New Research into Songbird Decline
Although it is widely accepted that, in some situations, predators of nests, chicks and full grown birds do affect the abundance of avian prey species, until now the evidence that such effects are widespread amongst songbirds has been weak, having been based on a relatively small number of studies.
In this recently published paper, the authors look at how prey and predator numbers have changed over nearly forty years, to see if there are associations between trends for particular prey species and their predators. This is the most sophisticated, long-term and large-scale analysis of its kind ever undertaken. It examines the effects of three avian predators of juvenile and adult birds (Buzzard, Sparrowhawk and Kestrel) and of four avian and one mammalian nest predators (Great Spotted Woodpecker, Magpie, Jay, Carrion Crow and Grey Squirrel). Grey Squirrel data were only available for the period 1995-2005.
This robust study found that:
- For 22 of the 29 potential prey species examined there was no statistically significant link between the increase of predator numbers and the decline of prey numbers. Thus, for the majority of prey species examined, the study provides no evidence that population changes have resulted from changes in predator numbers.
- Amongst the seven species in which there were significant negative effects of particular predators, the relationships that are most worthy of further investigation are associations between the increase in the number of Sparrowhawks during the period 1967- 2000 and declines in the abundances of Bullfinches, Tree Sparrows and Reed Buntings. These associations may help to identify priorities for future work on the effects of predation on songbird populations, in as much as they relate to the suite of prey species considered here.
- Across the suite of prey species covered, predator effects were negative for three species; Sparrowhawk, Kestrel and Grey Squirrel. This suggests that studies of predation by Sparrowhawks, Kestrels and perhaps also Grey Squirrel should be priorities for future work.
- There were a large number of positive associations between predators and prey, suggesting that predator numbers have largely increased as the amount of prey has increased. This is particularly the case for native avian nest predators (Great Spotted Woodpecker, Magpie, Jay and Carrion Crow). Although this largely exonerates these predators, as driving declines in the numbers of songbird species at a national level, it does not preclude individual predators having local effects.
Dr Stuart Newson (Lead author, Senior Research Ecologist at the British Trust for Ornithology) said: “At the heart of this piece of work, is a shared concern about the decline of species such as Bullfinch and Yellowhammer. For the majority of the songbird species we examined, there was no evidence that increases in common avian predators and Grey Squirrels are associated with large-scale population declines. However, by looking at changes in predators and prey at a very large number of sites, we have identified some predator/prey relationships that may be usefully studied further. This research relies upon a huge amount of information that has been collected by thousands of BTO volunteers over a period of forty years.”
Clive Sherwood, Chairman of SongBird Survival said: “This is the first major project that we have funded and we are pleased to have worked on it with the BTO. It will take the debate forward and clarifies some priorities for future research.”
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