Invasion of the rough- legs?

PUBLISHED: 10:42 02 November 2007 | UPDATED: 23:08 28 May 2010

Certain species of migrant bird feature more strongly in some autumns than others. How successful a breeding season it has had (influencing the overall numbers of the bird), the weather and food availability are key elements of a species arrival. So far

Certain species of migrant bird feature more strongly in some autumns than others. How successful a breeding season it has had (influencing the overall numbers of the bird), the weather and food availability are key elements of a species' arrival.

So far this autumn, large numbers of siskins have been featuring strongly with hundreds, even thousands, being seen in many parts of the UK. Hopefully, there will be a strong arrival in our part of the world soon and riverside alders and birches will be awash with these pretty birds.

These tiny green, yellow and black finches are woodland birds, but are fond of garden bird feeders, coming to feed on peanuts and a variety of seeds.

I spent most weekends in October on the Norfolk Coast and the sight and sound of small flocks of siskins passing west along the coast was a frequent occurrence wherever I happened to be walking. Siskins call with a gentle 'tsoo' interspersed with a variety of harder twittering notes that they can't seem to get out of their beaks quick enough. It is this call that gives away the passage of birds overhead, so one worth familiarising yourself with.

This autumn's crop of siskins is likely to have originated in Scandinavia. Chances are that food is scarce in their homeland, so there has been an 'irruption' of siskins heading west to our shores in search of a supply of winter food.

Another bird that looks like it may be arriving in above average numbers is the rough-legged buzzard. This is the Arctic tundra breeding version of the common buzzard (a year-round resident in the UK that is becoming a more frequent sight in the Fens with each passing year as they spread east).

'Rough-legs' only arrive here in good numbers (and I'm only talking 2-300 birds) every 10 years or so. This happens when the combination of a good supply of lemmings (principal food for the birds) gives rise to a good breeding season and the adult birds being able to raise several youngsters. Next crucial factor is a shortage of lemmings that forces the birds to cross the North Sea and come to us.

It is a little early to call it an invasion year (the last was in the autumn of 1998), but I have noticed that several have been seen along the east coast.

The last time an invasion happened, three of these superb birds made a rare appearance in Cambridgeshire and spent the winter on Aldreth Fen to delight us local birdwatchers.

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