What triggers the physical seasonal change in animals?

Question

For example, birds molt in spring and fall. Dogs shedding fur, etc. I'm looking for a detailed answer besides "cold weather". Are there other factors such as light exposure, barometric pressure, etc.

Answer 1

I would expect light exposure variance has much to do with it. Look at how it effects circadian rhythm, for instance. Probably begins a cascade of varying hormonal changes.

Answer 2

This isn't very detailed but, it's an internal clock . every thing has one,it tells you when you need sleep!Ya know what I mean?