Intraspecific kleptoparasitism in flock-fishing Great Crested Grebes (Podiceps cristatus) and Great Cormorants (Phalacrocorax carbo) – a cost to participants?

Intraspecific kleptoparasitism was studied in flock-fishing Great Crested Grebes Podiceps cristatus during May and June 2011 and in Great Cormorants Phalacrocorax carbo during September 2011 and August 2012 at Lake Krankesjön, South Sweden, for a total of c.11 h for grebes and c.6 h for cormorants. Only attempts to steal large fish (relative to a bird’s gape) were recorded, because small fish mostly did not elicit kleptoparasitic attempts and those seen appeared never to be successful. Attempts to steal large fish were rare, 0.14 h-1 grebe-1 (N = 96) and 0.39 h-1 cormorant-1 (N = 163). In both species, the host lost its fish in about 20% of cases. Based on the observed low parasitism and success rates, it is concluded that kleptoparasitism was a minor cost of participating in flock-fishing