A study was done a while ago concerning the migration of Brown Trout as to when they would start spawning. The temperature of the water only rose 2° from the time the first fish started moving upstream until the final fish started to move. I found that extremely fascinating. Their temperature sensors were so close to that of each other, but varied enough to protect the species if something happened in a particular stretch of that 200 mile migration. If they all moved at precisely the same time, and something catastrophic happened in that exact location of the river, the entire species could be wiped out.
The same thing applies to hummingbirds. From the time the first Ruby Throated hummingbird starts to migrate north until the last one starts to move, can be just a 1° difference in the tilt of the earth, or just the slightest tweak in their sensory receptors. If they were all created exactly the same, the consistency between them all could prove catastrophic in the same way as the fish migration. That slight adjustment between each one of the birds allows the migration to run approximately one month long, and separates them as much as 1000 miles in distance, of all those going to the same location.
What's also fascinating is that there's a location for each one of them. They can all look nearly identical, but the personalities and differences between these birds allows them to spread across the entire continent.
This adult male Ruby Throated hummingbird is going through an early molt. It can start as early as August and continue through November. He's not the handsomest male in the garden, but during his prime time he's as good looking as the best of them.
N.E. of Edmonton, Alberta. August 2016.