Carry-over Effects in Arctic-breeding Shorebirds: A Cross-species Perspective

Avian studies have long been limited to single populations at a single time and place. However, such studies overlook carry-over effects, where conditions in one season cause fitness consequences in subsequent seasons. As technological advances make it possible to follow individual birds over a full year, it has become clear that carry-over effects can have fitness implications, and are therefore important to consider. In this thesis, I use tracking and physiological data from 14 species of Arctic-breeding shorebirds to link reproductive and timing variation to a bird's earlier experiences. In general, carry-over effects appear to influence important metrics of breeding and timing in Arctic-breeding shorebirds. The timing of nest initiation was influenced by both carry-over effects, measured by a bird's migration timing, and local weather conditions. While patterns were generally consistent across species, variation in the influence of carry-over effects among species merits further research. Tracking data also showed that delays in one season continue into the next, although seasonally variable mitigation means that birds generally reduced the extent of delays, potentially at a physiological cost. Winter is the exception, as birds appear able to fully "reset the clock" during this period, preventing delays from accumulating across years. Winter levels of the stress hormone corticosterone (CORT) in feathers showed a positive relationship with nest success in the subsequent summer, supporting the idea that high levels of CORT may not always imply that an individual is struggling, or at least that the relationship between CORT and fitness may be complex. This thesis is one of the first multi-species studies of carry-over effects, and is unparalleled in number of species and sample size within the carry-over effect literature. It is additionally novel for the multiple methods used to assess carry-over effects across a similar group of species. The importance of carry-over effects demonstrated within this thesis highlights the need for using a whole year approach to assess what influences variation in fitness, especially in migratory species. Doing so will improve our ability to identify and understand the causes of factors affecting demographic rates and driving declines across taxa.