Pollen Digestion by Darwin's Finches and Its Importance for Early Breeding

Abstract

This paper examines the role of pollen digestion in the early breeding of Darwin's finches. Pollen is a rich source of protein, but the extent to which birds digest pollen has been debated. On Isla Daphne Major, Galapagos, Geospiza scandens, the Cactus Finch, and G. fortis, the Medium Ground Finch, consume pollen and nectar of the prickly pear cactus, Opuntia echios. I found that >90% of ingested pollen was digested during passage through the gut of adult and nestling G. scandens and adult G. fortis. This value is twice as high as the maximum reported for birds, and comparable to the most efficient digestion known for specialist marsupials. Nectar is ingested with pollen by the finches and may stimulate germination of pollen in the crop, facilitating digestion. Opuntia pollen grains germinated in vitro in a nectar and water solution. Breeding of finches typically follows shortly after the arrival of the first heavy rains of the year. In 6 of 12 years, however, some G. scandens pairs bred and successfully fledged young before onset of the rains. At these times, Opuntia flowers were abundant, arthropods were scarce, and the diet of G. scandens was predominantly Opuntia pollen and nectar. As a result of breeding early, pre—rains breeders produced more clutches and fledged more young per year than post—rains breeders. Although G. fortis take pollen and nectar from Opuntia flowers and efficiently digest pollen grains, they feed on flowers to a much lesser extent than G. scandens, and did not breed before the rains. These results have implications for similar vertebrate pollinator and plant systems in arid tropical regions.