Dispersion of lapwing nests in relation to predation and anti-predator defence
The role of nest predation in influencing the breeding dispersion of lapwings was studied experimentally in north-east Scotland. Nests were significantly aggregated with respect to the distribution of suitable habitat, and within such areas, although about 20% of all nests were at least 200m from neighbours. Predation was not significantly greater on these single nests than on those in aggregations, and was not related to group size. Group mobbing response to a nest predator that represented little danger to adult lapwings, such as the crow, included close dives, and physical contact. Response to a more dangerous predator, such as the fox, was restricted to group aerial circling, and ground distraction displays by individuals at least 5m from the predator. The effectiveness of group response to the crow was related to the size of the nesting aggregation, but was not in the case of the fox. Predation rates on artificial nests increased linearly with the distance from an active lapwing nest to a radius of 50m, the limit of the defended zone, and lapwings excluded crows from the area adjacent to their nests. The risk of predation on undefended artificial nests was lowest for single nests, and highest for those within 25m of other artificial nests. Lapwings thus appear to rely on active group defence against less dangerous predators, favoured by nesting in aggregations, and passively through the spacing of cryptic nests, to reduce predation by more dangerous predators. Significantly more lapwings than expected nested within 30--100m of conspecifics, in the range of densities that I predicted would have the lowest overall risk of nest predation from both predator types. Lapwings also nested significantly farther than expected from all trees, and even farther from trees containing crow nests, the centres of activity of territorial crows. This probably resulted from a mechanism of differential reproductive success rather than actual avoidance, and lapwings nesting close to crow nest-trees were not highly aggregated to benefit from group defence. Thus nest predation is of prime importance in influencing lapwing nest dispersion, in relation to the location of both conspecifics and predator activity centres.