Meeting Abstract
Food supply is a major driver of timing of breeding, yet individuals exposed to the same resource environment (e.g. shared feeding areas) often vary in their reproductive phenology. For example, individuals in poor body condition will breed later than those in good condition. At the physiological level, perception of food supply and its environmental correlates can be integrated directly into endocrine axes. Similarly, internal energetic state is incorporated into the endocrine system, inducing foraging behavior when energy levels are low. We hypothesized that if energy status limits reproduction phenology, then individual variation in reproduction phenology should correlate with net energy gains. To test this hypothesis, we conducted an experiment on female black-legged kittiwakes (Rissa tridactyla) nesting on Middleton Island, AK, during the prelaying period. Following a 2×2 design, we manipulated energy gains and costs via food supplementation and a short-term weight handicap (alongside controls). We measured baseline luteinizing hormone, testosterone, and corticosterone before and after a four-day GPS-accelerometer deployment, conducting a hormone challenge (luteinizing hormone releasing hormone) at the final recapture. Energy intake (supplementation) but not expenditure (handicap) influenced baseline reproductive hormones. Fed birds were less likely to forage at sea than unfed birds, and we found an interactive effect of food and handicap on at-sea activity budgets. Feeding advanced laying phenology, but short-term handicapping did not delay laying. By integrating endocrinology with movement ecology, we are able to determine how differences in energy status can lead to variation in reproductive phenology.
