Seasonal occupation of breeding areas

Every bird student has noted the feverish impatience with which certain species push northwards in spring, sometimes advancing so rapidly upon the heels of winter as to perish in great numbers when overtaken by late storms. (Frederick C. Lincoln 1935.)

With the approach of spring, as days lengthen and temperatures rise, migratory birds begin to move from lower to higher latitudes in order to re-occupy their breeding areas. One species after another spreads in wave-like manner towards higher latitudes, progressively re-settling areas vacated since the previous autumn. When individuals reach their home areas, they establish territories and acquire mates in preparation for breeding. After breeding is completed, they withdraw to lower latitudes where they spend the non-breeding period. These large-scale distributional changes enable birds to exploit the surge in fresh food supplies produced each spring and summer at high latitudes, yet avoid the shortages of winter.

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