Scientists Say: Irruption
There’s a word for when we see bird species out of their normal range

Some winters, birders in the United States get to see snowy owls far outside their normal range. These birds have irrupted.
Chuck Homler d/b/a Focus On Wildlife/Wikimedia Commons
Irruption (noun, “ear-RUPT-shun”, verb “to irrupt”)
The word for when a group of organisms suddenly increases in number. This could mean that a population of animals has bred rapidly. It could also mean that a species is being seen where it is not usually found.
In a sentence
There may be a winter irruption of songbirds farther south than usual if seed crops in the north can’t support their numbers.
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Power Words
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irruption (v: to irrupt) A sudden increase in a population. This may mean a larger number of animals in their usual geographical range, or a population observed far away from where they are normally seen.