Nest parasitism, promiscuity, and relatedness among wood ducks

Nest parasitism is a common reproductive strategy used by many species of cavity nesting birds. Among these, the wood duck (Aix sponsa) is known to have evolved very specific strategies of when and whom to parasitize that is often based on population and/or environmental queues. Here, we investigated the genetic relationship of two female wood ducks competing over an artificial nesting box in Delaware, including the continued incubation of one female despite the death and body remains of the other female throughout the incubation process. We test whether such an extreme case of nest parasitism can be explained by relatedness, egg lineage composition, or a combination of other factors. To do so, we extracted genomic DNA from blood and tissue of the females, as well as chorioallantoic membranes of all viable and inviable eggs. Subsequently, we assessed relatedness among females and eggs based on hundreds of nuclear loci and the mitochondrial control region. We concluded that (1) the two incubating females were entirely unrelated, (2) the single clutch is in fact represented by a minimum of four unrelated females, and (3) a single female can lay eggs sired by different males. The latter finding is the first direct evidence for successful extra-pair copulation in wood ducks. With decreasing costs and increasing effectiveness, genomic methods have the potential to provide important insights into more complex ecological and evolutionary tactics of such populations.

An experimental test of information use by North American wood ducks (Aix sponsa): external habitat cues, not social visual cues, influence initial nest-site selection

Birds may use a variety of cues to select a nest site, including external information on habitat structure and nest site characteristics, or they may rely instead on social information obtained directly or indirectly from the actions of conspecifics. We used an experimental manipulation to determine the extent to which a California population of the wood duck (Aix sponsa) used social information gleaned from visual cues inside nest boxes that might indicate the quality or occupancy of that site. Over two nesting seasons, we manipulated the contents of newly installed boxes to simulate one of three states: (1) presence of wood duck eggs, indicating current use of a nest site; (2) presence of down and shell membranes, indicating a previously successful nest; and (3) control nests with fresh shavings indicating an unused box. In addition, we measured habitat characteristics of the area surrounding each box to assess the use of external, non-social information about each nest site. We found no evidence that females laid eggs preferentially, or that conspecific brood parasitism was more likely to occur, in any of the treatments. In contrast, nest site use and reproductive traits of wood ducks did vary with vegetation cover, and orientation and distance of the box from water. Our results suggest that personal information, not social information, influence initial nest site selection decisions when females are unfamiliar with a site. Social cues likely become increasingly important once nest sites develop their own history, and a population becomes well established.Significance StatementIn selecting a nest site, birds may use many types of information, including habitat characteristics, their own previous breeding experience, or social cues inadvertently provided by other individuals of the same or different species. We examined information use in a Californian population of wood ducks by experimentally manipulating the visual cues within nest boxes and found that females did not use internal box cues to direct their nesting behaviors, appearing to rely on key habitat characteristics instead. These results contrast with previous studies of this system, suggesting that females may change the cues they use depending on their prior experience with a particular area. In the nest-site selection literature, there appears to be a divergence between research on passerines versus waterfowl, and we advocate unifying these perspectives.

Survival, Harvest, and Lincoln Estimates of Wood Ducks Banded in Ohio

Wildlife agencies have carefully managed wood duck Aix sponsa populations in part through harvest regulations since the early 1900s. However, unlike many other waterfowl species in North America, waterfowl managers largely do not know breeding population size. Lincoln–Petersen methods based on harvest and band recovery data are an attractive alternative to air or ground count-based surveys for estimating populations of wood ducks by state and federal agencies that are tasked with sustainably managing harvest opportunities for this species. We used banding and recovery data to estimate annual survival rates, harvest rates, and population size in late summer (August–September) using Lincoln–Petersen methods for wood ducks banded within Ohio from 1990 to 2017. Sex, age, and daily bag limits best explained survival rates of wood ducks banded in Ohio, with lower survival rates in years with more liberal bag limits. Lincoln–Petersen estimates of population size ranged from 116,992 to 632,462 annually, and we detected a significant declining trend in population size through time. Mean harvest rates of wood ducks banded in Ohio ranged from 0.069 (adult females) to 0.121 (hatch-year males), and we detected a significant increasing trend in harvest rate through time for adult male, hatch-year male, and hatch-year female wood duck cohorts. Aerial surveys in other Great Lakes states provide comparable population estimates with our Lincoln–Petersen estimates for Ohio and also show a declining trend in population size. We recommend continued investigation into the use of Lincoln–Petersen techniques for estimating wood duck population size throughout the Great Lakes region. If declining population trends are not unique to Ohio, waterfowl managers may need to further assess the potential impact of increased harvest rates on population size to ensure sustainable harvest into the future.