Interesting thread, Birdwatcher. Thanks for starting.
Do you have any experiences with the dates of the past years when migration starts/started. Are there years when the bird start to migrate earlier or later in the U.S. and Canada?
The general trend is earlier, at least in the spring. The time of return follows a bell curve, especially in songbirds where timing and direction of migration appears to be mostly genetic in origin.
Come back too early and you prob’ly die, and get removed from the gene pool. Come back too late and the best territories are taken and/or your young don’t have time to mature and put on enough fat for their own fall migration so the genes die with their young also.
Always a crap shoot, but always enough variation that it happens every year and a subset of the population is doomed to fail. In a warming climate those that get here somewhat earlier have an advantage and leave the most offspring which likewise tend to return early. Climate cools and the opposite happens.
So it’s a self-adjusting system, regulated by the death of the losers.
