Wednesday, 2 March 2011

Teacher teacher!

It is on those days when the first touch of spring-like warmth can be felt that many of our resident birds are spurred into song. Perhaps the most familiar and noticeable of these early springtime songsters is the Great Tit, whose ringing ‘teacher-teacher’ song can be heard from woodland, hedgerow and garden from February onwards.

With a broad black stripe running down his chest and widening to meet his legs, the male Great tit is arguably the most handsome of our tit species. This stripe plays an important role in Great Tit behaviour. While you and I might be able to distinguish between a male and female (her stripe is narrower and peters out about halfway down the belly), female Great Tits can detect subtle variation in the male stripe, which acts as a badge of social status. A male of high rank will make a better mate, providing access to more resources and so increasing the female’s chances of rearing a brood of chicks. Great Tits invariably have just the one brood a year.

Although the basic ‘teacher-teacher’ structure of the song is familiar to most birdwatchers, there are variations on this theme, with older birds tending to have a more diverse repertoire. One old male was found to use some forty different variations of the ‘teacher-teacher’ theme. The amount of song will be greater on warmer days and will increase as spring takes hold more firmly. Song output declines once the female begins incubating her clutch of eggs. Incubation does not begin until all of the eggs have been laid and the clutch is complete.

Interestingly, the female will begin to roost in the nest cavity from once the first egg has been laid. Although she sits on the nest she will not be generating sufficient heat to start incubation. This only happens once her brood patch develops. This is an area of bare skin, rich in blood supply and hot to the touch, that forms just before the clutch is completed. The combination of the brood patch and shifting patterns of time spent off the nest means that she can regulate the temperature of her clutch, keeping the eggs at 35.4 degrees C.

As if the effort of producing the eggs was not enough, the real work will begin once the eggs hatch and the chicks emerge. Each chick weighs just over a gramme when it emerges from the egg and it will increase to some fifteen times this weight when it is ready to leave the nest. One researcher calculated that the effort of feeding a brood of Great Tits was equivalent to a human parent bringing home more than 100kg of shopping every day for just over two weeks!

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