As climate change worsens, spring temperatures come earlier in the year, cueing tree swallows to build their nests sooner. This leaves baby swallows vulnerable as the inevitable cold snap happens. “It’s getting warmer overall. They’re thinking, OK, it’s a good time to breed, to lay my eggs,” said Lily Twining of the Max Planck Institute for Animal Behavior in Germany. Related: Birds are dying mid-air possibly due to climate crisis effects But late-season cold snaps can still happen.