Snow may cover the ground where you live, but your birds are likely basking in the warm sunshine of their wintering grounds. They will need to perfectly time their departure to ensure an adequate food supply and favorable environmental conditions when they return to you. If they wait too long, those resources might be depleted, or they might not have enough time to raise their young.
With no weather forecast, how do they decide when to initiate migration? Birds have well-developed internal clocks and evolved to use changes in daylight (photoperiod) as a cue for migration preparations. Other environmental factors at their non-breeding or stopover sites may influence the timing of migration, including an inability to find food needed to fuel migration.
The global climate is changing and while birds are migrating earlier, they may not be able to adapt quickly enough to stay in sync. Researchers are documenting the departure dates from non-breeding grounds, whether birds use staging areas to monitor environmental conditions, how individual birds are adapting, and if that impacts breeding success.