Interactive effects of concealment, parental behaviour and predators on the survival of open passerine nests

Abstract

1 A simple model of relative effects of parental behaviour (parents present vs. absent) and nest concealment on probability of nest predation was evaluated by measuring survival of paired natural/artificial nests of four open nesting passerines over 3 years. 2 The ratio of rodent to corvid predation (i) decreased from yellowhammer (small eggs, ground/near ground nests) through blackcap (small, shrub) to song thrush and blackbird (medium sized, shrub/subcanopy); (ii) was highest in years when rodent abundance peaked – this effect was clear in yellowhammer, detectable in blackcap, but not found in thrushes. An inverse relationship was found between mean annual nest survival and abundance of the major nest predator for each species. Predators differed between poorly (corvids) and well-concealed (rodents) nests in blackcap. 3 The effects on nest survival differed among species, including: positive effect of parental behaviour combined with neutral effect of concealment (thrushes); independent positive effects of behaviour and concealment (yellowhammer); neutral effect of behaviour combined with positive effect of concealment (blackcap). These patterns are consistent with hypothesis that relatively larger species with conspicuous nests (thrushes) should either engage in more vigorous nest defence or their defence is more efficient, compared with smaller species with less conspicuous nests (blackcap). 4 The positive relationship between nest concealment and survival resulted either from an effect of nest site per se (yellowhammer) or from an effect of parental behaviour (blackcap) that changed from negative (nest disclosure) to positive (nest defence) between poorly and well-concealed nests, respectively. The view that lack of a within-species relationship between nest concealment and survival (thrushes) results from parental behaviour compensating for predation risk associated with poor nest concealment was not supported. 5 This study demonstrated (i) multiple interactions among factors influencing the rate of nest predation, both between and within species; (ii) potential bias associated with drawing general conclusions from small-scale experiments.