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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 relatively unchallenging 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 environmental and reproductive effects on immunity. In this study, we measured inter- and intra-annual variation in complement, natural antibodies, PIT54 and leucocytes in crossbills across four summers (2010-2013) and multiple seasons within 1 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 variation across multiple seasons within a single cone year was less pronounced and lacked a dominant predictor of immune investment. However, we found weak evidence that physiological processes (e.g. reproductive condition, moult) or abiotic factors (e.g. temperature, precipitation) affect immune investment. Collectively, this study suggests that a reproductively flexible organism may be able to invest in both reproduction and survival-related processes, potentially by exploiting rich patches with abundant resources. More broadly, these results emphasize the need for more longitudinal studies of trade-offs associated with immune investment.

This document provides a concise set of rules and guidelines for lead researchers conducting fieldwork in the natural sciences. The rules emphasize the importance of early planning, personnel decisions, training, safety, and creating a positive work environment. The document also highlights the need to consider social issues and potential hazards in the field, as well as the importance of data management and self-care. The rules are based on the authors' collective experience and aim to assist researchers with limited field experience in successfully conducting their field season. [Extracted from the article]

Many animals rely on photoperiodic and non-photoperiodic environmental cues to gather information and appropriately time life-history stages across the annual cycle, such as reproduction, molt, and migration. Here, we experimentally demonstrate that the reproductive physiology, but not migratory behavior, of captive Pine Siskins (Spinus pinus) responds to both food and social cues during the spring migratory-breeding period. Pine Siskins are a nomadic finch with a highly flexible breeding schedule and, in the spring, free-living Pine Siskins can wander large geographic areas and opportunistically breed. To understand the importance of non-photoperiodic cues to the migratory-breeding transition, we maintained individually housed birds on either a standard or enriched diet in the presence of group-housed heterospecifics or conspecifics experiencing either the standard or enriched diet type. We measured body condition and reproductive development of all Pine Siskins and, among individually housed Pine Siskins, quantified nocturnal migratory restlessness. In group-housed birds, the enriched diet caused increases in body condition and, among females, promoted reproductive development. Among individually housed birds, female reproductive development differed between treatment groups, whereas male reproductive development did not. Specifically, individually housed females showed greater reproductive development when presented with conspecifics compared to heterospecifics. The highest rate of female reproductive development, however, was observed among individually housed females provided the enriched diet and maintained with group-housed conspecifics on an enriched diet. Changes in nocturnal migratory restlessness did not vary by treatment group or sex. By manipulating both the physical and social environment, this study demonstrates how multiple environmental cues can affect the timing of transitions between life-history stages with differential responses between sexes and between migratory and reproductive systems. [ABSTRACT FROM AUTHOR]

It is well documented that irruptive and nomadic migrants move in response to resources that are distributed unpredictably in space, time or both. Increasing evidence, however, suggests that irruptive and nomadic species may use seasonal timing mechanisms to prepare for migrations, despite the more facultative nature of their movements. Here we use data from free-living and captive Red Crossbills Loxia curvirostra, a typical irruptive nomad, to examine three hypotheses regarding control of facultative migration: (1) the facultative migration hypothesis, which states that both preparation and departure decisions are regulated by resource availability; (2) the seasonal preparation hypothesis, which states that preparation is initiated by seasonal factors (i.e. endogenous rhythms and/or photoperiod) but that departure decisions are dependent on local resource availability; and (3) the seasonal migration hypothesis, which states that both preparation and departure decisions are initiated by seasonal mechanisms and are independent of local food resources. Red Crossbills in North America are thought to make temporally consistent spring migrations in anticipation of conifer cone maturation. In this study, fat deposits of free-living Red Crossbills peaked in May and June, exceeding even winter deposits. In agreement with the field data, captive birds on a natural photoperiod with constant food and temperature showed a peak in fat deposition and activity levels in June. These findings are consistent with the seasonal preparation and the seasonal migration hypotheses and contribute to a growing literature that suggests that facultative migrants may prepare for movements in similar ways to seasonal migrants. [ABSTRACT FROM AUTHOR]