Sex-biased dispersal, post-fledging care and juvenile movements in the white-throated dipper

Roskell, Philip and Sharp, Stuart (2022) Sex-biased dispersal, post-fledging care and juvenile movements in the white-throated dipper. Masters thesis, Lancaster University.

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Abstract

Dispersal is a crucial life history trait which has profound consequences for individual fitness and population dynamics. Yet, our understanding of the spatial and temporal processes of dispersal remains poorly understood, largely due to the logistic difficulties of following and monitoring wide-ranging individuals. White-throated dippers are specialised aquatic passerines which are well-established as an important indicator of river and stream water quality, and they provide an ideal model system for studying dispersal due to their linear territories. However, little is known about the factors that underpin variation in natal dispersal distance. Furthermore, as with many passerines, behaviour during the post-fledging period has rarely been studied despite the likely influence on dispersal. Here, these issues are investigated using a long-term study population of dippers in the UK. In Chapter 2, the impact of early life conditions on natal dispersal is investigated, including the timing of breeding, brood size, population density, and body condition. The analyses also accounted for sex, a well-known determinant of dispersal distance in birds. Indeed, as with many species, sex was the most important predictor of dispersal distance in dippers, with females generally travelling further than males. However, there also appears to be a weak effect of condition, suggesting that individuals in better condition disperse shorter distances. This may reflect the benefits to settling on territories near to the natal area, including familiarity with nesting and feeding sites. No other early life effects were detected. In Chapter 3, observations during the 2021 breeding season were used to measure provisioning rates and track movements of juvenile dippers during the post-fledging period. Brood division appeared to be widespread within this population, and comparisons of mean feeding rates suggests that males contributed more to feeding the fledglings than females. Post-fledging provisioning rates were generally higher than nestling provisioning rates. On average, juveniles were first observed outside of their natal territory 30 days after fledging, but some had left as young as nine days old. Soon after leaving the natal territory, juveniles were highly mobile and able to travel relatively long distances; many individuals were observed together with birds from other broods. Together, these observations provide some of the first descriptions of behaviour during the post-fledging period in this species, and the implications for dispersal are discussed. In the final chapter, the key messages of the thesis are reviewed alongside avenues for future research, with an emphasis on the need to further study the early life determinants of dispersal and behaviour during the transience stage. Combining long-term studies of marked individuals with advanced tracking technologies offer perhaps the best opportunities.