Ecology is not a dirty word

This post is from my PhD student Rebecca Peisley, who I co-supervised with Gary Luck. Rebecca submitted her thesis earlier this year. Find the paper here: The role of avian scavengers in the breakdown of carcasses in pastoral landscapes.  Also read Rebecca’s previous post on her other PhD work in apple orchards here.

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Animal carcasses such as deceased livestock, road-kill or culled animals are a fact of life in agricultural landscapes, and can encourage increased and unwanted visits from wild dogs and foxes and also contribute to the spread of disease. Scavenging birds in these landscapes can remove carcasses, and therefore provide an important ecosystem service for farmers.

Perhaps the most well-known examples of carcass removal services in agricultural landscapes, which are worth millions of dollars each year, are those provided by vultures in Europe, Asia and Africa. The removal of carcasses by vultures near human settlements can reduce the number and visitation rates of rabid dogs that would otherwise be attracted to the carcasses, and in so doing, reduce the spread of rabies to the human population.

Unfortunately, vulture populations have suffered severe declines in recent years, and do not occur worldwide. However, other bird groups such as raptors and corvids are also common scavengers and a diverse assemblage of these occur in Australia. Like vultures, the scavenging behaviour of raptors and corvids has the potential to reduce the prevalence of unwanted pests such as red foxes and wild dogs in the landscape, and also reduce the spread of diseases such as blowfly strike. However, the benefits of carcass removal in agroecosystems in Australia are not widely recognised and have not yet been quantified.

Therefore, as part of my PhD research, my aim was to measure the contribution that scavenging birds made to the removal of carcasses in pastoral landscapes in south-eastern Australia. I specifically wanted to find out what bird species scavenged on carcasses in farmland, and how much of the carcass they could remove. I also wanted to know how changes in environmental features of the landscape in which these birds were scavenging influenced the value of the ecosystem service they could provide.

To answer these questions, I set out multiple pairs of rabbit carcasses, each of known weight, across a pastoral landscape for a period of one week. One carcass of each pair was left uncovered and therefore freely accessed by scavenging birds, while the other carcass was covered by a wire cage and could not be scavenged. I then set up motion-sensor cameras to continuously monitor each carcass over the week, in order to identify what and how frequently each bird species visited them. At the end of the week I collected and re-weighed all the carcasses. If scavenging birds were contributing to carcass removal, I would expect the uncovered carcasses to have lost more weight than the caged carcasses. This difference in weight-loss would give me a measure of the ecosystem service being provided by scavenging birds. 

My results confirmed that the rabbit carcasses left open to scavenging birds had a higher percentage of weight loss than the caged carcasses. Photos from the motion-sensor cameras showed that this was mostly due to scavenging by four raptor species. These were the brown goshawk (Accipiter fasciatus), whistling kite (Haliastur sphenurus), little eagle (Hieraaetus morphnoides) and wedge-tailed eagle (Aquila audax). The Australian raven (Corvus coronoides) was also a common scavenger.

(Click on the pics to enlarge)

I also found that when a raptor scavenged a carcass, more of the carcass was removed (16.7% removal in one week) compared with carcasses that were not scavenged by birds (6.6% removal). Therefore, raptors were responsible for removing 10.1% of the carcasses they scavenged.

However, a landscape is not a uniform area. Farms are surrounded by different land-uses ranging from other agroecosystems to native vegetation, and each farm will have its own management practices. Even conditions within a single farm vary, with each paddock differing in tree cover, crop types and grazing animals. Raptors scavenge over large areas: wedge-tailed eagles for example can have a home range over 30 km², and they are likely to encounter many different landscape features in just one flight. Therefore we cannot expect their activity within such a variable system to be uniform either.

I found that scavenging birds tended to find rabbit carcasses more quickly in open areas within the landscape that only had a few isolated large trees (paddock trees), and it took them longer to find a carcass as the amount of vegetation increased. Although this relationship was not statistically significant, it does make sense ecologically. Raptors like to hunt in open areas where visibility is greatest, and all of the species observed in my study nest in large trees. This underlines the importance of paddock trees as perch and nest sites for raptors in agroecosystems, which in turn can increase the waste removal services they provide.

My study is the first of its kind carried out in Australia and has only scratched the surface of possible benefits scavenging birds can provide to farmers. Now that we know raptors are major contributors to carcass removal in pastoral landscapes, maintaining key habitat features for these species, such as large isolated paddock trees, is essential for not only for raptor conservation but also maximising the ecosystem services they can provide.

© Rebecca Peisley