Nest protection in mallards Anas platyrhynchos:: untangling the role of crypsis and parental behaviour

1. The covering of clutches with nest material is generally considered to improve the thermal environment of developing embryos. Here we tested an alternative hypothesis: that this behaviour reduces the risk of clutch detection by predators and hence, fulfils a cryptic anti-predation function in a ground-nesting non-passerine bird, the mallard. In addition, we assess the anti-predation function of the direct presence of an incubating parent on the nest for the first time in a ground-nesting non-passerine bird. 2. We compared predation rates of real mallard nests with two types of artificial clutches: (i) covered with nest material and (ii) uncovered. In addition, the cryptic effectiveness of nest material, female body presence, and uncovered clutch were assessed using a simulated search for nests on photographs by human volunteers. This allowed us to evaluate separately the impact of overall crypsis (covering of the clutch by nest material and colouration of the female feather) and the direct protective capacity of the incubating female. 3. Our data demonstrate that in mallards, concealment of the clutch with nest material reduces the risk of nest predation. Although the incubating female seems to provide less effective crypsis to the nest than nest material alone, the presence of the female on the clutch enhanced nest survival, suggesting a significant anti-predation capacity of the incubating parent in this species. 4. Contrary to some previous studies, the relative effects of crypsis and parental anti-predation behaviour on nest survival did not differ with respect to nest concealment by surrounding vegetation.