Investigating brood parasitism in Wood Ducks

 
 

Project led by Nicole Odell

wood ducksParasitic wood ducks are faced with several factors affecting their success. Wood ducks begin incubating once the clutch is complete and as ducklings leave the nest within 24-48 hours after hatch, ducklings hatching after the brood leaves the nest do not survive. Therefore, parasitic females should lay their eggs before the host female begins to incubate to synchronize their eggs with the host eggs. However, females lay parasitically both before and after host incubation has begun and some of the eggs hatch synchronously with the host’s eggs. The explanation of what enables the parasitic eggs to develop and hatch synchronously with eggs laid days earlier is unknown. One possibility is that females are able to manipulate the development time of eggs. For example, recent studies have shown a correlation between testosterone concentration in eggs and embryonic development. We hypothesize that testosterone loading in eggs is used by females to ensure hatch synchrony and that parasitic females load eggs at a higher rate to catch-up to the host’s eggs. However, while yolk androgen levels may reflect egg order, parasitic females may not be able to manipulate yolk hormones in individual eggs in order to synchronize their eggs with the host’s. Therefore, another possibility is that females are unable to manipulate yolk androgen levels and that yolk androgen levels are a reflection of a female’s basal androgen levels at the time of yolk development.

DO WOOD DUCKS COUNT?

We conducted a field experiment to determine if female wood ducks use clutch size as acue to the nest stage or the value of a potential host nest. We established pairs of new nest boxes containing 5, 10, 15 or 20 decoy eggs. Egg-laying was monitored throughout the breeding season and nesting behavior was noted. Nests with small numbers of decoy eggs (5-10) were more likely to be parasitized (p<0.001).

eggsclutch size

graphYOLK ANDROGENS IN WOOD DUCK EGGS

In a separate study, we investigated whether wood ducks (Aix sponsa) alter androgen concentrations in their eggs as a function of their behavioral strategy (parasite or host) and whether yolk androgens influence hatch synchrony. Egg biopsies (n=150) were taken from eggs prior to the start of incubation, using nondestructive techniques, to determine the concentration of androgens. Maternity was assigned using behavioral observations and dna analyzes of blood samples from eggs and ducklings. We found that androstenedione concentrations differed significantly between females, with parasitic eggs having higher concentrations.

RESULTS

Our results suggest that female wood ducks can distinguish between nests containing different numbers of eggs and use this information to guide their decision to lay in the nest and, potentially, to incubate the combined clutch. The ability to count has been found in a variety of taxa and at least one other brood parasitic species (American coots) have been reported to count eggs to manipulate clutch sizes (Lyon 2003 Nature 422: 495-499). By assessing the number of eggs in a host nest, a potential brood parasite may be able to better evaluate the laying stage and/or prospective value of a nest. For some females, nests with small number of eggs were sufficiently valuable that females incubated the clutch, despite the fact that other eggs were already present.

Furthermore, hosts showed higher androgen concentrations in later laid eggs, which may assist in synchronizing the development and hatch of their own eggs. Parasitic eggs had higher androgen concentrations than host eggs, and eggs laid after the initiation of incubation had higher hatch success than expected, suggesting a possible role for androgens in hatch synchrony.  Our study suggests yolk androgens may play a role in hatch synchronization in precocial birds and a role in assisting conspecific brood parasites in parasitizing the host female. While host females may utilize androgens to influence hatch synchrony, conspecific brood parasites may coopt this ability to influence offspring development and increase the success of their eggs and exploit their host.

FUTURE DIRECTIONS

Duckling Hormones to Adult Behavior
As wood duck females are highly philopatric, this species offers the potential to look at both duckling hormone levels and the resulting adult behavior. This allows the possibility to examine whether high androgen levels promote a specific adult behavioral strategy, such as being parasitic. One hypothesis is that females may alter egg yolk hormone levels in an effort to produce ducklings with a particular phenotype. 

Additionally, a possible continuation of this study would use testosterone and/or estradiol implants to manipulate females and eggs.  Such manipulations would allow a further investigation of (1) whether a female is capable of active manipulation of yolk concentrations, (2) the long term affect of yolk androgens on ducklings, and (3) the possible examination of whether the organization/activational hypothesis is applicable to CBP.

Revised October 2, 2009