Intraspecific parasitism and nest-site competition in wood ducks

Abstract

We investigated the behavioural mechanisms and adaptive significance of intraspecific brood parasitism in wood ducks, Aix sponsa, by observing a colour-marked population in northeastern Illinois for seven breeding seasons (1989–1995). The birds nested in boxes that were dispersed widely and mounted high on tree trunks, mimicking the distribution and locations of natural nesting cavities. During 158 mornings of observation, 103 parasitic eggs were laid: 44 (43%) by females that had nested in the population previously and 59 (57%) by new recruits. Parasitism was a facultative behaviour, and its occurrence varied with a female's age and contents of the box in which she had previously nested. Returning females (2–7 years old) usually laid again in last year's box, even if another female already was laying there. Each female behaved as if the box were hers. Indeed, jointly nesting females behaved so similarly that it was impossible to differentiate the ‘host’ from the ‘parasite’. Eventually one of these females was forced to depart, often after a fight. In 66% of cases the earlier-arriving female prevailed. Displaced females left behind eggs that were ‘parasitic’ in the definitional sense (i.e. incubated by another female), but parasitism actually was a consequence of being evicted from a favoured nestbox. Displaced females usually laid and incubated a complete clutch in a nearby box; the following year, they again attempted to nest in the box they had used initially. New recruits to the population were younger (1–2 years old), and they often did not nest. However, they did search widely for nesting cavities. When a new recruit found a suitable site that contained an active nest, she sometimes laid a few parasitic eggs there and, in subsequent seasons, attempted to nest in the box she had parasitized. Natal females often returned to and laid in the box from which they hatched, unless it was occupied by their mother. Adult females also actively avoided parasitizing close kin. In general, a nest cavity that was previously used successfully may be worth returning to, and fighting for, because its location is known and its quality has been proven. Scarcity of preferred nesting sites is probably the key ecological factor underlying all four unusual reproductive behaviours that characterize female wood ducks and other cavity-nesting waterfowl: natal philopatry, nest-site fidelity, aggressive competition for nest sites and high levels of intraspecific parasitism.