What factor primarily dictates the late summer breeding schedule of the American Goldfinch?

Answer

The maturation of seed-bearing plants like thistle and milkweed

The late breeding schedule, usually June or July, is timed to coincide with the production of fibrous seeds by plants like thistle and milkweed, ensuring abundant food for the young.

What factor primarily dictates the late summer breeding schedule of the American Goldfinch?
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