Relationship Between the Reproductive Ecology of the White-crowned Pigeon and the Fruiting Phenology of Tropical Hardwood Hammock Trees

Bancroft, G.; Bowman, Reed; Sawicki, Richard; Strong, Allan; National Audubon Society

The ecology of White-crowned Pigeons in Florida is tightly tied to the abundance of fruits in the tropical forests. Four species, Metopium toxiferum, Guapira discolor, Ficus aurea, and F. citrifolia dominated the diet of birds during the nesting season. The two Ficus species produced fruit during all months of the year but fruit production tended to be greatest during the summer. G. discolor produced ripe fruit from late May through early fall and its production was closely associated with recent rainfall. M. toxiferum fruit began to ripen in July and ripe fruit remained on the trees through the winter although fruit loss was often substantial by early fall. The number of pigeons in the keys increased in April with increasing fruit abundance, reached a plateau extending from June through August and attained the annual peak in September. Pigeon numbers in Florida dropped precipitously each October when over 90% of the pigeons migrated out of Florida for the winter. The September peak included a large number of flying young from the preceding nesting season. Pigeons nested on half the mangrove islands in Florida Bay and on a few islands in Card and Barnes Sound. No pigeons were found nesting on mainland Florida and only one nest was found on the mainline Keys in 6 years. Nesting distribution in Florida Bay was nearly non-overlapping with the distribution of raccoons. Raccoons presumably restrict pigeon distribution by preying on nest contents or adults. Pigeons laid clutches of eggs from early May through early September. The onset of nesting and especially the termination of nesting were linked to fruit availability. The number of clutches laid during the season was also linked to fruit availability. Fruit was especially abundant in 1988 and more clutches were laid than in any other year. The number of new clutches laid per week showed a bimodal pattern during the season. An initial peak occurred in late May to mid June. The number of new starts per week decreased during late June to early July before increasing to the seasonal peak in late July-early August. The second major wave of nesting was tied to the ripening of M. toxiferum fruit. In 1988, when M. toxiferum ripened earlier and was extremely abundant, the second nesting wave began earlier and lasted longer. The availability of M. toxiferum fruit also had an impact on nesting success. Within a season, nesting success increased after M. toxiferum became available. The highest nesting success was recorded in 1988 when M. toxiferum was very abundant. Egg size, growth rates of young and young survivorship within nests increased when M. toxiferum became available. Two factors seem to explain the importance of M. toxiferum. First, the tree is still relatively abundant in the Upper Keys. Second, its fruit is extremely high in lipid content and low in water content resulting in substantially more energy content in a given volume of this fruit than any other fruit species available to pigeons. White-crowned Pigeons remain threatened in Florida because of human developmental pressures on their foraging habitats. M. toxiferum trees are generally removed from suburban areas because they cause severe skin rashes on most people. We recommend that the FGFWFC work with Monroe County to develop a series of criteria for encouraging or requiring the planting of alternative fruit sources. We strongly recommend that G. discolor and B. ovata be planted whenever possible. We also recommend that a network of hammocks be maintained throughout the Keys to provide a range of foraging areas, especially for recently independent young.