Tuesday, 5 April 2016

Bulging Siskins.

The drop in Siskin numbers that I commented on at the end of my last post was short lived and the number visiting the feeders in the garden went back up to around 15 on 2nd April and numbers have remained there or thereabouts since. I managed to get a net up for a short time on the 3rd and 4th and again late this afternoon which resulted in a combined total of 35 different individuals being caught (29 new birds and 6 retraps). This shows there are more around than counts alone would indicate and or that there is plenty of turnover due to birds migrating through the area.

A fairly average looking male Siskin for comparison with the bird below (both caught today).
This is a particularly bright and intensely coloured individual.
The retraps were interesting in that only one had been ringed relatively recently and that bird had been ringed on 23rd March. The other retraps involved 3 that were ringed in February (11th & 24th x 2) and another from early March (5th) but most surprising of all was a bird that had been ringed on Billinge Hill on 9th October last year, a distance of around 2km from the garden, and it was thought to have been migrating south at the time. Did this bird stay and winter in the region or is it returning north along a similar migration route to the one it used to go south in the autumn?

The retraps from February and early March are intriguing as their weights haven't changed all that much and the little change there has been can be attributed to normal diurnal variation rather than any pre-migratory fattening. Age doesn't seem to be a factor as one of these long stayers is an adult male so perhaps they are just going to be late migrants that will fatten up in due course.

Despite some retraps showing little or no weight gain the proportion of heavy birds in catches has been increasing with 8 of the 9 birds caught this afternoon having substantial fat deposits and a weight range of 15.1g to 16.3g. These very heavy birds are likely to be going back to breeding grounds on the continent rather than in Scotland. The lighter bird from today was one of the retraps referred to above and weighed 13.1g at 18:30 compared to 11.9g at 07:30 when ringed on 5th March. This change of 1.2g fits in with normal diurnal weight changes and had it been caught at the same time on both days it may well have weighed the same, or very nearly so.

 Fat Female Siskin
The lump is the birds crop full of seeds but it is also covered in a layer of fat which extended across the back of the neck.

Blowing back the feathers on the throat reveals the tracheal pit bulging with fat (appears pinky-orange compared to the deep red of the breast muscle).

Blowing back the feathers of the belly also reveals a thick bulging layer of fat.
These heavy birds aren't going to stay around for much longer so it will be interesting to see what happens over the rest of this week. Will numbers finally dwindle or will other birds replace those that leave?

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