A bird pollinator shows positive frequency dependence and constancy of species choice in natural plant communities.

Animal pollinators mediate reproduction of many plant species. Foraging theory suggests that animal pollinators exhibit preferences for common plant species in natural communities (positive frequency-dependent foraging) and temporary single-species specialization (flower constancy) during foraging bouts. Positive frequency dependence may favor common plant species; flower constancy may enhance conspecific pollen transfer particularly in rare plant species. Previous experimental studies suggest that avian pollinators are unlikely to exhibit these behaviors. We studied foraging behavior of Cape Sugarbirds (Promerops cafer), the main avian pollinator of many Protea species, using focal-plant and focal-bird sampling, assisted by high-resolution maps of the spatiotemporal distribution of Protea individuals and their flowering status. We found that Sugarbird's visitation preference increased with species' relative floral abundance, and that individual Sugarbirds tended to visit single species in sequence. Flower constancy during foraging bouts was significantly higher than expected from random plant-animal encounters at the scale of pollinator movements. Positive frequency dependence may favor the reproduction of abundant plant species while flower constancy may be particularly important for rare plant species. This first simultaneous study of both behaviors in a natural plant-pollinator system shows that bird pollinators exhibit both types of behavior and, in this way, possibly influence plant community structure.

Polyandry in coal tits Parus ater: fitness consequences of putting eggs into multiple genetic baskets.

Females of many species mate with multiple males within a single reproductive cycle. One hypothesis to explain polyandry postulates that females benefit from increasing within-brood genetic diversity. Two mechanisms may render sire genetic diversity beneficial for females, genetic bet-hedging vs. non-bet-hedging. We analysed whether females of the socially monogamous coal tit (Parus ater) benefit via either of these mechanisms when engaging in extra-pair (i.e. polyandrous) mating. To obtain a measure of within-brood genetic diversity as a function of paternal genetic contributions, we calculated a sire diversity index based on the established Shannon-Wiener Index. In 246 broods from two consecutive years, sire genetic diversity had no effect on either the mean or the variance in brood fitness measured as offspring recruitment within 4 years after birth. The hypothesis that benefits of increasing sire diversity contribute to selection for female extra-pair mating behaviour in P. ater was therefore not supported.