Pre- to post-fledging carryover effects and the post-fledging ecology of the Dickcissel (Spiza americana)

The post-fledging period—defined as the time between when a bird leaves its nest and disperses or migrates—is a critical stage in the avian life cycle. Past research has identified survival during this stage as a key component in population growth and maintenance of avian species. Therefore, understanding causes of variation in fledgling survival may be of critical importance in conserving avian biodiversity. I examined the post-fledging ecology of the Dickcissel (Spiza americana), with emphasis on pre- to post-fledging carryover effects of nestling traits and fledgling habitat use. Specifically, I sought to address the following questions: (1) Do body condition and wing development at fledging predict juvenile survival during the post-fledging period? (2) What is the relative influence of body condition and wing development on cause-specific mortality (predation and exposure) of fledglings? (3)What is the relative influence of body condition and wing development on cause-specific mortality of fledglings compared to other potential factors? (4) What post-fledging micro-habitat characteristics do fledglings prefer to use? (5) Are habitat characteristics selected by fledglings the same as those selected for nesting by breeding adults? (6) Do habitat characteristics selected by fledglings benefit fledgling survival? From May to August of 2014 and 2015, I radio-tagged, quantified body condition and wing development at fledging, and monitored the survival of 104 fledgling Dickcissels in two grasslands of central Illinois, USA. For fledglings that died, I attempted to identify the cause of death: either death due to predation or death due to exposure. While documenting fledgling survival, I also quantified vegetation characteristics at 323 fledgling locations, 323 random locations, and 52 nest locations of 60 different fledglings. Additionally, I used automated radio telemetry systems (ARTS) to document fledgling activity rates continuously during the post-fledging period. I found pre- to post-fledging carryover effects of body condition and wing development at fledging, in which traits were positively associated with survival during the early part of the post-fledging period. Survival benefits of each trait depended on cause-specific sources of mortality, such that individuals in better body condition were less likely to die from exposure while those with more advance wing development were less likely to be preyed upon. Fledglings in better condition and with more advance wing development were comparatively more active and mobile earlier in the post-fledging period, suggesting they were better able to evade and/or hide from predators. Fledglings preferred areas with greater vegetation density (higher, denser, and more concealed) which were positively correlated with post-fledging survival. Preferred habitat of fledglings did not differ from nesting habitat. Collectively, my results add to a growing literature on the post-fledging ecology of birds, document several ways by which young songbirds mitigate the high risks of mortality during the early post-fledging period, and highlight important considerations for wildlife programs designed to conserve avian biodiversity