To understand the patterns of fruit-bird interactions and to identify species with significant roles that are irreplaceable in these interactions (key species), we classified plant types according to traits relating to frugivory by birds, and analyzed the relationships between plant types and frugivorous birds in a primary lucidophyllous forest in Japan. At the 4-ha study site, 111 plant species were bird-dispersed and 15 common bird species were frugivorous. The growth form of plant species was divided into overstory, understory, and liana. The phenological pattern of fruiting was divided into “summer”, “fall”, and “persistent” from the temporal pattern of the seed rain. Fruits were classified in terms of size, as small, a size widely eaten by birds, and large, a size that is difficult for small birds to eat. Seventeen types of plant were identified in the study site, which were classified according to growth form, phenological pattern, and fruit size. Of these fruits, 14 species were considered to be major species, that is species that are both abundant and important for certain birds, and a further 20 species were identified as complementary species, that is species that compensate for a low diversity or for a temporal lack of the major species. Of the birds, eight species were considered major dispersal agents. The patterns of relationship between fruits and birds overlapped in various ways. No strong relationship in which species of fruits and birds are dependent almost entirely on each other were found. An important species set composed of three key species (Eurya japonica, Cleyera japonica, and Cornus controversa) and a group of summer fruits provided continuous and familiar food for many bird species. The patterns of relationship suggest that conservation of the overall composition of fruit types improves the stability of food resources for birds and facilitates dispersal success for the plants themselves.
We investigated the seasonal correspondence between the fruiting phenology of fleshy-fruited plants and the abundance of frugivorous birds, in a temperate forest, in central Japan. The majority of fleshy fruits ripened in the fall when frugivorous birds were most abundant. This correspondence occurred earlier than in a warm temperate forest, located in southern Japan; these relationships in East Asia coincide with those of the temperate regions of North America and Europe. We also examined whether the abundance of frugivorous birds led to profitable effects for seed dispersal among fleshy-fruited plants. The Brown-eared Bulbul Hypsipetes amaurotis was considered to be an important seed disperser for many fleshy-fruited plants because of its high frequency of occurrence throughout the fruiting season, and its large gape size, which allowed it to swallow all sizes of fruits found at the study site. Although numerous Brown-eared Bulbuls and other frugivorous birds were present in the fall, fruit removal rates in fall-fruiting species were not always higher than in summer-fruiting species. The abundance of frugivorous birds alone could therefore not adequately explain the concentrated fruiting phenology among fleshy-fruited plants.
Seed dispersal of Japanese stone pine Pinus pumila by the Eurasian Nutcracker Nucifraga caryocatactes was studied at Mt. Apoi in Hokkaido, northern Japan. The seed foraging and caching behavior of diurnal birds and mammals was observed, and the relative importance of each species for pine seed dispersal was examined. All mature cones disappeared from the pine shrubs by mid-October each year regardless of the cone crop size. Eurasian Nutcrackers, Varied Tit Parus varius, Eurasian Nuthatch Sitta europaea, Red Squirrel Sciurus vulgaris, and Siberian Chipmunk Tamias sibiricus were all potential seed dispersal agents, however, observations revealed that nutcrackers carried 96% of all seeds transported from the pine trees. The nutcracker carried 142 seeds on average (max. 209) in one trip. Nutcrackers mainly carried pine seeds into their mixed coniferous forest, breeding habitat, where stone pines cannot normally become established and cached them in the soil there. Nutcracker caches averaged 12 seeds with a maximum of 51 seeds. Pine seedlings were found growing in clusters (in groups of two or more trees). The number of seedlings per cluster closely resembled the number of seeds in nutcracker caches. Results suggest that most stone pine seedlings originated from nutcracker caches. Eurasian Nutcrackers thus play an important role in the regeneration of Japanese stone pine despite their small number of caches.
Seed retention time (SRT) of 16 fruit species in the guts of the Brown-eared Bulbul (Hypsipetes amaurotis), a major fruit consumer in central Japan, was studied to examine the relationship between SRT and fruit characteristics, i.e. fruit size, seed size, seed weight, and water content. Caged bulbuls were videotaped after feeding on fruits, and the time of defecation of each seed was recorded. Most seeds were always defecated in fecal pellets, with the exception of Aucuba japonica (the largest of the seeds studied), a seed of which was regurgitated on one occasion. Bulbuls defecate large seeds more rapidly than small seeds. The SRT of the last defecated seed, mean SRT, and standard deviation of SRT were significantly negatively correlated with seed size, fruit size, and seed weight, while SRT of the first defecated seed and water content were not correlated with any of the fruit characteristics examined. This suggests that Brown-eared Bulbuls are somehow able selectively to eliminate bulky seeds from the gut rapidly in order to overcome digestive limitations. If birds would prefer fruit species with large seeds that they can regurgitate and with short seed retention times in the gut, the results suggest that large seeds have the advantage of quantity of seed dispersed. Small seeds retained in the gut for longer have the advantage of being carried further and thus can achieve greater dispersal distances and more diverse destinations. The evolutionary interaction between fruiting plants and avian seed dispersers, may affect the diversity of fruit characteristics mediated by the length of retention time in a bird's gut.
In the Madagascan rainforest, the role of the Velvet Asity Philepitta castanea, an endemic frugivorous bird, in the regeneration of five understory shrub species (Myrsinaceae and Rubiaceae) was examined during the dry season (August to October). Effective dispersal distance was 33.3 m/h. Based on seed retention time in captivity, more than 85.7% of regurgitated seeds and all defecated seeds were estimated to be transported outside the crowns of mother plants. Seeds passed by the Velvet Asity germinated less successfully than unmanipulated (control) seeds in four out of the five species of shrubs. The reduced germination rate of processed seeds was partly due to the non-adapted morphology of the Velvet Asity as a seed disperser, in particular its voluminous, thick-walled, muscular gizzard. The narrow, slightly decurved bill and the semi-tubular tongue with vibrissae at the tip of this bird are normally features of insect- and/or nectar-eaters. Moreover, since manual removal of fruit pulp decreased the germination rate of seeds, the shrub species studied may not have developed adaptations for seed dispersal by animals. The most probable explanation for this situation is that the Velvet Asity has shifted relatively recently to occupy the niche of a fruit-eater of the understory and as yet insufficient time has passed for a sophisticated relationship with fruiting plants to have coevolved.
Head bobbing patterns of walking Black-winged Stilts Himantopus himantopus and eight species of herons were studied. Though several of the species studied had been previously reported as non-bobbing birds, all nine species usually head bobbed while walking during our observations. The head-bobbing pattern most frequently observed was ‘one bob per step’ in which a bird bobs its head once for each step it takes. In several species, one of two other patterns was also observed. The ‘one bob per two steps’ pattern was observed in three species of herons when they were walking slowly, and the ‘two bobs per step’ pattern was observed in Black-winged Stilts. Non-bobbing walking was observed in Japanese Night Herons Gorsachius goisagi walking at relatively fast speed during foraging, and in two other species of herons when they were not foraging. Head bobbing may be affected by walking speed and by whether birds are foraging or not.
In birds with biparental care, two parents cooperate to provide the appropriate amount of care for the young. Evolutionary stable strategy (ESS) models predict that cooperation can be stabilized when parents respond to reductions in care by their partners by increasing their effort, while not fully compensating for the reduction. To examine whether parents adjust their effort according to their partner's contribution and what cues the parents use in the bargaining process, we manipulated parental care in the Barn Swallow Hirundo rustica. Twenty-eight pairs were randomly assigned to three groups: (i) reduced male parental care (7 pairs), (ii) control (13 pairs), (iii) reduced female parental care (8 pairs). Parental care was manipulated by attaching small weights to the base of a bird's tail feathers. The manipulation successfully reduced parental provisioning in the handicapped birds, while still maintaining biparental care. Regardless of sex, however, handicapping of individuals led to no compensatory responses by the mates. The handicapped birds spent more time resting, causing lowered provisioning rates. Males with a handicapped female decreased their provisioning rates to guard the resting females against extra-pair males. Since the provisioning parents in the three groups seldom met at their nest, it is unlikely that parents monitor their partner's provisioning rate directly. We predicted that parents would adjust their provisioning rates according to the begging behaviors of their nestlings. However, no significant relationship was detected between the begging intensity (begging level and calling duration) and travelling time. Moreover, there were no significant differences in either begging level or calling duration among the three groups. Since various factors affect the provisioning rate in a handicapping manipulation, our data do not support the prediction derived from ESS models of biparental care.