LOCATIONS AND NUMBERS OF DOUBLE-CRESTED CORMORANTS USING WINTER ROOSTS IN THE DELTA REGION OF MISSISSIPPI

Monthly counts of Double-crested Cormorants (Phalacrocorax auritus) we re made at is night roosts in the Delta region of Mississippi from Oct ober through May during winters 1989-90 and 1990-91 as a basis for est imating wintering populations. Populations peaked at 24,660 birds in A pril 1990 and 26,893 in March 1991. Numbers of Double-crested Cormoran ts differed among counts (N=9, P=0.037) and roosts (N=13, P=0.016) in 1989-90, but only among roosts (N=17, P=0.0004) during 1990-91. There was no difference (0.1323>P>0.1223) between adjacent evening and morni ng counts (N=18) at the same roost, numbers of Double-crested Cormoran ts counted entering and leaving roosts were highly correlated (tau=0.9 47, N=28, P <0.005) with cormorants counted in photographic slides tak en after Double-crested Cormorants arrived in the evening and before t hey departed in the morning. However, direct counts made at the roosts and from slides were different (0.0002>P>0.0001). Roost descriptions are given and influences on population fluctuations are discussed.