I got out early that morning, as late July in south Florida can be miserable, but birds were already up and moving. Many July mornings are unremarkable, but this morning I started hearing the fight calls – zrees and zips — of overhead Blue Gray Gnatcatchers and Yellow Warblers. They were already starting to migrate! As the light got better, I could even see some of the tiny Gnatcatchers moving to the south overhead. Their flight was so slow and wind-buffeted that it was hard to believe they could migrate anywhere.
Migration is a chancy, expensive endeavor. Most of our midwest birds have favored it because it gives them areas with less competition during the nesting season. Yes, most of our migrants are actually tropical refugees – species that have moved north to get the freedom of longer summer days and less crowded habitats. Tanagers, orioles, raptors, cuckoos, even wood warblers, come from basically tropical families that often have more species still in the tropics. It’s a great strategy, but the cost is a tough and often-dangerous long-distance trip, twice a year.
– Photo Patrick Carney
Since migration is so taxing, you might expect it to only be a feature of medium and large birds. Only they should have the body reserves and the swift flight capable of making the long journey and avoiding predators. Yet some very small birds manage to do it, often in surprising fashion. Most of us know a bit about warbler migration – they gorge on food in the morning, hide and rest in the afternoon, fly at night – but there are smaller birds than warblers that make these phenomenal journeys. How do tiny birds manage to pack in enough food and evade predators on the long journeys south and back?
Think tiny, and you immediately think of hummingbirds. The vast majority of hummer species stay in the tropics, but a few species, including our familiar Ruby-throated, undertake dramatic migrations. The outlines of Ruby-throated Hummingbird migration are now somewhat clear to us: the birds ‘tank’ up on nectar in the eastern U.S., then make a nearly nonstop direct flight south, along the coast or over the Gulf of Mexico or the western Atlantic to reach Mexico, Central America and the Caribbean Islands (nicely summarized in the video below).
Ruby-throats convert and store a huge amount of nectar as fat, and their speedy, direct flight over the open ocean helps them mostly avoid predators. Land-bound migrants, like the western Rufous Hummingbird, often take elaborate migration routes to coincide with particular blooming flowers in the western U.S. (described in this article). As hummingbirds, they still can outfly most potential predators, but must have access to flowers to keep their energy stores high. But how they remember a complicated land route is still an amazing mystery.
Another startlingly tiny migrant is the Golden-crowned Kinglet. Not only are they migrants, but they follow the classic perching bird migrant strategy of flying at night to avoid predators. Add in the fact that they are temperate migrants, so that their migration is later in the Fall (October and November) and doesn’t go as far south (Ohio and the southern U.S. are their winter destinations), so that they are exposed to colder temperatures, and their migration seems even more extraordinary. What we do know about Kinglets is that their metabolism is unusually high, even by perching bird standards. This allows them to function at cooler temps, but means they are almost always hunting for food during the day. Anyone who has followed these hyperactive little sprites can easily appreciate that fact (a brief summary is at this website). Kinglet migration seems to be burning the candle at both ends – not only do they migrate at night, but they hunt food for most of the day, with only short periods of rest. How can they get by with such little rest? We don’t know.
Yet a different set of tricks is used by Blue-Gray Gnatcatchers. These tiny birds are the sole migrant species in a largely tropical family, and they seem to have devised their own migration playbook. They apparently don’t migrate at night – they’re not heard on night sound monitors, nor do they show up much in birdkill data for tall buildings. Instead, they seem to migrate early in the morning – they are often seen or heard flying overhead then – and they start very early in the year, often in mid-July. With their weak flight, this makes their migration a series of brief hops, which will take them from the upper Midwest south to their wintering grounds along the Gulf Coast. Birds from the Northeast can often accumulate along coastal migrant traps (described at this webpage). They never seem to fly very high or far, which may limit their exposure to predators as they are relatively slow fliers. Like kinglets, they seem to be active nearly all day during migration, probably padding their energy reserves for next morning’s flight. With so many non-migratory relatives, Blue-gray Gnatcatchers seem to have made some astonishing adaptations to reach us here in Ohio.