Saturday, 26 May 2012

Weather affecting Blue Tit brood size and timing in 2012


11 eggs - some hidden by feathers 2011

Weather affecting Blue Tit brood size and timing in 2012

Bue Tit Cyanistes caeruleus
Average weight 11 grams
Average Lifespan 3 years
We have certainly noticed at birdboxesonline this year's weather has been a contributing factor to the timing of our Blue Tits nesting. We had eleven Blue Tits in one of our camera boxes last year and all fledged on 17th May 2011, this year they laid 9 eggs of which only six hatched and survived but they have still not fledged at the time of writing this. 26/5/12. On average it seems they are 2 weeks behind last year - certainly in Surrey and Sussex.

9 eggs ( 3 hatched ) in this picture 2012

The birds time their nesting so there are plentiful supplies of green caterpillars about. The yellowness of the males chest bib is an indication of the amount of caterpillars he has eaten - this is due to the carotene pigments in them - the females are more attracted to yellower bib. The caterpillars can be found on many Trees plants and bushes and it is a very common site to see a bird hanging upside down from a twig to forage for them they need them to be plentiful! Each chick can eat over 100 per day - no wonder the female looks tatty and tired after they have fledged!
The average clutch of Blue Tit eggs tend to be about 10 - 12 although I have heard from others as well as my own studies, this year's eggs seem to be lower in numbers. The UK generally had above average weather in March 2012 but went on to have the wettest April in over a century.
How this will effect the population generally is a bit more guess work - 70 - 80% of blue tits die each year, a huge mortality rate and most of that figure is made up of young birds - it's a brutal Darwinian process.. However Blue Tits can live a long time 15 - 20 years in extreme rare cases. So will the population dip slightly or will the amount of breeding pairs remain roughly constant? If they can survive through the first year they have a knack for survival and are more likely to survive to breed again? This paper has noted a higher chance of breeding females being predated and the males possible countering that with feeding rituals. I suspect that this year will make little difference on the population as a whole. However a few more terrible springs may be anotherstory.


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