Friday, 11 March 2011
I Don't Just Do Crossbills !
As many readers may know House Sparrow numbers are in sharp decline and the species is now Red listed. In my own garden in central Aberdeen I used to get up to about 60 at once but here too the numbers seem to have decreased in the last few years, and there is maybe a population of around 40 to maybe 60 in the area. Last year I know that they had several broods so juvenile survival must be very poor, either that or the birds are dispersing locally. I have ringed about 90 House Sparrows (with little effort) and have so far only had two retraps, and reckon at the moment there are only two or three metal ringed birds visiting.
So, I have just started a RAS project organized and sanctioned through the BTO to study my local House Sparrow population - RAS stands for ' retrapping adults for survival'. By colour-ringing the birds and reading the combinations in the field it is possible to get 're-traps' without physically re-catching the birds. By ringing enough birds and by getting as many re-sightings of individuals as possible we can deduce the mortality within the population. Over and above this I am going to colour ring as many of the newly fledged juvenile birds as possible to try and work out some numbers for juvenile mortality (and the time scale of this), as well as measuring any localised dispersal. The presumption is that the Sparrows will continue to visit the feeders so should be seen again (if they are not predated or die from disease). Dispersal can be measured by checking feeders and Sparrow perching sites in the neighbourhood. Disease may also be a significant factor in mortality- I rescued a male House Sparrow in December last year suffering from avian botulism, but sadly this bird expired ( horrible stuff, I hope none of you have to deal with that). I should add that this bird was a mile from my house so not my feeding regime causing the problem.
The bird in the photo is an adult male A01, now getting the distictive black bill of the breeding season. I am using Interrex acrylic rings designed and manufactured in Poland. The rings are sealed with cement to safely secure them and to prevent removal and are fitted so they can't extend below on to the foot or above the tarsus 'knee' joint. They are easily read with binoculars on perching birds ( when facing the right way !) or can be image grabbed from digital photo sequences.
I know many birders don't rate Sparrow but I can't understand this - they are very 'happy' social finches, the males striking when in breeding plumage and I for one can't imagine the dull silence that would prevail if there were no House Sparrows chirping in my garden. Let's hope the streets and fields don't fall silent.