Fruit production, migrant bird visitation, and seed dispersal of Guarea glabra in Panama

The relationship between bird visitation and the size of the available fruit crop was studied at an understory tree ( Guarea glabra Vahl, Meliaceae) in the tropical wet forest of Barro Colorado Island, Panama Canal Zone. Twelve resident species and seven North American migrant species fed on the Bright orange arilloids (seeds with arils), which were not depleted during the normal fruiting period. The number of individual visitors, the number of visiting species, and the number of seeds removed increased linearly with the size of the available fruit crop. The proportion of seeds removed did not increase with the size of the available fruit crop, indicating that dispersal is a function of the number of fruit available and not a disproportionate function of large fruit displays. Four species of North American migrants ( Myiarchus crinitus, Catharus ustulatus, Vireo olivaceus , and Vermivora peregrina ) accounted for 70% of the visits and 60% of the seeds removed from the trees. No resident species visited Guarea as frequently as any one of these migrants. There was no indication that any single visitor was dependent on this tree for nutrition, nor that the tree was dependent upon any single species for dispersal. We hypothesize that the fruiting season of G. glabra is adaptively synchronized with northward migration of opportunistically frugivorous North American birds.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47724/1/442_2004_Article_BF00348067.pd

Digestive plasticity in Mallard ducks modulates dispersal probabilities of aquatic plants and crustaceans

1. The consequences of plastic responses of the avian digestive tract for the potential of birds to disperse other organisms remain largely uninvestigated. 2. To explore how a seasonal diet switch in Mallard (Anas platyrhynchos L.) influences their potential to disperse plants and invertebrates, we recorded the retention time of markers, following exposure to two diets of contrasting digestibility (trout chow vs seeds). 3. We then recorded the retrieval and germination of Fennel Pondweed (Potamogeton pectinatus L.) seeds and Brine Shrimp (Artemia franciscana Kellogg) cysts ingested by the same birds. 4. Gut passage rates of markers were increasingly longer in birds on the seed-based, high-fibre diet and shorter in birds on the animal-based, low-fibre one. 5. Propagule digestibility, and thus survival to gut passage, differed between diet groups, with more seeds and fewer cysts retrieved from ducks on the animal-based diet. Germination decreased with retention time, but was not affected by diet. 6. Differences in passage rates of markers but not of seeds and cysts suggest no change in dispersal distances of plants and invertebrates between seasons, while differences in digestibility would affect the numbers of propagules dispersed. [KEYWORDS: Artemia franciscana ; diet switch ; endozoochorous dispersal ; Potamogeton pectinatus]