Study: For southbound Ruby-throated Hummingbirds, experience really matters

3/9/2016 | 0

Ruby-throated Hummingbird by Lora Render.

Ruby-throated Hummingbird by Lora Render.

Considering that Ruby-throated Hummingbird is a familiar garden bird across eastern North America, eagerly awaited each spring and doted upon (and photographed) all summer, you’d be forgiven if you assumed that there were few unanswered questions about the bird’s migration and stopover ecology. Yet, surprisingly, little is known.

We know that Ruby-throats spend the winter along the Gulf and southern Atlantic coasts and at the tip of Florida, and we know that most overwinter in Central America, but it’s not clear which route they follow to get there each fall — that is, whether they fly around or over the Gulf of Mexico.

Now, thanks to research just published online in The Auk: Ornithological Advances, the journal of the American Ornithologists’ Union, we know they are more than capable of making the 1,000 km nonstop flight across the Gulf.

Ruby-throated Hummingbird. Photo by B. Dossman.

Ruby-throated Hummingbird. Photo by B. Dossman.

According to the researchers — doctoral candidate Theodore J. Zenzal and Distinguished Professor Frank R. Moore, of the Department of Biological Sciences at the University of Southern Mississippi — the tiny birds can fly 2,261 kilometers (more than 1,400 miles) without a break.

Zenzal and Moore captured and banded 2,729 hummingbirds at Bon Secour NWR, at the entrance to Mobile Bay in southern Alabama, during fall migrations from 2010 to 2014. Then they entered the birds’ masses and wingspans into a computer to estimate flight ranges. Older birds and males, the researchers concluded, were able to travel farther than younger birds and females.

Fall migration peaked on September 25, the researchers found, as Ruby-throats moved through the area over 62 days between late August and late October. Older hummingbirds arrived earlier than younger birds, and older birds showed up in better condition. Moreover, Ruby-throats that were leaner upon arrival tended to take on more fuel and remain in the study area longer than birds that arrived with larger fat stores.

Adult hummingbirds arrive early, suggest the authors, because they leave northern breeding areas earlier in the season than younger birds, possibly because males provide no parental care. And even if adults and young birds depart the breeding grounds at the same time, write the researchers, “differential passage may reflect the fact that younger birds travel more slowly than adults (e.g., spend longer at each stopover site, make additional stops en route, or take a less direct migratory route).”

Older birds are also socially dominant and less likely to become disoriented while migrating, write Zenzal and Moore. In addition, more experienced birds may forage more efficiently.

One of the study’s more eye-popping results was that the number of young hummingbirds captured was 28 times greater than the number of old birds. The disparity, speculate the authors, might suggest differences in migration routes or in habitat use along the Gulf coast. “Young birds may also rely on the coast for orientation as a leading line, whereas older birds might take a more direct route.”

The researchers found little evidence of sex-dependent migration during autumn, contrary to the well-known pattern each spring. “During autumn, there may be little pressure for a particular sex to minimize time spent on migration compared to spring,” write Zenzal and Moore, “but we know little about the wintering ecology of Ruby-throated Hummingbirds.” — Chuck Hagner, Editor

Read the paper

Theodore J. Zenzal, Jr., and Frank R. Moore (2016) Stopover Biology of Ruby-throated Hummingbirds (Archilochus colubris) during Autumn Migration. The Auk: Ornithological Advances, Volume 133, pp. 237–50.

See photos of Ruby-throated Hummingbird.

eBird maps: When to look for Ruby-throated Hummingbird each spring.

Study: Ruby-throated Hummingbirds are arriving earlier, migrating slower.

How banders along the Gulf coast are rewriting what we know about hummingbirds in winter.

 

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