A debate has been raging in the Hampstead Conservation Area about whether small garden birds are suffering from the increase in populations of predatory birds such as sparrow hawks, jays and magpies.
The Frog has some thoughts on this.
The picture is a mixed one. Some species have increased in numbers in Inner London, e.g. long-tailed tits and goldfinches, while others, such as the house sparrow, whose decline is well known, have decreased. The increase in the numbers magpies, carrion crows and sparrow hawks is due to a number of factors.
Sparrow hawk populations in Britain (and across Europe) suffered a catastrophic crash in the 1960s as a result of pesticide poisoning. Since the banning of DDT (and others), they have made an excellent comeback, and are now relatively common, in London and elsewhere. There was for many years a pair breeding in old poplars to the east of Fitzjohns Avenue. Since their main prey is small birds, the raising of a brood of young sparrow hawks must involve the deaths of many such small birds. However, the studies I have read of sparrow hawks' predation (mostly in Woodland), have shown the predominant prey species to be great tits, and blue tits. Anecdotally, blue tits are said to be decreasing a little, but great tits are as common as ever, and one of the commonest birds in Hampstead. Both species have 2-3 broods a year, numbering half a dozen or more, under good conditions, so factors such as heavy rainfall during breeding weeks (which will be fatal to whole broods) will have more impact on population than predation. Moreover, studies of predator-prey populations have usually shown that predator breeding success is determined by the availability of prey, not the other way around.
Jays and Magpies
Populations of magpies have also increased greatly in London in recent decades. Reasons for this are more complex than for sparrow hawks, and centred mostly outside the city. The huge increase in road killed animals in the countryside has made it much easier for young magpies to survive their first winter. Also, magpies are no longer slaughtered by farmers and gamekeepers the way they used to be. As a result of these, and doubtless other factors, magpies are now very common in London, and as major nest predators, probably have an effect on breeding success among small birds. Jays, also important nest predators, have not increased their numbers to the same extent, but I have often seen these, too, raiding nests in Hampstead gardens. However, I belive that grey squirrels, and domestic cats, are more important killers of small birds. Squirrels raid the nests, for both eggs and young birds, and I think everyone who owned a cat has relieved it of few more or less mauled young birds, at the time these are taking their first clumsy flights. Grey squirrels are heavily implicated in the catastrophic population crashes of several woodland bird species- the beautiful hawfinch and the spotted flycatcher to name but two- but these birds have now declined so much, that I think we can regard them as already extinct as breeders in our area. But you asked about magpies and jays. I have seen magpies "working" a hedge for nesting birds. A family party of magpies hops, walks and flies down both sides and on top of the hedge, to flush out nesting small birds, and then they rob the nests. Magpies and jays are members of the crow family (Corvidae), and other members of this family have been implicated in the decline or extinction of a huge range of species worldwide. For example, I have seen studies showing how crows have decimated tortoise populations in the western USA, and sociable plover populations in central Asia. In all these cases, the crow populations have increased due to human influence, (providing road killed animals, garbage, etc.), and these high populations have used their leisure time to harass the local tortoises, plovers etc.
In short, an increase in jay an magpie populations cannot be good news for small birds, but many other factors are also of importance.