Behavioural tactics at nest visits differ between parasites and hosts in a brood-parasitic duck

Whether conspecific brood parasitism is adaptive is a matter of debate. It may just be accidental when suitable nest sites are scarce. I tested this and other hypotheses, using video recordings and quantitative behavioural observations at nestboxes of individually marked common goldeneyes, Bucephala clangula. In 11 of 13 parasitized nests filmed, parasite behaviour differed markedly from that of hosts during most of the egg-laying sequence. Hosts typically started and laid the most eggs in the nest, covered the clutch when leaving it, and deposited down. Parasites covered the eggs poorly and did not deposit down. Hosts spent increasing time on the nest over the laying sequence, whereas parasites did not. Parasites tended to lay eggs later than hosts in the morning, possibly in response to nest guarding by hosts. When in the nest, hosts usually prevented females from entering, but parasites rarely did so. Of 84 parasitic eggs, 85% were laid by 'true parasites, and 15% by females that behaved like hosts but were eventually displaced by another female. Most (73%) of the 'truly' parasitic eggs were from females that were marked as adults previously, and 27% from new recruits. Parasitism in this goldeneye population is thus usually an alternative behavioural tactic, distinctly different from the behaviour of hosts, and not simply a side-effect of competition between females over the same nest.