The fall migration of Ruby-throated Hummingbirds

What’s “wrong” with this picture?

eBird map of Ruby-throated Hummingbirds, August-September, 2010-2019

In this map from eBird, each of the purple squares represents sightings of one or more Ruby-throated Hummingbirds reported during the peak of fall migration (August-September) over the last 10 years. The darker the purple, the more reports of Ruby-throated Hummingbirds from those blocks. Though the U.S. has many more eBird contributors, certain parts of Mexico and Central America, including major population centers and tourist destinations, get at least some birding coverage. That accounts for much of the uneven distribution of purple squares on this map, but there’s something more troubling about it.

According to conventional wisdom, Ruby-throated Hummingbirds cross the Gulf of Mexico in fall migration, heading from the southern coast of the U.S. to the Yucatan Peninsula and then south into Central America (note the purple squares in Costa Rica, a major wintering area for the species). Considering the huge concentrations of southbound Ruby-throateds along the northern and western Gulf Coast in September, resident and visiting birders in the Yucatan Peninsula could hardly miss seeing these trans-Gulf migrants after they make landfall. That is what’s “wrong” with this picture: There are only a handful of light to medium purple squares in the Yucatan region.

Why aren’t there more and darker squares? It’s not that there aren’t enough birders in the Yucatan Peninsula and even out in the Gulf itself in August and September. In the map detail below, the gray, blue-gray, and green-gray squares indicate August-September checklists that didn’t include any Ruby-throated Hummingbirds:

eBird map detail showing no-data checklists between the Mississippi River delta and the Yucatan Peninsula in August and September.

eBird map detail showing locations of August-September checklists with no Ruby-throated Hummingbirds reported; the Mississippi River delta is at the top, the Yucatan Peninsula at the bottom.

There are gray squares covering most of the northern peninsula as well as ones out in the Gulf representing boats and offshore oil and gas platforms, popular resting spots for exhausted migrants. If tens of thousands of Ruby-throated Hummingbirds were crossing from the southern U.S. to the Yucatan Peninsula in August and September, more people should be seeing (and reporting) them along their routes and when they reach land. The inevitable conclusion is that they’re just not there, at least not in numbers big enough to get noticed.

Romance vs. reality

The trans-Gulf migration of Ruby-throated Hummingbirds has a powerful hold on our imaginations. Millions of other birds of dozens of other species make the same journey, but none so tiny and seemingly fragile as the Ruby-throated. The basic facts of this feat—that birds smaller than your thumb can and do cross more than 500 miles of open ocean—have become a matter of hummingbird dogma, leading to the overgeneralization that the trans-Gulf route is equally important in both spring and fall migration (a few people believe that all Ruby-throated Hummingbirds cross the Gulf in both directions). Even many hummingbird researchers have embraced the image of tens of thousands of southbound Ruby-throateds striking out across the Gulf as though it was established scientific fact. It’s not.

Despite hedging on the issue in the first edition of A Field Guide to Hummingbirds of North America, I’ve long been skeptical of the importance of trans-Gulf migration to southbound Ruby-throateds. The main reason is geography. The Yucatan Peninsula may be a convenient launching point for birds headed north from Central America, but this narrow wedge of land would be a tricky target for southbound birds. A migrant whose course is off by just a few degrees east would end up out over the open Caribbean, with few dots of land for rest and refuge. A westward drift could add more than 200 miles to the journey, severely testing the limits of the birds’ endurance and fuel storage.

The other reason is weather. Fall is hurricane season, and storms traversing the Gulf present not only a formidable barrier to southbound migrants but an opportunity for birds traveling around the Gulf to use the storms’ counterclockwise winds to boost flight performance. A study of southbound juvenile Ruby-throated Hummingbirds on the Alabama coast (Zenzal 2016 and 2018; summary) found correlations between departure timing and wind direction, with a significant preference for winds from the east.

The major conclusion of the Alabama study was that juvenile Ruby-throateds use overland routes around the Gulf rather than crossing it. Adults were rarely encountered at the study’s coastal site, leading the researcher to speculate that experienced migrants took off across the Gulf from points farther inland. Given the scarcity of Yucatan sightings on the eBird maps above, it’s far more plausible that the vast majority of adult Ruby-throateds are also taking overland routes, though apparently farther inland than inexperienced juveniles. Both the trans-Gulf spring migration and western circum-Gulf fall migration are visible in this mesmerizing animated eBird map (currently not available for embedding):

Ruby-throated Hummingbird Abundance Animation

There’s more than one way across (and around) the Gulf

This doesn’t mean that Ruby-throated Hummingbirds don’t cross parts of the Gulf in fall migration, just that they may be doing it differently than most people imagine. In a study of migrating birds on and around offshore oil and gas platforms in the northern Gulf of Mexico (PDF; Table 9.3), detections of Ruby-throateds in fall had a distinct western bias, with two platforms off the central and southern Texas coast accounting for 58% of the sightings. The scattering of sightings on eastern platforms near the delta of the Mississippi River may represent a few birds that actually did strike out for the Yucatan Peninsula, but the birds detected further west were at least as likely to have been cutting across the northwestern Gulf on a southwesterly trajectory toward southern Texas and northeastern Mexico.

There’s also a second circum-Gulf route that almost no one talks about. It involves a shorter ocean crossing with the potential for multiple stopovers, and it may be a safer and more expedient route to Central America for Ruby-throateds from the Atlantic region. The southern tip of Florida is only about 475 miles from the nearest point on the Yucatan Peninsula, compared to over 550 miles from the Mississippi Delta, over 600 miles from Fort Morgan, Alabama, and over 640 miles from Rockport, Texas. Cuba sits between, providing a convenient place for less fit birds to rest and refuel. The first fall sightings of Ruby-throated Hummingbirds in southeastern Mexico are often on the northeastern tip of the Yucatan Peninsula, suggestive of birds that took the eastern circum-Gulf route.

Why it matters

It’s only in the last few years that we finally have enough evidence from enough sources to address the question of whether Ruby-throated Hummingbirds follow similar migratory routes in spring and fall. Citizen science projects such as eBird, research such as the two studies cited above, and re-encounters with banded birds are all contributing to a clearer picture of migration, which in turn helps us to understand how we can help them survive and thrive in an increasingly hostile world. Ensuring a future for Ruby-throated Hummingbirds and millions of other migrants will require international cooperation to preserve and manage critical stopover habitats around the entire Gulf of Mexico. Understanding when and how birds are using the landscapes they encounter in their travels helps us prioritize these conservation efforts where it will do the most good.

References

Russell, R.W. 2005. Interactions between migrating birds and offshore oil and gas platforms in the northern Gulf of Mexico: Final Report. U.S. Dept. of the Interior, Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, LA. OCS Study MMS 2005-009. 348 pp.

Zenzal, T.J. Jr. 2018. Migratory hummingbirds make their own rules: the decision to resume migration along a barrier. Animal Behaviour 137.

Zenzal, T.J. Jr. 2016. Stopover Ecology of Ruby-throated Hummingbirds (Archilochus colubris) During Autumn Migration. Dissertations. 348. https://aquila.usm.edu/dissertations/348

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Interview on BirdCallsRadio

Recently I sat down for a long, fun chat with Mardi Dickinson of BirdCallsRadio. We talked about everything from hummingbirds (of course) and field guides to birding in Arizona and my favorite tropical destinations to making polymer clay jewelry and translating Mayan glyphs.

The episode is now available for your listening pleasure. Click the photo to go to the episode’s Web page and listen via your browser or subscribe and listen via iTunes.

A cardinal of a different color

A male Northern Cardinal with a rare mutation has become an Internet sensation!

Former shelter kitty Lucky Wilbury, who is currently recovering from a life-threatening bladder blockage.

To commemorate this avian celebrity (and gently rib certain curmudgeons in the birding community), I created this homage to Andy Warhol’s colorful silkscreen portraits of celebrities. It’s now available in my Mountain-Gem Arts store on Zazzle on men’s, women’s, unisex, and kids’ T-shirts, sweatshirts, hoodies, and more in a variety of bright, medium, and dark colors, including many bird-friendly options. A cropped version including the left and center panels is available for your wall and as a 2-inch square button to adorn your Tilley hat or birding vest (or as one of your minimum 15 pieces of flair).

Proceeds from sales of this design (and everything else in my Zazzle shop) will help defray the cost of recent lifesaving veterinary treatment for my indoor-only rescue kitty, Lucky Wilbury.

 

Hummingbirds of Arizona Tour

This August I’ll be co-leading the semiannual Hummingbirds of Arizona Tour for the Southeastern Arizona Bird Observatory. The 7-day, 6-night itinerary starts and ends in Tucson and covers the top hummingbird destinations in this hummingbird-rich corner of the Southwest: Madera Canyon, Patagonia, the San Pedro River, the Huachuca Mountains, and Cave Creek Canyon. Our featured lodging will be Casa de San Pedro Bed & Breakfast, a beautiful and extremely comfortable inn adjacent to the San Pedro Riparian National Conservation Area that is the location of one of SABO’s two hummingbird banding stations.

Up to 15 hummingbird species are possible in mid-August, including southwestern specialties such as Lucifer, Magnificent, and Violet-crowned; southbound Calliope, Rufous, and Allen’s; and, with luck, irregular wanderers from Mexico such as White-eared and Plain-capped Starthroat. Though hummingbirds will be the focus of this tour, we won’t neglect the songbirds, birds of prey, butterflies, wildflowers, and other natural treasures that make this corner of Arizona such a popular destination for birders and naturalists of every stripe. Our field trips will cover a wide range of habitats, from the cactus forests of the Sonoran Desert to the cool pine-fir forests atop our “sky island” mountains.

My co-leader will be my husband and colleague Tom Wood (right), founder and director of SABO, and we’re looking forward to showing a small group of hummingbird admirers around our favorite birding destinations while sharing some of what we’ve learned about hummingbird identification, behavior, ecology, and conservation.

If you’re saying to yourself, “Arizona in August? Is she insane??” hear me out. August is the lushest, greenest month of the year in southeastern Arizona. Monsoon thunderstorms that begin in early July create a “second spring,” bringing the deserts and canyons to life with birds, butterflies, and wildflowers. It’s also the peak of hummingbird migration, when maximum numbers and species diversity are present.

The tour is August 13-19 (Sunday-Saturday) and follows the Southeast Arizona Birding Festival in Tucson. The limit is 8 participants, so reserve your spot now! For more details and/or to make a reservation, please visit the tour page at SABO’s Web site.

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One hummingbird species, two maps

Hummingbirds.net Ruby-throated map Mar 2012

The migration map from hummingbirds.net through March 2012.

eBird Ruby-throated map Mar 2012

The same time period on the eBird map.


The best way to follow the spring migration of Ruby-throated Hummingbirds is the map at hummingbirds.net, but if you also follow eBird you might notice that sightings posted there tend to lag a bit behind. This annual discrepancy became a full-blown controversy in 2012, when sightings reported to hummingbirds.net galloped far ahead not only of eBird but of any similar period since 1996, the year hummingbirds.net creator Lanny Chambers began tracking Ruby-throated spring migration.

-w-250The reasons behind the 2012 discrepancy had more to do with different birding styles than with that spring’s odd weather or the birds themselves.  To clear the air and build a bridge between the mainstream birding and hummingbird specialist communities, I wrote an analysis for the May 2014 issue of the American Birding Association‘s Birder’s Guide to Conservation and Community. Whether you’re not yet a member of ABA or missed that issue of the Birder’s Guide, you can read “Parallel Universes” for free right here (page 46).

Thanks to Birder’s Guide editor Michael Retter and all the hardworking folks who make ABA such a great organization, and to Lanny Chambers and his dedicated network of hummingbird watchers.

Join me in Trinidad & Tobago!

I’m thrilled to have an opportunity to co-host (with my husband and colleague Tom Wood) a tour of Trinidad & Tobago June 18-26, sponsored by the Southeastern Arizona Bird Observatory and arranged by Caligo Ventures. This island nation off the Caribbean coast of South America is home to over 400 species of birds, including 19+ species of hummingbirds, as well as a variety of other tropical wildlife from dainty butterflies to gigantic Leatherback Sea Turtles. Here’s a short introduction to the world-famous Asa Wright Nature Center, where the group will stay for five nights:

I’ve wanted to visit Trinidad and Tobago for over 30 years, ever since I read David Snow‘s studies of the White-bearded Manakin (Manacus manacus) at Asa Wright Nature Centre after returning from my first trip to Belize. I had spent many hours studying the previously undocumented courtship behavior of the closely related White-collared Manakin (Manacus candei), and Snow’s landmark work helped me understand what I had observed.

There are still spaces available on this tour for a few enthusiastic nature lovers (the limit is 10 participants). For more information, visit SABO’s Trinidad & Tobago info page or contact Caligo Ventures at 800-426-7781 or by e-mail.