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Gulls on film: roadkill scavenging by wildlife in urban areas

The impact of roads on wildlife (both directly through wildlife-vehicle collisions, and indirectly due to factors such as habitat fragmentation) has likely increased over time due to expansion of the road network and increased use and number of vehicles. In the UK, for example, there were only 4.2 million vehicles on the roads in 1951, compared to 37.3 million by the end of 2016. With more than a million wild vertebrates estimated to be killed by traffic daily, roadkill is now a higher cause of mortality for vertebrates in the US than hunting. We can learn a lot about animal ecology and behaviour by monitoring these flattened fauna, including their distributions, especially in the case of difficult-to-survey animals such as polecats (Mustela purotius); for example, 51% of records on a recent UK-wide survey were of roadkill animals. Carcasses of roadkill animals are also frequently collected on behalf of schemes such as Cardiff University Otter Project and the Predatory Bird Monitoring Scheme to monitor wildlife disease, and the presence of pollutants such as pesticides and heavy metals.

Monitoring roadkill can allow us to discover more about the impacts of roads on wildlife; roadkill counts can provide us with estimates of the number of animals killed on our roads each year and can allow us to measure the effectiveness of mitigation measures (such as wildlife crossings) that are installed to try and reduce this impact. The observed numbers of roadkill, however, may be an under-estimate because of the actions of scavenging animals that are eating or removing carcasses before they are counted. In our study, we aimed to quantify just what that shortfall was and find out which urban species are doing the scavenging. We wanted to know how quickly “roadkill” was removed, which species were removing it, and whether this differed between city parks and highly urban areas.

We used motion-sensitive camera-traps baited with chicken heads as proxy “roadkill” to attract scavengers. This provided us with a standard bait size (the bait weighed approximately 50g, equivalent to a large vole). Twelve locations around the city of Cardiff, Wales were baited with roadkill and filmed during this study, half of which were “residential” (within 50m of housing), and the other half were parkland. We filmed each site a total of ten times, combining to make a total of 120 filming sessions. The cameras were capable of filming using infra-red light, which allowed us to film both nocturnal and daytime scavengers. Using these remote cameras prevented any unnecessary disturbance to the animals and enabled us to gain exact times at which our scavengers snatched the roadkill.

We wanted to know how quickly “roadkill” was removed, which species were removing it, and whether this differed between city parks and highly urban areas.

Seven different species removed the roadkill; two species of gull; herring gull (Larus argentatus) and lesser black-backed gull (Larus fuscus), carrion crow (Corvus corone), Eurasian magpie (Pica pica), red fox (Vulpes vulpes), domestic dog (Canis familiaris), and domestic cat (Felis catus). Mice (Apodemus sylvaticus), and a brown rat (Rattus norvegicus) were also observed snacking on the roadkill but did not remove it completely.

Of the 120 corpses, 90 (76%) were removed within 12 hours. Birds were the more frequent scavengers, removing 51 of the carcasses, compared to 28 by wild and domestic mammals (in some instances, the scavenger was not caught on camera). Scavengers removed our roadkill remarkably quickly; 62% of carcasses were taken within two hours! The early bird gets the roadkill, it seems; scavenger activity peaked between 7-11 a.m. (sunrise was around 7:40 a.m. during the study), with over half (53%) of roadkill removed between these four hours. Most roadkill could therefore be removed before it could be observed during daytime roadkill surveys. Indeed, we calculated that roadkill could be under-estimated by a factor of up to six times due to the behaviour of scavengers.

The quick removal of roadkill, as well as the variety of species observed feeding on it, shows that many species are behaviourally adapted to scavenge on roadkill in urban environments. Roadkill is clearly an important source of food for scavenging animals, and by removing carcasses from our towns and cities (alongside other human-related food sources), scavengers in our towns and cities are providing valuable ecosystem services. Is it perhaps time to re-think the “pest” reputation of urban wildlife such as gulls, corvids, and foxes?

Featured image credit: Street. Public domain via Pixnio.

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