Australian National University, Canberra: Completed

Fiona Fraser

Summary | Habitat preferences | Variation in home range size | Reliance on specific grasses | Nest failure | Research use | More information |

Fiona Fraser at the base of a radio-tracking tower listening for the location of a radio-tagged partridge pigeon Photo: PWCNT

Summary

This PhD project was submitted October 2000 and was approved by examiners.

The Partridge Pigeon Geophaps smithii is now extinct from vast areas of its pre-European range. It is one of many granivorous and ground-feeding savanna birds identified as having declined in northern Australia since European colonisation and Aboriginal displacement. The mechanisms of decline of these species are poorly understood, but are probably associated with widespread changes to the savanna understorey due to altered fire regimes and the impacts of pastoralism.

The research was field-based, primarily within Kakadu National Park, on ungrazed land and an adjacent fenced farm subject to grazing by buffalo. Aspects of the ecological requirements of the red-eyed subspecies of the Partridge Pigeon G.s.smithii were explored, including structural and floristic habitat associations at varying spatial scales, variation in home-range sizes, seasonal availability of food, diet, nesting habitat, nest success and, main causes of nest failure. The focus was on patterns of habitat use which were related to variation in the savanna understorey associated with grazing by introduced herbivores and burning practices. The main findings of the study are summarised below.

Tracking the birds by radio transmitter is an important part of the study—one such transmitter can be seen on the top of the bird Photo: Fiona Fraser

Habitat preferences

A structurally patchy savanna understorey, at a relatively intricate scale, may be important for this species. In all seasons, partridge pigeons prefer to feed in areas that have an open ground layer (e.g. following fire), which may make movement easier and seed more visible, so increasing foraging efficiency. However, these birds were more likely to nest at sites where there was high vegetation cover, and these vegetated areas were also often used as roost sites and when retreating from disturbance. Home ranges varied seasonally between 8 - 31 hectares, and creating structural heterogeneity at this scale may best be achieved through fire management which ensures small-scale patchy fires.

Although in the short term these birds prefer to feed in recently burnt or open patches, some of the important perennial grasses that provide seed in the early wet season have delayed or reduced seed set if burnt earlier in the year. Growth and seed production of these perennial grasses can have complex relationships with fire and some grow faster and produce more seed if unburnt. In the longer term, year round resource needs may best be met where the fire regime is variable: some small scale fires throughout the year opening up feeding sites, some areas left unburnt to provide shelter, and longer term unburnt areas to allow for growth and seed production of perennial grasses. Minimising large scale hot fires that can destroy seed and result in a structurally uniform understorey layer is probably also important.

Variation in home range size

There was marked seasonal variation in seed availability, with seed abundance peaking in the early dry season, and then being gradually depleted by granivore activity and fire throughout the dry. Seed was scarce following early wet season rains which caused germination of most remaining surface seed. Partridge Pigeons evidently responded to the reduced early wet season food supply. Radio-tracking data show dramatic increases in home-range size at this time from about 8 ha in the mid dry season to 31 ha in the early wet.

Reliance on specific grasses

Seeds of certain perennial grasses and some sedges are available early in the wet season when seed is otherwise scarce. Availability of these seed types, in particular the perennial grass species Alloteropsis semialata and Chrysopogon fallax (Golden beard grass) which set seed set seed very early in the wet may be crucial for partridge pigeon survival at this time. This study found perennial grasses (including the two key species described above) and perennial grass seed to be less abundant at grazed sites. Further, as partridge pigeons are relatively sedentary (smaller home ranges cf. many other tropical granivorous birds), seed must available within confines of early wet season home range (~31 ha).

Preference for Sorghum seed during nesting, although large patches of dense Sorghum were structurally unsuitable. The large seed of annual Sorghum spp. is plentiful and sought by partridge pigeons early in the dry season (coinciding with the main nesting period). Although a seasonally important food, dense sorghum grasslands tended to be avoided—Sorghum grasslands may be structurally most suitable for this species where they are less dense or more patchy, e.g. following early dry season or wet season burning.

Nest failure due to predation and fire

The main nesting season appears to be early in the dry season and most nests fail, primarily due to predation. This is not unexpected as they nest on the ground and birds in the tropics generally suffer a high nest predation rate. A smaller number of nests were destroyed by fire. Patchy, slow burning fires during the nesting season may lower the likelihood of nests being burnt and could increase the likelihood of chicks/juveniles being able to move away from fires.

Using the research

The results of this research have been used to trial a Habitat Suitability Index model for partridge pigeons during the nesting in Kakadu National Park. Park rangers and Traditional Owners have been involved in setting up and testing this model. Funding for this management model is being provided by the Key Centre for Tropical Wildlife Management, NTU.

An information sheet for visitors to Kakadu has been compiled and will be available in the park shortly. The sheet describes the ecology of the partridge pigeon and findings of this study.