extreme weather events, which can have significant impacts on the reproductive fitness of opportunistic breeding tropical bird species. This study focused on the White-shouldered Fairywren and found that males lost their sperm storage organ and experienced increased body molt during a drought period. Precipitation patterns prior to capture were associated with the size of the sperm storage organ and molt, and plasma androgens played a role in these physiological responses. Photoperiod also influenced molt, androgens, and male mass. These findings highlight the importance of understanding environmental regulation of life-history transitions in tropical taxa under a changing climate.