The growing season in Michigan is about five weeks ahead of schedule. Many insects are also making very early appearances. That, in a nutshell, is the problem with a stretch of warm weather that comes ahead of the arrival of many migratory birds.
In my previous post, I discussed the migration timing of Neotropical (long-distance) migrants -- those species that winter in the tropics. As I mentioned there, the annual cycle of migratory birds has been choreographed by evolution to provide maximum resources at all the proper times of year. Birds wintering in the tropics take advantage of the resources available there, and fatten up towards the end of our winter. This enables them to make the journey north, back to their breeding areas in North America. As they move north, they take advantage of the insects that are emerging to feed on the new flush of leaves or opening blossoms on trees and other vegetation. A few short weeks later, they rely on soft-bodied insects such as caterpillars to feed their young. In autumn, the bounty of fruit, seeds, and nuts enables southbound migrants -- many species switch from a diet of insects to a diet dominated by fruit -- to gain and maintain the fat they need to make the long journey back to the tropics for the winter.
Favorable weather patterns can facilitate migratory flights, provided birds are on the move. Since the migratory movements of birds are primarily triggered by photo-period (day length), many long-distance migrants have not yet departed the tropics.
These birds are now already very out-of-sync with the insect resources that they need on their return trip to their nesting grounds. The longer it stays warm, the farther out of sync bugs and birds will be. This mismatch will extend into the nesting season, and could have significant impacts on successful reproduction.
However, it's nearly inevitable that we will see a return to cold weather. As reported by the National Weather Service, the average date of last freeze in southeast Michigan is not until late April. And based on past years, most of southeast Michigan will have a hard freeze (28 degrees or lower) sometime during April.
Under this scenario, insects will become inactive, and many will simply die. They may perish before they have an opportunity to reproduce, or their eggs or early life stages will not be able to survive. If cold weather persists, it could also kill tender leaves, cause flowers to drop, or kill their pollinators. Migrants arriving during this period, or trying to reproduce afterwards, may experience a severe food shortage. Short-distance migrants such as robins, which have already started nest building, are likely to lose their first brood. These species typically have two or more broods in a season, and so may be able to raise some young later. But long-distance migrants are usually single-brooded, and have only one chance to nest successfully. If they are in poor condition they may not even attempt to nest.
A cold snap could also prevent plants from setting fruit (whether seeds, berries, or nuts). As noted above, these are essential crops for fall migrants, and a lack of resources on fall migration could cause outright mortality or late arrival on the wintering grounds, and consequently lower quality winter territories. In turn, reduced health in winter has carry-over effects that reach into subsequent seasons.
The birds most at risk are the long-distance migrants, particularly forest species. The Boreal Songbird Initiative has an excellent, referenced page dedicated to the effects of global warming on birds. For an example of research on how mismatches between food requirements and food availability impact long-distance migrants, see this paper: Avian population consequences of climate change are most severe for long-distance migrants in seasonal habitats. While this warm spell is a weather event, it is indicative of the problems associated with a changing climate.
Here in spring 2012, things are already seriously out of whack. At this point, the best we can hope for is a return to much cooler weather, but without hard freezes or measurable snowfall.
Update: Only four days later, we have had a freeze in southeast Michigan. Meteorologist Jeff Masters notes that it probably caused tens of millions of dollars of damage to Michigan fruit crops. Fortunately, it was only a single night, but we have a couple of months yet to go. Masters also lists other significant freezes -- including the one in 2007 which occurred in April. This preceded one of our poorest fall banding seasons, and I also discussed it in my summary of the spring 2008 survey season.