Clutch size and population stability in birds: A test of hypotheses

The general consensus of current theoretical models for the evolution of reproductive rates is that large clutches are selected for in unstable populations. Predictions from these theories on the geographical variation in clutch size in passerine birds are tested, assuming that breeding populations, often of low density, close to the borders of their distributional ranges, are more unstable than those, of usually higher breeding densities, in more central areas. The predictions were only found to be verified for the latitudinal variation in clutch size for hole-nesting species. For the same geographical gradient (Switzerland-Finland) for the open-nesters, and for all species for the altitudinal gradient (in Switzerland), no support was found for the hypotheses and, in fact, there was a tendency for the reverse trend, viz. that mean clutch size is positively correlated with population size. For instance, those few species which did exhibit an altitudinal increase in mean clutch size, from lowland to highland, are those generally characterized from their distributional ranges as 'highland' species.