To date, this year we have caught 24 adult Bearded Tits at Leighton Moss . Of the 24, 14 are males and 10 Females. Of the 14 males 5 are 3 years old or older but the oldest female is only in its third year. This is obviously a small sample so I looked at our records since 2000
In total up to the end of 2008 we had caught 1395 juveniles of which 723 were males and 672 females. Of these 298 males (41%) had been caught or sighted as adults. By contrast of the 672 females only 171 ( 26%) were caught or sighted as adults. So it appears as though males survive better in the first year.
I then looked at the numbers of each sex as to how long they live and the table below shows the results.
7 + years 1 Male
6+ years 2 Males
5+ years 5 males 2 female
4+ years 6 males 4 females
3+ years 18 males 9 females
Of course these results need testing statistically, but it does appears that males survive better than females.
John wilson
1 comment:
There's quite a lot of literature about differential survival rates among birds. But females usually disperse further than males, so you may simply be losing more females to the outside world and not getting many immigrants back in. But, generally, male passerines do live longer than females. That's because males dominate females when food is tight, and also because females get nobbled on the nests by weasels etc. But there can also be issues of catchability that can't easily be ignored. So simple % calculations are of limited use and a survival program such as MARK is needed to work out more robust figures that takes these variables into account.
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