Habitat geometry rather than visual acuity limits the visibility of a
ground-nesting bird's clutch to terrestrial predators.
Abstract
The nests of ground-nesting birds rely heavily on camouflage for their
survival, and predation pressures, often linked to human activity, are 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 nearby control sites. Importantly, our
findings show that habitat geometry – rather than predator visual
acuity restricts nest visibility to terrestrial predators, and that an
open field would actually be perceived as a closed habitat 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.