Why we have been unable to generalize about bird nest predation

Abstract

Nest predation is such an important determinant of the reproductive success of many birds, particularly passerines, that its dynamics are of great interest to ecologists and wildlife managers. The large body of literature devoted to assessing these dynamics, however, has not succeeded in producing much consensus as to what makes a nest more or less likely to be depredated. The idea that nest predation increases at habitat edges, for instance, was very popular for a time, but in fact most studies do not find such an ‘edge effect’, and in a few sites nest predation actually decreases towards the edge (Lahti, 2001). Perhaps the nearest we have come to a definitive statement about nest predation is that it is exacerbated by habitat fragmentation at the landscape scale (Donovan et al., 1995). This hypothesis has strong intuitive appeal: the nesting success of forest birds, for instance, seemingly must decline if their habitat is broken up into small pieces surrounded by a matrix known to harbor an abundance of generalist predators. Apparently, we cannot maintain even this moderate and unsurprising generalization. Spanhove et al. (2009) found, among several Afromontane forest fragments, that the lower the edge-to-area ratio (in other words, the larger a patch and less convoluted its shape), the more likely a forestdwelling passerine nest will be ravaged by predators. Thus, their focal bird, the white-starred robin Pogonocichla stellata, might actually benefit from a fragmented forest habitat, with more edge and lower habitat patch sizes. To some extent, at least in the largest patch in the study site, predation rates were highest in the less disturbed interior of the patch, where nesting tended to occur later in the season and under denser canopy and sparse herbaceous cover. Can landscape fragmentation really benefit some bird species? I believe that the answer is yes, but that conservationists and managers ought to regard this point as trivial, and that more productive questions in avian reproductive success lie elsewhere. The reality of our ecological situation, in Kenya’s Taita Hills as elsewhere, is that distinctive habitats for wildlife have been and often continue to be greatly shrunk and subdivided. Species like the whitestarred robin, which are common, have a broad range and inhabit a variety of forest types as well as shrublands, are not the species of concern in this situation. Many species might benefit from fragmentation, in which case we are in the serendipitous situation of having inadvertently managed a landscape in their favor. Managers should, of course, consider the nesting and other habitat requirements of a threatened species before drafting a plan; this goes without saying. The point here is that very few of these plans will ever need to be drawn up for birds that benefit from fragmentation. Nevertheless, studies that buck the general trends in ecological correlates of nest predation are valuable red flags for those who wish to understand the factors underlying the reproductive success of birds. From Spanhove et al. (2009), one can gather four candidate explanations for their surprising result and its apparent inconsistency with other studies: the use of artificial nests in many studies, different life histories or habitat requirements of the prey species, the scarcity of Afrotropical studies, and different nest predator assemblages. Dozens of studies of nest predation, including assessments of edge and other fragmentation effects, have used natural as well as artificial nests, with no clear distinction overall in their findings (Lahti, 2001), and so this factor is unlikely to be key. Nest site and other behavioral and lifehistory traits will certainly be important determinants of reproductive success, but studies during the last 40 years in the temperate zone have already assessed nest predation for a broad array of species with a range of nesting habits and life histories. This brings us to the scarcity of studies of tropical birds, and birds of the Afrotropics in particular. In recent years, nest predation studies on African birds have become more common (e.g. Schaefer et al., 2005; Hanson, Newmark & Stanley, 2007; Kotze & Lawes, 2007; Lloyd, 2007; Boukhriss, Selmi & Nouira 2009), but a tentative conclusion that one might draw from them as a whole is that general ecological correlates of nest predation are not