White-breasted Cormorant, Phalacrocorax carbo, breeding season timing and its control factors at a seasonally constant Kenyan lake

Environmental factors and breeding activity of the White-breasted Cormorant were documented over two breeding seasons (1995/96) at a new 900-nest colony near Lake Naivasha, Kenya (00° 46′S, 36° 22′E), a non-seasonal lake. Breeding of this species has been reported to occur year-round near the equator in East Africa. Other studies suggest that the White-breasted Cormorant breeds mainly during April–June throughout sub-Saharan Africa, irrespective of the local climatic regime or differences in photoperiod trend. At Lake Naivasha, breeding also occurred primarily during April–June. There was no evidence of a consistent seasonal increase in prey abundance during this period, and the timing was not linked consistently to any other environmental changes, such as amount of rainfall, lake water level or transparency, daily photoperiod, or ambient air temperature. Onset of the main rains, however, appeared to be a possible stimulant to breeding readiness in the population as a whole. It is suggested that breeding timing at Lake Naivasha may be influenced by a combination of the subspecies' nearly 12-month endogenous reproductive cycle and an annual trigger through a local environmental factor, possibly the onset of the main rains.