i catch most called in swarms 2nd to 3rd week of April...about 2 weeks before Tulip Poplar pops open. For my hives I have seen swarm cells as early as late February but like drobbins mentioned, probably will be later this year. I guess I usually see swarm cells appear around first week of April but I generally split in late March if the hive looks strong enough and I have drones starting to fly.