Birding Eggs and egg-laying
Very few birds nest all the year round, most have a restricted breeding season, which in Britain and the temperate world is in spring and summer. Dr David Lack has shown that breeding is so timed that the young are usually raised when their food is most plentiful. Various external factors which differ with latitude and with the environment control the breeding season. In the higher latitudes of Europe the day length is the most dependable factor. Here Dr Lack has shown that the average clutch size of the Robin and other species is larger in the northern parts of its range than in the southern. This is so that they can take advantage of the longer hours of daylight during the short breeding season. In our temperature zone increasing warmth in spring is apparently one of the factors which brings the birds into breeding condition. An example of this is that resident species like the Song Thrush and Blackbird, breed earlier in mild early springs than in cold late ones.
Natural selection has, broadly speaking, eliminated those birds whose breeding season did not coincide with the most abundant food supply. Local conditions, however, may vary the length of the breeding season from year to year and may also affect the number of broods raised by each pair.
If you are studying the life history of a small population of birds within easy reach of your home, you can find out for yourself when the breeding season occurs and how much it varies by recording the dates on which eggs are laid and by recording fairly simple data on the weather. The Nest Record Scheme of the BTO will provide you with forms to which you can finally transfer your field data: the nest record cards themselves tell you what data you need to collect, not only to record the breeding season but also details of the nest site, habitat and so on. You need to collect quite a lot of records but they will also be of use to the BTO in building up a national picture of the breeding seasons.
One of the key dates in a bird’s life is the laying of the first egg. Whereas in mammals the early stages of the development of the embryo take place in the mother’s womb, it would be inconvenient and very dangerous for a bird to carry the additional weight of five or six young in her body. She would be greatly handicapped as she flew, at risk from predators and if, by some mischance, she was caught and killed, her offspring would die too. By disposing of her eggs into her nest, she remains agile. If the eggs are lost to some predator, she has a chance to escape and produce a repeat clutch.
The essential parts of an egg are the yolk, albumen and shell with a membraneous lining. Just before the eggs leave the oviduct the colour which is provided by two basic pigments – a blue or blue green and a brown olive – is deposited on the shell. Spots are produced by the pigment gland while the egg is stationary in the oviduct, but if the egg is moving streaks are produced. The function of the colouring is chiefly to camouflage the eggs and many which are laid in the open, such as those of the Little Tern and Ringed Plover which are generally stone or dun-coloured with spots, are extremely difficult to see. Many others, such as the Lapwing, Partridge, Skylark and Meadow Pipit, have a buff-brown basic colour. Birds of prey produce a series of warm red-brown eggs which, unfortunately, prove very attractive to egg collectors. Those eggs which are well hidden in holes or in deep nests and do not need camouflage are often pale or even white, and it is thought that these light-coloured eggs might help the parent find them more easily in the dark hole.
- Birding: Bird Nests and Nest Building (birdwatchtips.info)
- Reproductive System of Birds (birdwatchtips.info)