Breeding biology and ecology of the great cormorant (Phalacrocorax carbo lucidus) at Lake Naivasha, Kenya
thesisposted on 15.12.2014, 10:33 by R. Brooks. Childress
This thesis examines breeding season timing and seasonal declines in reproductive success in piscivores nesting inland near the equator. Also explored are the effects of sexual character intensity and nest-site characteristics on breeding timing, fecundity and mate choice, as well as the effects of resource partitioning between two similar piscivores. P. carbo lucidus is a relatively common, but little-studied, piscivore of sub-Saharan Africa. Lake Naivasha (0Â° 46' S) is reputed to be seasonally-constant in fish production. The colony studied was newly-established in 1995, the first year of the two-year study.;At Lake Naivasha, P. carbo lucidus bred primarily during April-June. There was no evidence of a consistent increase in prey in the lake during this period, and the timing did not appear to result from any other consistent environmental-response adaptation. However, the onset of the main rains appeared to be an important stimulant. Based on studies by others, this subspecies breeds during April-June throughout sub-Saharan Africa, irrespective of the local rainfall regime or differences in photoperiod trend. While nesting at Lake Naivasha, a large portion of the colony appeared to forage at other locations. It is suggested that breeding timing at Lake Naivasha may be controlled by the combination of an increase in prey abundance generally throughout sub-Saharan Africa during April-June, the subspecies' nearly 12-month endogenous reproductive cycle, and the onset of the main rains.;A significant seasonal decline in reproductive success was observed. Combining the two years, the mean number of chicks fledged by the first third of pairs to start laying (2.9) was significantly greater than that of the last third (1.2). The primary reason appeared to be the greater age (and experience) of the early-breeding pairs. Egg or chick predation did not seem to be an important factor, even for late breeding pairs. Breeding interference by hungry chicks from earlier broods appeared to be an important factor limiting the reproductive success of late-breeding pairs.;In the first year of the new colony, the earliest-breeding pairs (first third) preferentially selected canopy-top nest sites (76%) vs. sites below the canopy (24%). They also built significantly larger nests than later-breeding pairs. Canopy-top locations and larger nests had significant positive effects on brood size and fecundity in the colony's first year but not in its second year..