Ecology of the Little Black Cormorant, Phalacrocorax sulcirostris, and Little Pied Cormorant, P. melanoleucos, in Inland New South Wales II.* Proximate Control of Reproduction

Abstract

Populations of P. sulcirostris and P, melano2eucos at a group of freshwater lakes were sampled approximately monthly between March 1972 and May 1975. Both species displayed strong gonadal cycles. In P, sulcirostris these were not regularly related to change in any one environmental component, though linear combination of a cyclic factor (temperature or daylength) with water level and food availability accounted for 85.2% of variation in the testis cycle. In a drought year significantly fewer males produced sperm, and the ratio of those producing much sperm to those producing little was significantly lower than in the two succeeding wet years. Eggs were laid in the wet years but not in the drought year, when there was no secluded, flooded timber in which to nest. In P. melanoleucos the date of onset of testis maturation was regular but that of regression was variable. Gonadal maturation was evidently initiated by increasing daylength and regression was initiated by low temperature, falling water level and perhaps lack of social stimulation. Eggs were laid in each year, but in the drought year not until the lakes refilled. Both major breeding seasons concluded abruptly with the widespread abandonment of eggs and young. In both species laying was completely synchronous within and slightly staggered between subunits of a colony, and very different between colonies.