Integration of photoperiodic and food cues to time changes in reproductive physiology by an opportunistic breeder, the red crossbill, Loxia curvirostra (Aves: Carduelinae)

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Abstract
Opportunistic breeders presumably use short‐term cues (e.g., food) instead of photoperiod to time breeding. This view cannot explain regulation of reproductive patterns in one opportunist, the red crossbill (Loxia curvirostra). Captive crossbills held on natural photoperiod, constant temperature, and ad libitum food express cycles of gonadal development, secretion of luteinizing hormone (LH), fat deposition, body mass, hematocrit, and molt. This shows that changing food availability (presumed to drive reproduction in these birds) is not necessary for expression of a seasonal cycle. However, changes in food availability can modulate this seasonal pattern. Reduced food intake attenuates long day induced LH secretion. Furthermore, long day stimulated birds show greater testicular development if shifted from restricted to ad libitum food than if provided ad libitum food continuously. Ad libitum food alone does not stimulate complete reproductive development in winter, when crossbills often breed opportunistically in the wild. Additional stimulation by other factors (e.g., social interactions) is critical at this time. Thus, crossbills regulate their basic annual cycle with photoperiod and superimpose opportunistic responses to supplementary cues (e.g., food and social factors) on this underlying seasonality. This permits crossbills to adjust to both seasonally predictable and unpredictable changes in their environment.
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