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the reproductive period of birds. Migratory birds return to their homes for nesting purposes, and settled types move to their nesting grounds. Nesting periods vary in length for various species of birds, and may also vary in the same species as a result of climate and food conditions.
Some birds nest once annually (monocyclic nesting), and others do so several times (polycyclic nesting). The latter is typical of birds dwelling in southern latitudes (with the exception of deserts and high altitudes). A number of changes take place in the organism of birds during nesting. Energy reserves build up in the form of fat, glycogen, and other substances. The dimensions and activity of the sexual organs increase, first as a rule in the male, and then in the female. The pituitary function changes, and in some cases, molting occurs and the so-called courting plumage appears. There are also behavioral changes, such as seasonal migrations, singing, mating calls, and nest building. The beginning of the nesting period is recognized to be the time of mating and of pair forming; the nesting period ends at the moment that the young birds begin to fly and conduct a more or less independent manner of life.
G. P. DEMENT’EV