Good topic.
I believe if it were only daylight, we would see more similar behavior from the birds year to year.
As an example, in California, we have been suffering drought conditions, and the previous two seasons, birds were well into the breeding routine by the time the season started. This year with a bit above average water, and all of the creeks, ponds, and springs at normal levels, the birds are just getting into the breeding cycle (from what I have witnessed).
This makes sense, as during drought years, birds would need to breed earlier to get ahead of the ensuing lack of summer water.
And as another example, it seemed to us that birds were done with breeding towards the end of the season, but he had a couple late storms... Right after this stormy weather we noticed a resurgence of breeding behavior... I would guess some hens lost some nests and tried to rebreed.
It is also my understanding that turkey poults are very susceptible to hypothermia following stormy (wet/snowy) weather... So if we are having a lot of spring rain, I would expect to see a bit of a later breeding cycle (someone correct me if I am mistaken please).
Also, I have noticed that birds in higher elevations tend to get into the breeding cycle later than at lower elevations (at the same latitude). This would make me think that temperature plays some role as well.
I would say that the amount of daylight is a key signal in instigating breeding behavior, but that weather and habitat conditions also play important roles.
