Survival, fidelity, and dispersal of Double-crested Cormorants on two Lake Michigan islands

Colony fidelity and dispersal can have important consequences on the population dynamics of colonial-nesting birds. We studied survival and inter-colony movements of Double-crested Cormorants (Phalacrocorax auritus; cormorants) nesting at Spider and Pilot islands, located 9 km apart in western Lake Michigan, during 2008-2014. We used live resighting and dead recovery data from both colonies, plus dead recoveries from throughout North America, in a multistate live and dead encounter model to estimate annual survival, inter-colony movements, plus temporary and permanent emigration to unmonitored sites. Annual survival averaged 0.37 (annual process variation, σ=0.07) for hatch-year, 0.78 (σ=0.08) for second-year, and 0.89 (σ=0.04) for after-second year birds. The best approximating model recognized only 2 age classes for transition probabilities, indicating little difference in fidelity and movement probabilities after the natal year. Annual fidelity to Spider and Pilot islands averaged 0.53 (σ=0.17) and 0.48 (σ=0.24) for second-year and 0.55 (σ=0.23) and 0.62 (σ=0.16) for after-second year cormorants, respectively, indicating substantial emigration for both age classes. For birds that dispersed, emigration was approximately equally divided among neighboring colonies, temporary emigration sites from which surviving birds subsequently returned, or permanent emigration sites from which birds never returned (but were still encountered through dead recoveries). Our results indicate that Double-crested Cormorants in the Great Lakes have tremendous potential to disperse, which may help to explain their rapid recolonization following historically low populations in the early 1970s.