From the email I've been receiving, it's obvious that lots of people are concerned about the April 2007 cold snap and its effects on the Ruby-throated migration. I don't have many specific answers about this unusual event, but I do have a few theories and opinions.
The stage was set by an unusually warm early spring, with no significant cold fronts during the entire month of March. As a result, although the arrival of the first migrants on the Gulf coast was normal, some birds made extraordinary speed northward with no bad weather to slow them down. In my opinion, the spring hummingbird migration is constrained by the availability of active insects, not by the blooming of flowers, and there were plenty of bugs for the earliest birds to eat. Freezing temperatures are not a direct factor in hummingbird mortality; healthy Ruby-throated tolerate nights in the teens easily, but freezing weather limits insect activity. Hummers can do without nectar, but they need bugs at least every few days or their nutrition suffers (including their tolerance for cold).
By the first week of April, a few birds had made remarkable progress, particularly into the midwest, which saw unseasonable highs in the 80s. Everyone knew a cold front was long overdue, and it was a nasty one, bringing thunderstorms, hail, sleet, and snow.
Hummingbirds do not migrate in flocks. Each bird has its own internal clock and migration schedule, probably inherited; the migration starts with just a few individuals in early March, builds over several weeks, then tapers off until it's essentially over by June. The dates on the map reflect the earliest birds, not the vast bulk of the population. Since they are spread out both geopraphically and temporally, the species limits its vulnerability to catastrophic conditions, including bad weather. This is also why you may not see any hummingbirds for weeks after the map shows sightings in your area--if a hummer passed through a yard two houses away, you probably wouldn't notice.
So, a very small percentage of the Ruby-throated population had the misfortune to be caught on the wrong side of the cold front. Will they survive? There's little research or precedent to suggest answers, but I would expect most individuals to find enough shelter and food to manage, while some others will not. There's an advantage to being first to arrive--a chance to claim the best breeding territories--but there's always a risk of outrunning the food supply. If climate change is moving toward earlier springs, these risk-taking hummers will be in the forefront of an evolutionary shift to earlier migration schedules. But climate deals with averages, over long periods of time, while weather has immediate and dramatic effects. Climate affects populations; weather affects individuals.
What can you do to help? Don't wait to hang your feeders until after you see hummingbirds. Let the map guide your timing. The standard 1:4 feeder syrup won't start to freeze unless nights drop below about 27 degrees Fahrenheit, and even a slushy feeder is better than none at all. You don't need to worry about insects or shelter, because hummers are adept at finding both on their own.
It's heartwrenching to think of hummingbirds dying from the cold, but remember only a small portion of the population is affected, and they're tougher and more resourceful than you might think. I'm more concerned about what's happened to the wildflowers on which the rest of the species will depend as they head north. The loss of flower resources might have a much harsher impact overall than the direct effects of cold weather on the leading edge of the migration. Remember that many hummingbirds never use feeders, so try to replace the freeze-damaged flowers in your garden as soon as you can.