Geese

I don't have an image or a sound recording for this, but one of the big excitements of the season is taking place daily now: when I walk Biscuit in the morning we see and hear formations of geese heading north. This is a moving, even thrilling, thing to experience.  I used the pronoun "we" deliberately in that first sentence: Biscuit will look up into the sky and watch as the geese pass. The group we saw this morning easily numbered over one hundred geese.

I've always assumed that these birds respond to the changing day length rather than to temperature: is that true?