Some members of the corvidae (the family of birds which includes crows and magpies) have been shown to be predators at the nests of songbirds. Many of these birds have flourished in recent years; the number of magpies in the UK trebled between 1970 and 1990. However it is unclear whether or not predation by this larger corvid population has contributed to the decline in songbird population.
Previous studies looking at national population trends have been unable to establish a clear link between songbird decline and corvid population growth. In studies where the population of corvids, and other predators, have been controlled, the effect on songbird populations has varied. It has been suggested that one reason for this unclear picture is that some individual corvids take many eggs and nestlings from songbird nests whereas other individuals take very few or none. If this is the case then songbirds will only benefit from corvid management if those particular corvids which predate a lot of nests can be accurately identified, and then selectively controlled.
A research project (funded jointly by the University of Exeter and SongBird Survival, in collaboration with the Game and Wildlife Conservation Trust) aims to explore this hypothesis. The project will investigate whether, and how, individual crows differ in their predation behaviour. A key focus will be on the differences between two main groups in the corvid population; territorial and non-territorial individuals. Within crow and magpie populations some birds will form breeding pairs and will establish and defend a territory during the breeding season (territorial birds) where as other birds may form pairs but will not be able to hold a territory (non-territorial birds). We may expect that territorial birds have the highest impact on songbird populations because they frequent and get to know particular areas, watching the nest building or chick provisioning activity of the songbirds, and so can easily find and predate nests. Alternatively, it may be that itinerant birds have the highest impact on songbirds, because they cover larger areas and so encounter more nests, even if they cannot exploit them so efficiently. However, it is likely that there will be factors aside from territorial status that influence predatory behaviour in individuals and the study will aim to explore this as well.
The project will involve tagging and subsequently tracking individual crows, both territorial and non-territorial, to see how they use their habitat and where they find their food. At the same time the local songbird population will also be observed, their nests will be located and monitored using cameras to see how many eggs and nestlings are taken and which species are taking them. Additionally, it is intended that other more advanced technologies, such as stable isotope analysis of feathers, will be used to aid the investigation into the corvids’ diet and foraging behaviour.
This investigation should provide insight into whether territorial and non-territorial birds have differing impacts on songbird nesting success, and whether each group are more or less susceptible to legal control methods. It is hoped that the results of this work will help inform more effective, better targeted, management of corvid populations, thus benefitting Britain’s threatened songbirds. This kind of research is designed to help us better understand why songbirds populations continue to decrease, and is vital if we are to halt, or even reverse, the loss of these birds from our countryside.