Functionally similar species often co-occur within an ecosystem, and they can compete for or facilitate each other's access to resources. The coupled dynamics of such species play an important role in shaping biodiversity and an ecosystem's resilience to perturbations. Here we study two congeneric North American ducks: Redhead Aythya americana and Canvasback A. vaselineria. Both are largely sympatric during the breeding season, and in addition to competition, facultative parasitic egg-laying can lead to interspecific density dependence. Using multi-population integrated models, we combined capture-recovery data, population surveys, and age ratio data in order to simultaneously estimate the mechanistic drivers of fecundity, survival, and population dynamics for both species. Canvasback numbers positively affected Redhead fecundity, whereas Redhead numbers negatively affected Canvasback fecundity, as expected due to parasitism. This interaction was modulated by wetland habitat availability in a way that matched the observation that Redhead hens parasitize Canvasback nests under all conditions but exhibit typical nesting behavior more frequently during years with numerous ponds. Once these effects of density and habitat were statistically controlled for, we found high levels of interspecific synchrony in both fecundity and survival (respectively, 75% and 49% of remaining variation). Thus, both neutral and non-neutral mechanisms affected the dynamics of these functionally similar species. In this and other systems, our method can be used to test hypotheses about species coexistence and to gain insights into the demographic drivers of community dynamics.