Habitat geometry rather than visual acuity limits the visibility of a ground-nesting bird's clutch to terrestrial predators

The nests of ground-nesting birds rely heavily on camouflage for their
survival, and predation risk, often linked to ecological changes from
human activity, is a major source of mortality. Numerous ground-nesting
bird populations are in decline, so understanding the effects of
camouflage on their nesting behaviour is of relevance to their
conservation concern. Habitat three-dimensional (3D) geometry together
with predator visual abilities, viewing distance, and viewing angle
determine whether a nest is either visible, occluded or too far away to
detect. While this link is intuitive, few studies have investigated how
fine-scale geometry is likely to help defend nests from different predator
guilds. We quantified nest visibility based on 3D occlusion, camouflage,
and predator visual modelling in northern lapwing, Vanellus vanellus, on
different land management regimes. Lapwings selected local backgrounds
that had a higher 3D complexity at a spatial scale greater than their
entire clutches compared to local control sites. Importantly, our findings
show that habitat geometry – rather than predator visual acuity –
restricts nest visibility to terrestrial predators, and that their field
habitats perceived by humans as open are functionally closed with respect
to a terrestrial predator searching for nests on the ground. Taken
together with lapwings’ careful nest site selection, our findings
highlight the importance of considering habitat geometry for understanding
the evolutionary ecology and management of conservation sites for
ground-nesting birds.