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What's up with the Spring 2012 ruby-throated migration?


What a strange spring! When the early reports started coming in, and from farther and farther north, I was skeptical. I get a few prank reports from time to time, and I just ignore them. But I couldn't imagine a hoax on this scale. When I followed up asking for more information, the responses were quite credible, sometimes with good photos. Something unusual was happening. Here's my theory:

Ruby-throated hummingbirds wintering in Central America have no idea what the weather is like north of the Gulf of Mexico; they head north in response to the increasing photoperiod (days getting longer) as the vernal equinox approaches.The first northbound ruby-throated arrived on the Gulf coast exactly when they always do, in the last week of February. Normally, March in the eastern U.S. has a cold front every few days, which typically convinces migrating hummingbirds to stop and wait out the headwinds, forcing the migration to a slower, stop-and-start pace.

This year, an unusual jet stream flow produced several weeks of strong south winds with no cold fronts, so the earliest hummers found no barriers to their progress, and some of them raced north. There are evolutionary advantages to being first to grab a territory, but the risks are huge and mortal. I suspect birds with the genetic instructions to push north as fast as they can are a very small minority, and in normal years don't get very far before perishing from freezing weather and lack of food. Since these unlucky individuals fall out of the gene pool, I'm guessing they tend to be young males on their first northward trip. There's little data to support that, though.

This year, the early birds hit the jackpot. Even if they encounter an early-spring snowstorm, the cold likely won't be deep or last long, within the survivability envelope for most individuals. This is how evolution takes advantage of new opportunities as they occur, even when they're only temporary. This is weather, not climate change. Time will tell whether it's a trend, or more like a 100-year flood.

The bulk of the population seemed to be taking a safer, more conservative approach, because that's what their genetic inheritance dictates. I'd expect most hummingbirds to arrive on their breeding sites around their normal dates, and not participate in March's mad dash northward. In fact, the sighting reports support this: the total number of reports is no more than usual for the date, and most of them are coming from the same places I would expect at that date in a typical year. By late April the migration seemed almost normal, perhaps a few days ahead of an average year. (All data are valuable, even if they don't make it to the map.)

My first hummer appeared on April 14, one day earlier than my previous earliest date. By April 27 I'd captured four birds to band, all males. However, two of the four were returnees that I'd already banded in August 2011 as immatures. What was I saying about young males on their first trip?


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