Nest-site limitations and colony development in tree-nesting Great Cormorants

The effects of nesting site availability on colony size were investigated i n a Great Cormorant (Phalacrocorax carbo) colony on dead trees in north-Eas t Italy. Trees have collapsed and colony structure has changed in recent ye ars. After a steady increase in size, the breeding colony decreased dramati cally as predicted by estimates of potential nesting space. After the forma tion of a new sub-colony in living trees, the colony grew again. Tree struc tures were used differentially, with low-quality sites occupied in years of strong space limitation. However, the proportion of nest sites that were u sed twice in a season increased only slightly as nesting space diminished, perhaps reflecting the high costs of breeding late. The mean brood size at fledging remained unchanged between the growing and declining phases, sugge sting that food availability to cormorants did not change, despite the appa rent de crease in fish harvest size. In addition, immature-plumaged cormora nts still attempted to breed when the mean nest-site quality was at its low est. This study provided an example of a breeding colony where yearly chang es in size were not a good indicator of its actual phase of development.