Poor choice of breeding habitat by red-necked grebes at fish ponds

摘要

Common Carp (Cyprinus carpio) ponds provide the prime breeding habitat for Red-necked Grebes (Podiceps grisegena) in Central Europe. Between 1993 and 2004, I studied the choice of breeding sites and the reproductive successes of grebes in relation to the age of the carp stocked in fishponds in SE Poland. The reproductive success of birds nesting in ponds stocked with the youngest fish (fry) was high, whereas pairs in ponds stocked with one year old (1+) - or older - carp suffered serious chick losses due to food shortages, in that over 1/3 of them were faced with total brood failure. Unexpectedly, early breeding pairs preferred ponds with 1+ fish, apparently deluded by the rich fish supply. Behavioural observations showed that fish formed a substantial part of prey provided to the flightless young. However, grebes are gape-limited predators and the range of fish which they are able to eat is limited to small-bodied fish. One-year old carp are too large to be swallowed by the chicks and the rapid growth of cultured fish also makes them unavailable for the adult birds in the later stages of the breeding season. Moreover, the numbers and the biomass of aquatic macroinvertebrates and tadpoles, the alternative prey to fish, were markedly smaller in ponds with older carp than in those stocked with fry. Most of the late nesting pairs (mainly replacement clutches) established territories in fry ponds. The poor habitat forecasting by the early breeding pairs may have demographic consequences for grebe populations since the early nesters are presumably the highest quality breeders.