Two Blue tits have been visiting Derek's garden since Christmas. Whether or not they are the pair that nested there last year, is impossible to say. The death-rate among Blue tits is very high. About 70% of adults die each year so it is very likely that at least one of the birds is 'new'. About three-quarters of Blue tits remain paired to the same mate from one year to the next so, if both of last year's birds are still alive, they are, more than likely, still together. Having bred successfully last year a pair will want to return to the same nest-box.
The tits began visiting Derek's box in early April and were soon bringing material for a nest. Blue tit nests are built by the female. This year's one is particularly bulky and untidy, making it difficult for the camera to see deeply into the cup.
The first egg, we think, was laid on the 20th April and followed by seven more, one each day, until the full clutch had been laid. An adult Blue tit weighs about 11 grams. Each egg weighs more than a gram, so that the complete clutch weighs almost as much as the parent. A Blue tit has to eat about her own weight in food each day while she is laying. The male helps out by bringing her juicy insect larvae. It used to be thought that this 'courtship feeding' was just a prelude to mating but now we know that the male's gifts of food are a vital source to extra nutrient for her. While she is still producing eggs, she must feed so often that she has no time to incubate. The development of the chicks within the eggs stops temporarily.
Incubation began on 27th April. Only the female Blue tit incubates. She sits for most of the day and all of the night, keeping the eggs warm as the nestlings develop inside them. Building the nest in a box and lining the cup with hair and feathers means that the eggs are well insulated and she can afford to leave them for short periods. A Blue tit mother may spend 20% or more of her time hunting and feeding on caterpillars.
The eggs hatched on the 11th May. Baby tits are blind, naked and helpless. When they detect the shadow of a parent above them, they open their bills for food. The nest helps them to keep warm, they sleep most of the time and no energy is wasted, a most efficient system. Small birds can't control their body temperatures. The mother has to ensure that they don't chill and so she spends much of her time in the nest. Meanwhile, the male continues to feed her. From now on the pair must work frantically to feed their chicks.
In the early stages of their development, most of the nourishment the chicks receive is converted into body tissue but, as the babies develop they become more active and more of the food resources are converted into energy. A coating of downy feathers helps to keep the growing chick warm and gradually they begin to control their body temperature. The eyes open and the legs become strong enough for them to move about in the nest. Then the outer feathers begin to appear. What appear to be little rows of whitish organ pipes develop on their wings. These are the sheaths out of which the long flight feathers, the bird's equivalent of fingers, will emerge. The big breast muscles with which the birds will flap their wings are now developing rapidly.
Then the babies are become 'imprinted' on their parents. Imprinting is a strange form of learning in which the birds come to know their own species. When they in turn come to breed, this knowledge will ensure that they select other blue tits as partners and not members of another species.
Nesting often end in failure and this year's blue tit nest was almost such a case. Towards the end of the second week, one of the nestlings died. During the third week, three more succumbed, then three more. The last chick survived and, finally, at approximately 7.45pm on Wednesday 30th May 2007, 19 days after hatching, it fledged. It is impossible to know what caused the demise of the others, but the most likely candidate is starvation. The supply of caterpillars, bees and spiders may not have been sufficient to sustain the whole brood. Blue tits are synchronised hatchers; all the chicks emerge at the same time. This means that food is distributed more or less equally among all of the babies. If there is an acute shortage, all of them will be weak and, as almost happened in this case, fail to survive. The loss of the other chicks in our nest may have been timely; it meant that there was enough food to sustain the one successful chick.
Birds which hatch their eggs asynchronously have an advantage in times of scarcity. Their first chick, hatching a few days before the second, gets all the available food until it can eat no more at which point the next chick is fed and, in due course, the third and so on. In this system, cruel as it may seem, the younger chick, or chicks, may die if food is scarce. The older one then eats the body of its sibling so that no resources are wasted.