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.

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.

Are there studies on how red dyes affect hummingbirds?

Feeder solution colored with cherry juice concentrate

Hummingbirds don’t need a colored solution, but natural coloring from fruits and vegetables (in this case, tart cherry juice concentrate) is a far safer alternative to the petroleum-based dyes found in liquid food coloring and commercial “instant nectar” products.

Primum non nocere. [First, do no harm.]
—Thomas Sydenham, “The English Hippocrates” (1624–1689)

A recent visitor to the Field Guide to Hummingbirds Facebook page asked a question that comes up frequently in discussions of the use of petroleum-based food dyes in hummingbird feeder solutions:

Have any scientific studies been conducted to determine the effects of these chemicals on hummingbirds?

Some people are surprised to learn that the answer is an emphatic “NO.” Despite oft-repeated (and oft-debunked) urban legends that the San Diego Zoo, Audubon Society, or some other trustworthy source tested red dye on hummingbirds and found one or more specific effects (liver damage, kidney damage, cancer, tumors, “birth defects,” weakened eggshells, or, in some versions of the story, no harm at all), there is no evidence that any such testing has ever been conducted on hummingbirds by anyone anywhere.

Considering the challenges of such a study, direct testing of dyes on hummingbirds is not likely to happen:

  • Hummingbirds, like most wild birds, are protected by the Migratory Bird Treaty Act and various state laws, and animal research is subject to its own set of federal, state, and institutional regulations, so the first hurdle would be to get all the necessary permits and permissions to capture wild hummingbirds, confine them at a research facility, and perform lethal testing on them.
  • Getting the permits would require the researchers to prove that they could provide proper housing and care for the duration of the study. Hummingbirds are difficult and expensive to house and feed, and it could cost many thousands of dollars to keep dozens of them alive and otherwise healthy for the duration of testing.
  • The size and composition of the study population is very important. If the number of birds is too small, the results may not be clear, or critics may dismiss them as not statistically significant. If only one species is tested, critics may argue that the results may not apply to other hummingbirds. If only males are tested, critics may contend that the dye could affect females differently or not at all and therefore have little effect on populations.
  • To document the effects of dye consumption on the birds, the researchers would have to “sacrifice” and dissect them. It would be difficult to find qualified researchers interested enough in hummingbirds to perform such a study, qualified to undertake or supervise daily care of dozens of captives, and willing to kill them all to get the data. Imagine, too, how the hummingbird-loving public would react to learning that dozens of hummingbirds had been taken from the wild, dosed with dyes, killed, and dissected, even if the results might help to protect the health and safety of millions of other hummingbirds.

“Skeptics” who insist that such studies are required to “prove” that petroleum-based artificial dyes harm hummingbirds are setting the bar far higher than we do for our own health. No one in their right mind would suggest that we evaluate the safety of food additives, drugs, etc. by exposing human subjects to potentially harmful doses. Instead, we rely on the results of testing on lab animals and cell cultures to indicate whether and how various chemicals may affect human health.

Just because artificial dyes have never been tested on hummingbirds doesn’t mean they’ve never been tested. In fact, there are plenty of published studies on the effects of FD&C Red No. 40 and FD&C Red No. 3 to help us make a compelling case against exposing hummingbirds to high doses of these dyes, without taking the life of a single bird.* The real challenges are to increase awareness of this evidence among backyard enthusiasts and persuade manufacturers of commercial “instant nectar” products to use safer alternatives.

Unfortunately, the people who make money selling “instant nectar” products have proven extremely resistant to pressure from both hummingbird experts and the general public, and there are no regulations governing the use of human-approved food additives in foods intended for wild animals. Too many companies put profits ahead of everything, including animals’ lives and federal laws, but persistent, science-based outreach promoting responsible hummingbird feeding will help to shrink the market for artificially dyed “nectar” products and protect the health of the birds we love.



* One “cruelty-free” alternative for directly assessing the effects of dyes on hummingbird health is to look for dye residues and signs of related genetic/cellular/tissue damage in birds that are either found dead or die after being brought to wildlife rehabilitators. These sad cases have provided valuable insights into the diseases and parasites that afflict hummingbirds (see Hummingbird health: pathogens and disease conditions in the family Trochilidae. Loreto A. Godoy, Lisa A. Tell, Holly B. Ernest, J Ornithol (2014) 155:1-12. 
DOI 10.1007/s10336-013-0990-z).