Satellite Tracking of Juvenile Reddish Egret (Egretta Rufescens) Dispersal and the Site Fidelity
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Information concerning all age classes of a species is essential for a thorough understanding of population dynamics, especially in taxa such as waterbirds that exhibit delayed sexual maturity. Data on the life history of juvenile Reddish Egrets (Egretta rufescens) are scarce, with many aspects of its ecology lacking information completely. Information pertaining to post-fledging dispersal is integral to the development of effective conservation planning. In addition to providing this basic information about the species, knowledge of differences in movements between the two plumage morphs could illuminate additional mechanisms that influence the maintenance of plumage dimorphism. For this thesis project, I measured the movements, dispersal and survival of juvenile Reddish Egrets using satellite telemetry. Data on 25 juveniles from the Texas coast were collected from mid-June 2010 to October 2011. Initial dispersal events were erratic and unpredictable as the birds explored their surroundings, but movements decreased as the winter of 2010 began and individuals settled on wintering grounds along the Texas coast, as well as in Florida and Tamaulipas. In general, movements were less in spring and summer 2011 relative to the previous fall and winter, as most individuals did not have a second exploratory period after their first winter. While most birds remained in the general vicinity of their natal region, the Laguna Madre, longer dispersal events also occurred as some individuals moved to Louisiana and Florida. Site fidelity (19.52%) was comparable to other juvenile waterbirds, and the survival rate of the deployed transmitters suggests that juvenile mortality may be high (Ŝ = 0.534 from June 2010 – October 2011) in this population. Comparisons between the two plumage morphs, males and females, and natal colonies all yielded non-significant results, indicating that the particular circumstances of early life (initial competitive ability, inherent boldness, initial growth rate, etc.) are probably more important than plumage morph, sex, natal colony or body size in determining the dispersal habits of a particular individual. As a mostly resident species and obligate habitat specialist, Reddish Egrets are particularly vulnerable to the effects of sea level rise, coastal development, and the predicted increase in frequency and intensity of severe weather systems in the Gulf of Mexico. If foraging grounds are becoming increasingly unavailable, inexperienced juveniles may have difficulty establishing territories, leading to higher mortality due to exhaustion, starvation or predation. This hypothesis holds implications not just for Reddish Egrets, the rarest heron in North America, but for the entire Laguna Madre ecosystem.