Do wood ducks use the quantity of eggs in a nest as a cue to the nest's value?

Breeding birds use a variety of cues to choose a nest site. For conspecific brood parasites, the number of eggs in a host nest may be an important indicator of nest stage (laying or incubating) and the resulting prospective value of the nest. In precocial birds, such as wood ducks (Aix sponsa), a parasitic female should lay her eggs during the host's laying stage to ensure hatch synchrony with the host. Incubation and hatching success may also be compromised in large clutches. Accordingly, parasitic females should respond to the number of eggs already present in a potential hosts' nest and should preferentially lay eggs in nests with smaller clutches. We conducted a field experiment using simulated nests containing different numbers of host eggs to test this hypothesis. When offered a choice of nests containing clutches of 5, 10, 15, or 20 eggs, females were significantly more likely to lay eggs in the 5- and 10-egg treatments, laid more eggs in total in the smaller clutch treatments, and were more likely to incubate the nests in the 5- and 10-egg treatments. These results indicate that wood ducks are responsive to quantitative cues, such as the number of eggs in a nest, although we do not yet know if they are able to do so directly by numerical discrimination (i.e., counting).