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.