Is it normal for a substantial number of bees to die off in the Spring?

As a rookie (my first year), I captured a swarm almost three weeks ago. I placed them in the hive box and all has been well. The bees have been active and building combs. The weather was warm and sunny here in Oregon for the first two weeks with abundant food sources, but the last week has been rainy and chilly (low 40's at night). I see this morning that a substantial number of bees are dead on the floor of my hive. What am I to make of this? Anything I should do (including removing the carcasses)?