Patterns of annual and seasonal immune investment in a temporal reproductive opportunist

Historically, investigations of how organismal investments in immunity fluctuate in response to environmental and physiological changes have focused on seasonally breeding organisms that confine reproduction to seasons with mild environmental conditions and abundant resources. The red crossbill, Loxia curvirostra, is a songbird that can breed opportunistically if conifer seeds are abundant, on both short, cold, and long, warm days, providing an ideal system to investigate interactions between immunity, reproduction, and environmental fluctuations. In this study, we measured inter- and intra-annual variation in complement, natural antibodies, PIT54, and leukocytes in crossbills across four summers (2010-2013) and multiple seasons within one year (summer 2011-spring 2012). Overall, we observed substantial changes in crossbill immune investment among summers, with interannual variation driven largely by food resources, while seasonal variation was less pronounced and lacked a dominant pred...