Effect of breeding timing on White-breasted Cormorant (Phalacrocorax carbo) reproductive success at a seasonally constant Kenyan lake

White-breasted Cormorant (Phalacrocorax carbo) breeding timing and reproductive success were documented in 1995 and 1996 at Lake Naivasha, Kenya (0degrees49'S), considered to be seasonally constant. In both years, pairs breeding earlier fledged significantly more chicks per breeding attempt than pairs breeding later. Smaller brood sizes later in the season were responsible for 81% of the difference; nestling loss represented 19%. Sixty-three percent of the total nestlings lost were from the nests of pairs breeding late in the season. Starvation appeared to be the primary cause of nestling loss, but there was no evidence of a consistent seasonal decline in prey availability. Neither predation nor sibling aggression appeared to be a major factor, although the reproductive success of late-breeders was almost certainly reduced by conspecific interference from fledglings of early-breeding pairs. The likeliest explanation for the seasonal decline is that older, more experienced (and therefore more successful) birds tend to breed early in the season.