THE EGG, AND THE YOUNG BIRD

Our birds have now completed a definite stage in their breeding cycle. The perils of the migratory journey have been avoided or overcome ; successful pairing and mating has taken place ; and a nest stands completed and awaiting the deposition by the hen bird of the first egg of the clutch. We have seen how the external environment of the birds has, mainly through visual stimili, brought their bodies towards the correct state for breeding. As this state approaches, the rudimentary eggs within the ovary begin to develop, and they steadily enlarge and commence to absorb yolk. Development continues and the rudimentary eggs pass towards and into the oviduct where they are finally fertilised by the male sperms. Thereafter processes along the oviduct deposit the albumin (egg-white), a soft skin of keratin (which is the same substance as feathers are composed of), and finally the hard chalky shell, which is white or pigmented according to the species of bird. The egg is laid by the hen bird broad-end first. Normally a whole string of rudimentary eggs is produced in the hen bird and there are usually many more to be found in the oviduct than are represented by the normal clutch of the bird. The interesting question then arises, " Why is it that there is a normal number of eggs in the clutch of any one species, which it is unusual for a bird of that species to exceed ? " The actual number of eggs in the clutch of different species varies enormously, from the single egg of the guillemot, laid on the bare ledge of a sea-cliff, to the fifteen to twenty laid by the partridge. It is possible that the number of eggs in the clutch is correlated with the mortality rate of the species, since a subtle balance must necessarily be set up if the species is to survive at a given strength in numbers, but such adjustment can only take place over periods of evolutionary time, and the idea that game birds have large clutches because their mortality rate is high from artificial causes due to shooting, represents far too narrow an attitude. What has been definitely proved however, is that birds tend to have bigger clutches in the northern part of their breeding range than in the southern part. This is very probably due to the added hours of daylight in higher latitudes, which give the adult bird a longer time in which to search for food for the young, and thus increase somewhat the chances of the successful rearing of a slightly larger brood. We have remarked that normally, when the hen bird has completed her clutch, she stops laying, and proceeds to incubate the eggs until they are successfully hatched. As we have seen, this does not mean that the hen bird produces within her body just that number of rudimentary eggs which are represented by the normal clutch, and no more. On the contrary, many more eggs are produced within the hen bird's ovary, but they are re-absorbed as soon as the clutch is complete. There must, therefore, be some controlling factor which inhibits egg-laying when the complete clutch has been produced, and once again, the answer probably lies in an inherited visual pattern of an egg-clutch of a given size. If this be so, then consistent removal of an egg from an uncompleted clutch ought to force the female bird to go on laying for a prolonged period, in an effort to make up the appropriate number of eggs demanded by the clutch-pattern. That this actually happens is, of course, well known, since the methods of egg-collectors were bound to demonstrate it time and again. Hundreds of examples exist of cases where birds have laid double or treble the normal number of eggs, when an egg is consistently removed each time, and in a certain case, as many as seventy-one eggs, laid in seventy-three days were recorded. We do not yet know, however, whether it is possible to get all species of birds to go on producing eggs in an effort to complete the egg-clutch, but there is little doubt that the majority of birds will do so. Once again, one has to be on guard against too rigid an attitude, for sometimes, under quite natural conditions and for no apparent reason, a hen bird will lay a clutch which falls short of the normal number.