Post-fledging ecology of birds: emergent patterns, knowledge gaps, and future frontiers

The post-fledging period - the time between a juvenile bird leaving its nest and dispersing or migrating from its natal site - is a critical yet challenging aspect of avian ecology to study. Past reviews have advanced our knowledge of the post-fledging period by focusing on a particular topic or taxon but have yet to describe patterns of survival and behaviour across the entire post-fledging literature. Here we review research on the post-fledging ecology of birds across taxa to (i) synthesize emergent patterns, (ii) highlight critical knowledge gaps, and (iii) identify promising future frontiers in research. Our review revealed a general dichotomy in the post-fledging literature, with studies on altricial and precocial birds using different terminology and rarely citing one another. Consequently, we compared the post-fledging ecology of altricial and precocial birds while synthesizing patterns and knowledge gaps in the literature. We showed that altricial and precocial birds share many similarities in their post-fledging behaviour, movement, habitat use, parental care, and survival, despite key differences in their ontogenies (e.g. development in the egg and nestling period length). In particular, altricial and precocial birds showed similar trends in fledgling survival with age (positive association), exhibit similarities in selecting post-fledging habitat (often selecting for denser, more complex vegetation that provides cover), and in movement strategies (moving less at younger age to avoid predator detection). Differences were also apparent, with altricial and precocial birds differing in key traits that carry over from the nesting stage to influence fledgling survival (e.g. mass versus wing development). Nevertheless, differences in descriptions of the post-fledging period have, in part, resulted in studies on altricial and precocial birds being disconnected in the literature, highlighting a need for a unifying, standardized terminology for the period after birds leave the nest. Comparative studies and meta-analyses across the altricial-precocial spectrum are also needed to bridge the two worlds. The post-fledging field has grown steadily over the past few decades, but this life-history stage remains understudied for nearly 90% of avian taxa and important frontiers remain largely unexplored: understudied geographical regions and taxa, brood parasites, ecophysiology, impacts of urbanization and climate-induced environmental change, and impacts of technology used to study this period. Our review emphasizes the importance of the post-fledging period for avian population dynamics and life histories, and highlights the great potential of research into this key developmental stage to advance our understanding of the ecology, evolution, and conservation of birds.