Fire, grazing and partridge pigeons

Partridge pigeons are one of a large number of tropical seed-eating birds whose abundance and distribution have declined this century. Fiona Fraser, one of the TS-CRC’s PhD students, has been studying the needs and habits of the bird to gain an understanding of why this may be so. Her work will also be used to help understand why other birds in the savannas are facing decline.

Geographic distribution | Signs of decline | Feeding habits | Food shortages? | Focus of research | Pigeon Facts |

Geographic distribution

The partridge pigeon occurs in tropical woodland in the top quarter of the Northern Territory and is distinguished by bright red skin around the eyes. There is a geographically separate sub-species in the Kimberley, which is characterised by yellow skin around the eyes. Serious problems have emerged for these birds, as with all seed-eating birds in the savannas, despite European settlement occurring relatively recently compared to southern parts of Australia (Don Franklin SL, Paradise falters for seed-eating birds , Issue 6). As Don Franklin pointed out, a large number of these birds have problems proportionally more severe than for any other faunal group in the tropical savannas, and more severe than for seed-eating birds in other parts of Australia.

Signs of decline

Clearly something significant is happening in the savannas that adversely affects animals that depend on seed and feed on the ground.

Speculation as to why many of these species may have declined includes changes in the savanna understorey vegetation brought about through grazing or altered fire regimes. Partridge pigeons nest on the ground, and increased nest predation may also have lead to their decline.

In this research, the partridge pigeon is being used as a case study to explore the possible effects of grazing and altered fire regimes on some of the bird’s basic ecological needs. These needs include availability of seed and particular habitats for nesting or roosting.

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

Feeding habits

Partridge pigeons feed upon the seeds of a vast array of annual grasses, perennial grasses, herbs and sedges in the savanna understorey. They consume a very diverse range of seed species (more than 60 seed species found in their diet so far), which easily exceeds the diversity of seeds eaten by the gouldian finch, another, although more famous, declining granivore.

Despite this diverse intake of seeds, partridge pigeons possibly confront a food shortage shortly after the start of the wet season. Following the first big rains, much seed on or just under the soil surface germinates and so is no longer available for consumption. There are, however, some perennial grasses that set seed following these rains, and these could constitute a crucial food supply during this time.

Generally, one possible indication of a seasonal food shortage is that the foraging range for a species increases. To meet their daily food requirements, individuals have to forage further afield. Data collected from radio-tracked individuals in Kakadu and from a nearby grazed property indicate that partridge pigeons use two to four times as much area (during a one-week period) after the start of the wet season compared to the mid dry season.

Food shortages?

These results give preliminary support to the notion of an early wet-season food shortage. It is believed gouldian finches also have to deal with a seed shortage at this time of year and rely on locating patches of seeding perennial grasses. However, given the relatively sedentary disposition of partridge pigeons—the average weekly home range is 19 ha—they don’t have the luxury of flying large distances in search of perennial grass patches as gouldians do.

Instead, they must increase their home range in the same general area they ordinarily inhabit. Except when water runs dry, partridge pigeons tend not to entirely leave their home range in search of resources—possibly due to a general saturation of preferred habitat types in the region.

This suggests they may rely on a patchiness of resources in the understorey savanna at a much smaller spatial scale than do gouldian finches. Traditional Aboriginal burning practices may best maintain this sort of detailed spatial and temporal scaling of the understorey. In turn, this may suggest that the decline of this species could mirror the changing fire regimes associated with Aboriginal depopulation and non-indigenous land-management priorities.

Focus of research

This research is examining how fire regimes and grazing can affect understorey vegetation and seed availability at a relatively fine spatial scale. For example, wet season fires, given their small size and cooler burning temperature, can create a locally patchy understorey. They also tend to remove speargrass from the understorey and favour perennial grasses. Alternatively, a late dry-season fire can create a vast uniform understorey, which appears as an endless sward of speargrass. Speargrass seed, however, is an important component of the partridge pigeon diet over the dry season, so possibly a combination of early dry-season and wet-season burning may best suit partridge pigeons.

Much more analysis is necessary before robust conclusions can be drawn about the range of influences on food availability and their significance. But by looking at species that use the savannas and their resources in different ways, we hope to identify common features that point to appropriate management. Developing an understanding of the ecological requirement of a species such as the partridge pigeon, and how to manage the savanna to satisfy their needs, is an important part of the larger picture of sustaining the tropical savannas.

Partridge pigeons

• walk almost everywhere
• move in small loose groups, constantly cooing to each other
• are capable of short, rapid bursts of flight
• have a loud, rapid wing action: it’s one of the noisiest pigeons in take-off
• they often walk to waterholes in large groups (maybe 30 in a single group) in the dry
• breed mostly in the early dry
• pull together a few strands of grass to make nests
• males and females share nesting duties
• both parents feed the young chicks on a special form of milk produced in the crop