Different hormone levels can be measured in the blood in adults and subadults. Adults have much higher hormone levels, making them anxious to get north and breed. The younger subadult birds don't have those high hormone levels and thus will take their time leaving Texas. But it's not always that simple. Sometimes subadult whooping cranes will migrate with sandhill cranes and thus be heading north in March in front of many adult whooping cranes.
Adults can start the migration at vastly different times.
For example, one year we had one breeding whooping crane pair start the migration in early March, and the last breeding pair left Aransas on April 20. That's quite a span of time. You don't have to worry about the cranes that leave late. Those late migrants usually make a more rapid trip to Canada so that all the adult pairs will get up to Canada in time to nest. Next week I should know just how many whooping cranes are left at Aransas.