PHYSICAL AND BIOLOGICAL FACTORS AFFECTING SEASONAL DOUBLE-CRESTED CORMORANT DENSITIES ON OKLAHOMA RESERVOIRS

We censused Double-crested Cormorants (Phalacrocorax auritus) on 8 res ervoirs in north-central and northeastern Oklahoma from October 1992 t hrough April 1994, excluding the months when the species was absent fr om the area (generally May-August). We used simple and multiple regres sion to identify physical and biological factors associated with seaso nal densities of Double-crested Cormorants on reservoirs. Densities of Double-crested Cormorants varied between years and among reservoirs; they were absent during at least 1 census period, typically winter, on 5 of the 8 reservoirs. Densities of Double-crested cormorants were hi gher in 1992-93 than in 1993-94, which appeared to be related to low t emperatures in 1993-94 that froze some study reservoirs and limited fe eding opportunities. Size of a reservoir, the area of its drainage bas in, and availability of forage fish were associated positively in simp le regression models (adjusted r(2) = 0.215-0.471) with densities of D ouble-crested Cormorants in autumn 1992 and winter 1993. The density o f boat ramps, an index of human disturbance, was correlated negatively in simple regression models with density of Double-crested Cormorants in autumns 1992 and 1993 and spring 1993, but only 4.4-13.5% of the v ariability in cormorant density was accounted for by density of boar r amps. Multiple regression models in autumn 1992 and winter 1993, both of which included size of the reservoir, accounted for 51.9% and 60.4% of the variability in densities of Double-crested Cormorants, respect ively. Although we identified what appeared to be fundamental associat ions of physical and biological factors with densities of Double-crest ed Cormorants, they should be viewed as foundations for future hypothe ses regarding habitat selection during migration and winter residency.