Seed dispersal by male tooth-billed bowerbirds, Scenopoeetes dentirostris (Ptilonorhynchidae), in north-east Queensland rainforests: processes and consequences

Moore, Geoffrey James (1991) Seed dispersal by male tooth-billed bowerbirds, Scenopoeetes dentirostris (Ptilonorhynchidae), in north-east Queensland rainforests: processes and consequences. PhD thesis, James Cook University of North Queensland.

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Abstract

The seed dispersal service of male Tooth-billed Bowerbirds (Scenopoeetes dentirostris), hereafter Tooth-bills, was investigated from June 1987 to December 1989, in upland rainforest near Paluma, North-east Queensland. Male Tooth-bills form "exploded leks" during the display season from September to January. Within leks, males establish traditional court sites on the forest floor. Males return to the same court site each display season, with some males known to attend the same court site for 13 years. If a male is displaced or lost from a court site, another male will establish at that site, maintaining lek cohesion. Leks and court sites within are therefore spatially and temporally stable.

Spatial distributions of male Tooth-bills and canopy gaps were examined in two 10ha sites, one logged and the other unlogged. Leks were positioned along ridge lines or on hill tops. Within leks, males were spaced regularly at about 50m intervals. Canopy gaps were randomly distributed across both sites, and were more abundant in the unlogged site. Three of 32 male Tooth-bills continued to maintain courts within or immediately adjacent to canopy gaps.

Fruiting phenologies were monitored at both sites using both fruit fall traps and transect walks. Male Tooth-bill display occurred during the first peak of fleshy fruit production from September to December. The number of species fruiting and the abundance of fruits produced varied between seasons and sites. A total of 61 plant species were deposited on the forest floor during the study.

Detailed observations on the behaviour and movement of four male Tooth-bills were made over the 1989 display season using radio-tracking and observations from hides. Males spent 50% of daylight hours calling from favoured song posts 1-6m above the court. The remaining period was spent actively foraging for fruits, collecting leaves for the court or interacting with other Tooth-bills. The median duration of bouts away from the courts was 12.4mins, while the median duration of bouts at the court was 6.2 mins. The median distance of foraging bouts away from the court ranged from 40-86m, with maximums ranging from 311-391m. The mean home range of males over the display season was 9.6ha. Analysis and modelling of foraging behaviour sequences suggest that more seeds were deposited within the court area (68%) than would be expected from time spent at the court (46%).

Male Tooth-bill diet was monitored using fruit fall traps beneath the favoured perches of ten birds. Over three display seasons, 51 species of fleshy fruited plant species were consumed and deposited in large seed piles beneath favoured calling perches. Of these, 40 species were large fruits containing large seeds (>4mm). Five families comprised 95% of the large fruit recorded in the diet. These were (in decreasing importance) the Elaeocarpaceae, Lauraceae, Symplocaceae, Myrtaceae and Rutaceae. Members of the Moraceae were also well represented in the diet. Gut treatment by male Tooth-bills was gentle, to the extent that members of the Rutaceae passed through the gut unchanged with the pericarp still intact. Seeds of all other species passed through the digestive system undamaged and with the pericarp variously digested.

Germination trials were conducted on 36 plant species consumed by male Tooth-bills. Of these, 53% of species germinated with varying success, with members of the Loganiaceae, Myrtaceae and Lauraceae exhibiting the highest germination success. Comparisons of germination response were also made between bird dispersed and non-bird dispersed seeds. In a comparison of nine plant species, male Tooth-bill dispersal enhanced germination in seven species and shortened the germination time in six species. Examination of the germination response of seed passing through Cassowaries (Casuarius casuarius) and Shining Starlings (Aplonis metallica) provided similar results. Fast germinating seeds, produced in the late dry season, may benefit most from seed dispersal by male Tooth-bills, and seeds with extended dormancy may benefit least.

Seeds and seedlings deposited in clumps require the ability to survive density dependent mortality. Experiments were conducted to compare seed predation in Tooth-bill seed clumps with other microsites on the forest floor. Experiments tested the effects of seed species, density, microsite and year on seed predation intensity. Litsea connonsii and Acronychia acronychiodes seeds were predated heavily while most other species were rarely predated. Predation of Litsea seeds was greatly reduced when seeds occurred at lower densities. Seed predation intensity of Litsea seeds also varied dramatically between years, with high seed predation in 1988 and significantly less seed predation in 1990. Reduction in seed predation coincided with a decline in rodent seed predator numbers. Seeds in Tooth-bill seed piles were predated with similar intensity to other sites on the forest floor.

Experiments were also conducted to compare seedling survival in Tooth-bill seedling clumps with other microsites on the forest floor. Microsite had no significant effect on seedling survival for either Acmena resa or Syzygium johnsonii. Seedlings grew very little in the first year of life. Seedling density had a significant positive effect on seedling growth in height, but a significant negative effect on growth in number of leaves. The only seedlings to gain leaves in the first year of life were those in canopy gaps.

Beneath the closed canopy, seedlings of Acmena resa and Guioa acutifolia persisted as bouquets of seedlings within Tooth-bill seed piles for four years after deposition. Shade tolerant Chionanthus axillaris saplings were more abundant in Tooth-bill court areas than in random sites within the forest, also suggesting effective recruitment within these areas.

These data suggest that clump dispersal of seeds by male Tooth-bills is an effective mode of seed dispersal for a number of plant species within tropical rainforests. Mechanisms of recruitment are discussed in relation to fluctuations of fruit production, seed predators and the availability of safe sites for germination. Advantages to the plants and birds are also discussed. This study consummates a link between a known seed disperser and the fate of seeds dispersed, a connection not previously reported for tropical rainforests.

Item ID: 33787
Item Type: Thesis (PhD)
Keywords: bowerbirds; fruiting; fruits; leks; Scenopoeetes; seed dispersal; seedlings; seeds; stagemaker bowerbirds; tooth-billed bowerbirds
Date Deposited: 21 Jul 2015 05:05
FoR Codes: 06 BIOLOGICAL SCIENCES > 0608 Zoology > 060801 Animal Behaviour @ 50%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060201 Behavioural Ecology @ 50%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960806 Forest and Woodlands Flora, Fauna and Biodiversity @ 100%
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