Why do starlings flock in such large numbers in the
winter? Flocks never seem that big in the summer.
While some starlings do not migrate, many of them migrate from north to
south and back. This normally occurs in large numbers. When you see big
flocks of birds in colder weather, they are probably heading south (late
fall) or north(early spring). Once they reach a warm area they tend to
spread out into smaller groups. If the entire group stayed together it would
be difficult for them to find enough food, water, and shelter for each bird.
If small groups break off and spread out, then food, water, and shelter will
be available to each bird in the small flock. Flocks also help with safety,
as there are many eyes to be watching and with larger groups it provides
heat when they are all together. My guess is that the reason you see such
large flocks in winter compared to summer is that in the summer the smaller
groups have already dispersed.
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Update: June 2012