Long-Term Dynamics of Bird Dispersal Network Structures Associated with the Functions in Bird Migration and Fruit Abundance

In temperate zones, the complex network of seed dispersal by migrant birds is formed and the structure is dynamic on long time scale. Over 12 years, we examined interannual variability of structures of bird dispersal networks and factors affecting them by observing the characteristics of fruit abundance, bird migration and bird dispersal interactions in central Japan. The fruit abundance exhibited a remarkable fluctuation across years, with the number of fruiting trees and matured fruits fluctuating repeatedly every other year, leading to the periodic fluctuations. The abundance of migrants was also fluctuated. According to the abundance of fruits and migrants, the 12 years as study period was classified into three types. The seed transporting frequency and the dispersal networks were investigated by collecting faeces of migrants. Of the 6652 samples collected from 15 bird species, 1671 (25.1%) included seeds from 60 plant species. Main dispersers were composed of Turdus pallidus, T. obscurus and Zosterops japonicus. The structures of bird dispersal networks were highly nested over 12 years, suggesting the networks are stable. Specifically, the nested structure developed in years when fruit abundance was low. GLM analyses showed the abundance of migrants, particularly T. pallidus and T. obscurus, had strong positive effects on construction of nested structure. The development of nested structure may be caused by the fact the two Turdus species were more frequently functioning as generalist dispersers when fruit abundance was lower. Our study revealed one of the mechanisms determining the structure of bird dispersal network on long time scale.

Nocturnal scent in a ‘bird-fig’: a cue to attract bats as additional dispersers?

The plant genus Ficus is a keystone resource in tropical ecoystems. One of the unique features of figs is the diversity of fruit traits, which in many cases match their various dispersers, the so-called fruit syndromes. The classic example of this is the strong phenotypic differences found between figs with bat and bird dispersers (color, size, and presentation). The ‘bird-fig’ Ficus colubrinae represents an exception to this trend since it attracts the small frugivorous bat species Ectophylla alba at night, but during the day attracts bird visitors. Here we investigate the mechanism by which this ‘bird-fig’ attracts bats despite its fruit traits, which should appeal solely to birds. We performed feeding experiments with Ectophylla alba to assess the role of fruit scent in the detection of ripe fruits. Ectophylla alba was capable of finding ripe figs by scent alone under exclusion of other sensory cues. This suggests that scent is the main foraging cue for Ectophylla alba. Analyses of odor bouquets from the bat- and bird-dispersal phases (i.e. day and night) differed significantly in their composition of volatiles. The combination of these two findings raises the question whether E. alba and F. colubrinae resemble a co-adaptation that enables a phenotypically classic ‘bird-fig’ to attract bat dispersers by an olfactory signal at night thus maximizing dispersal.

Bird dispersal techniques

Conflicts between humans and birds likely have existed since agricultural practices began. Paintings from ancient Greek, Egyptian, and Roman civilizations depict birds attacking crops. In Great Britain, recording of efforts at reducing bird damage began in the 1400s, with books on bird control written in the 1600s. Even so, the problem persists. Avian damage to crops remains an issue today, but we also are concerned with damage to homes, businesses, and aircraft, and the possibility of disease transmission from birds to humans or livestock. Bird dispersal techniques are a vital part of safely and efficiently reducing bird conflicts with humans. The bird must perceive a technique as a threat if it is to be effective. No single technique can solve all bird conflicts, but an integrated use of multiple techniques, each enhancing the other, generally provides relief.

Effects of bird ingestion on seed germination of Vaccinium cylindraceum (Ericaceae), an endemic species of the Azores archipelago1This article is part of a Special Issue entitled “A tribute to Sam Vander Kloet FLS: Pure and applied research from blueberries to heathland ecology”.

In this paper we investigate the effects of bird ingestion on Vaccinium cylindraceum Smith seed germination characteristics and the effect of a lack of bird dispersal on seed germination in the field. To test the effect of bird ingestion on seed germination, we carried out laboratory germination trials using ingested seeds and manually depulped seeds. To test the effect of a lack of bird dispersal on seed germination, we carried out germination trials using intact fruits above or under a cap of dead leaves under field conditions. The number of damaged seeds per fruit differed between species. Nearly 92% and 70% of the seeds ingested by, respectively, Erithacus rubecula and Turdus merula passed intact through the birds’ digestive tracts, and the germination characteristics of these seeds were not significantly affected. Only 16% of the seeds ingested passed intact through the digestive tract of Fringilla coelebs , and the germination ability of these seeds was significantly reduced. Uncovered intact berries in the field cracked, exposing the seeds to light, thus resulting in 18% of germination. Our results suggest that E. rubecula, T. merula, and F. coelebs can act as seed dispersers of V. cylindraceum. Also, V. cylindraceum seeds do not need to be ingested to germinate, but seed germination is enhanced as a consequence of seed exposure to light.