Data for: Environmentally-driven escalation of host egg-rejection decimates success of an avian brood parasite

The black-headed duck (Heteronetta atricapilla) of South America is the
only known avian obligate brood parasite with precocial offspring. In
Argentina, it relies on two species of coots as primary hosts, which
typically reject 35-65% of duck eggs. We show that environmentally-driven
increases in host egg-rejection behavior lead to substantial reductions in
the reproductive success of the brood parasite. Episodes of flooding and
vegetation loss caused dramatic shifts in host egg rejection behavior,
resulting in rejection (85-95%) of almost all duck eggs. Coots respond to
fluctuating water levels by building up their nest, raising their own eggs
but leaving duck eggs behind. Coots can apparently recognize parasitic
duck eggs, but large-scale rejection is triggered only when hosts must
actively make a choice. We use a simple population model to illustrate the
unique demographic challenges that black-headed ducks face with their
parasitic lifestyle, and to explore the potential impact of
environmentally-induced escalation of egg rejection. Using best available
estimates for key vital rates, we show that obligate parasitism may
provide a demographically precarious existence for black-headed ducks,
even under benign environmental conditions. Environmentally-mediated
increases in egg rejection rates by hosts could impact significantly the
viability of this enigmatic species of brood parasitic duck. Our results
demonstrate that egg rejection rates are not fixed properties of host
populations or individuals but are strongly influenced by social and
ecological factors. Shifts in these environmental drivers could have
important and unforeseen demographic consequences for brood parasites.