Monday, 3 May 2010

Songbird predators on trial again...

I suspect this might get wider coverage elsewhere, but it is important for us to be informed as we have contact with many folk. From personal experience I know this is a very popular topic with landowners when checking big boxes. Anyway, have a read - it's from the latest Notts Birdwatchers Newsletter.
Pete

Magpies, Jays & Grey Squirrels
by David Parkin
A question that birders often get asked is whether Magpies are killing all the garden birds. Certainly, there has been a marked increase in predators like Magpies, Jays, Carrion Crows and even Great Spotted  Woodpeckers, all of which are not averse to taking eggs or chicks out of their nests. And then there are Grey Squirrels: they may be cute, but don’t they take nestlings too? And what about Sparrowhawks taking Greenfinches off the bird table? There is no doubt that all of these predators have increased in recent years (perhaps as the number of active game-keepers has declined) at the same time as there have been serious declines in many of our common small birds. Several studies have tried to unravel the relationship between
song birds and these predators. In general, these have suggested that, while Sparrowhawks (for example) certainly do kill a lot of small birds, they do not seem to be directly responsible for the declines in (say) Tree Sparrows or Skylarks, and, despite the loss of nests in our gardens, predators like Jays and squirrels probably do not have a major impact on Blackbirds and Song Thrushes. Cats, and perhaps cars, may be much more significant.
However, there are many who disbelieve these findings, advocating a reduction in the number of raptors, arguing that they are seriously affecting their prey species. One particularly vociferous organisation is Songbird Survival, a charity that actively lobbies for reducing (killing) Sparrowhawks in particular. In an attempt to understand the relationship between songbirds and their predators, Songbird Survival financed a study by the British Trust for Ornithology (BTO) that sought to establish the facts. The BTO is an organisation that gathers
and analyses bird data, usually by harnessing an army of volunteers who go out into the countryside to undertake surveys like the Common Bird Census (CBC), the Breeding Bird Survey (BBS), the Wetland Birds Survey (WeBS), the Nest Record Scheme (NRS), as well as supervising bird-ringing and a host of other activities. Many Notts Birders will be involved in some of these activities. The BTO strives (and I believe succeeds) in being objective: putting it crudely, they get paid at the end of the month whether Sparrowhawks take Greenfinches or not.
Songbird Survival paid for a huge analysis of the relationship between the number of passerines and their predators, using data collected through the CBC and BBS since 1967. If an increase in predators is resulting in an increase in the predation of small birds, we might expect to see correlated changes in the number of predators and prey over time: in years when predator numbers were high, prey numbers should be low, and vice versa. Using data collected by BTO volunteers from England, the scientists examined the relationship
between the number of birds and two kinds of predator: those taking prey from nests (Magpies, crows, Jays, Great Spotted Woodpeckers and squirrels) and those taking adult birds or juvenile after fledging (Sparrowhawks, Kestrels and Buzzards). Using a set of very complex statistics, they looked for associations between the number of each predator and 29 species of actual or potential prey, ranging from Lapwings and Green Woodpeckers to Blue Tits and Yellowhammers. Because CBC and BBS data are gathered in different ways, they analysed the results from the two surveys separately.
The BBS has only been running since 1995, so the data are rather limited, and the number of results that were statistically significant was pretty close to that expected by chance. Of the seven most robust results, there was a small negative association between Grey Squirrel and Greenfinch, and a slightly larger one between Buzzard and Goldfinch. While one might expect that squirrels predate Greenfinch nests, it seems unlikely that there is much interaction between Goldfinches and Buzzards, and this finding must be due to statistical chance. The remaining robust results were all positive, and mostly involved Greenfinches which increased in frequency in common with Jays, Carrion Crows and Great Spotted Woodpeckers, suggesting that these were all responding to some similar environmental effects.
The CBC data extended over 34 years (1967-2000) so provide rather more robust results. Using these data, they found several associations; nine of these were negative where predators and prey moved in opposite directions, and 21 were positive where predators and prey increased or decreased together. The strongest results were a negative relationship between Sparrowhawk and Tree Sparrow, and to a lesser degree Lapwing, Bullfinch and Reed Bunting. Negative associations were also found between Kestrel and each of Lapwing, Starling and Tree Sparrow. However, increasing Sparrowhawk numbers appear to be associated with increasing numbers of Robins and Yellow Wagtails, and to have no effect at all on species like tits,
thrushes and warblers, all of which feature regularly in their diet. When they combined the results across all prey species, they found a negative relationship for both Sparrowhawk and Kestrel, but positive for the others; this result implied that if the predators doubled in number, the number of prey would reduce by about 1% (Kestrel) and 2% (Sparrowhawk). Such changes would be swamped by the effects of altered land use, climate and habitat.
It is, however, interesting that the changes were more pronounced in the CBC data, which come from an earlier time when the environment and its avian populations were changing rapidly. Now that things have stabilised a bit, the BBS data indicate that even these slight predator-induced changes have slowed down or stopped. Overall, it seems that the widespread decline in songbird numbers is not due to predators, but more likely to agricultural and climate change and our own destruction of the natural habitat of lowland England.

Ref:

Newson, S.E., Rexstad, E.A., Baillie, S.R., Buckland, S.T and Aebischer, N.J. 2010. Population change of avian predators and grey squirrels in England: is there any evidence for an impact on avian prey populations? Journal of Applied Ecology.

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