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Population Dynamics, Chick Diet, and Foraging Behavior of the Razorbill (Alca torda) at Matinicus Rock, Maine

During the summers of 2007-2009, I studied the population growth and reproductive and foraging ecology of the Razorbill (Alca torda) at Matinicus Rock (MR), Maine. This medium-sized marine bird in the family Alcidae (auks) was extirpated from the Gulf of Maine in the late 19th century by hunting, collecting, and colony disturbance. Following legislation protecting seabirds and their nesting habitats, the Razorbill has recolonized probable former nesting habitat in the Gulf of Maine during the past several decades. Six small colonies comprise the Maine population, which is listed as threatened and forms the southern extension of the species breeding distribution. In Chapter 1, I present a population model of the MR breeding colony, based on studies of population growth and reproductive success, and supplemented with previously collected data from the National Audubon Society Seabird Restoration Program (Project Puffin), with whom I collaborated. I also describe chick diet (supplemented with Project Puffin data) and draw connections between diet and reproductive success. I found that reproductive success was too low to account for the observed population growth rate, and conclude that the colony is a sink population supported by substantial immigration. Because annual fledging success was positively associated with prey quality, I suggest that substandard chick diet may contribute to the sink population dynamic via diet-driven depressed fledging success. In Chapter 2, I report on the foraging behavior of chick-rearing Razorbills fitted with bird-borne data-loggers at MR in 2008-2009. I describe diving behavior including depth, duration, and profile shape of dives, as well as diel patterns. Diving activity was restricted to daylight hours, and dives were shallowest and most frequent in the evening. Though generally similar to diving behavior reported at four European and Canadian colonies, Razorbills at MR performed three times as many dives per day as at the Gannet Islands, Labrador, and the mean dive depth was greater than three of four previous studies. Deeper and more frequent dives may indicate higher foraging effort and lower prey availability. Reproductive success would suffer if parents cannot buffer chicks against the effects of low prey availability through increased foraging effort or other behavioral modifications. Together, the pieces of our research indicate that prey availability may be negatively affecting reproduction and population growth at MR. Rapid colony growth cannot be explained by local reproductive success, and is likely the result of substantial immigration from other colonies. Chick diet is varied and includes multiple high-quality forage fish species, yet chicks also consume poor-quality prey (larval fish and euphausiids) that may signal periods of very poor prey availability. Frequency and depth of dives made by chick-provisioning adults are also suggestive of parents allocating extra effort to foraging, relative to other colonies.
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