With so much shipping around the world these days,
ship assistance is 'Natural'. Capture and escape or release isn't. Neither
is deliberate introduction . Even so, in most cases, it is difficult
to positively know the origin of individuals. As with the Canada Goose,
the question is, 'what is the more likely provenance?'. In the case of
the Tree Sparrow, it is a massive leap in range from any direction.
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