Hummingbirds: early to return, or not?

A gorgeous male Ruby-throated Hummingbird, Archilochus colubris, held by Bill Hilton. Bill caught this bird on April 28, 2010 in West Virginia at a banding demonstration for attendees of the New River Birding & Nature Festival. Its arrival in West Virginia was right on schedule. Male Ruby-throated Hummingbirds show up in this region around mid to late April, preceding the arrival of the females by eight days or so.

A friend forwarded me this map today, from the website http://hummingbirds.net/ Judging by all of the records on this map, there are plenty of contributors providing their hummingbird sightings, and the map purports to show the phenology, or timing, of hummingbird migration as the birds move north in their seasonal occupation of eastern North America.

But WAIT! I know it’s been an exceptionally warm spring, and some flora and fauna are well advanced beyond what would be the case in a “normal” year, but WOW! The map shows hummingbirds as far north as central New York, Michigan, and Wisconsin! Numerous records are sprinkled throughout the Upper Midwest, including Ohio. The map invites incredulity.

I have not heard of a single credible Ruby-throated Hummingbird report from Ohio as yet, and I try to keep my ear to the wall. The first Ruby-throats appear in the Buckeye State in the third week of April (exceptionally early records from around April 10), but they don’t become widespread and frequent until early May. That timing holds true for other states at this latitude, and it takes a bit longer for the birds to reach more northerly haunts.

While I have issues with the map above, I do want to say that the hummingbirds.net site is quite well done and does feature lots of good hummingbird information. I suspect that the records that comprise the hummingbird migration map are accepted from contributors at face value and plotted with no attempt to verify the record. I don’t want to come off as some sort of hummingbird scrooge, but extraordinary claims that contradict what it is known about the biology of a well-studied species should have solid evidence as backing.

This map is courtesy of eBird, and reflects Ruby-throated Hummingbird data that has been submitted as of today. The darker the purple, the greater the number of individuals that were reported. The eBird map shows just about what we would expect to see at this time – Ruby-throated Hummingbirds just beginning their spring infiltration of the United States, with nearly all reports from the Gulf States or lower Atlantic Coastal Plain region. Most Ruby-throated Hummingbirds winter in southern Mexico and Central America, and make the 500+ mile open water crossing of the Gulf of Mexico on their northward journey.

So why the huge disparity between the eBird map and the hummingbirds.net map (keeping in mind that eBird probably has far more contributors)? Well, Project eBird is quite diligent about reviewing data before publishing it. The project employs a small army of expert birders who vet unusual sightings – such as a mid-March hummingbird in the Upper Midwest – and if details are insufficient, the sighting does not appear in the literature. In short, eBird data is well reviewed and scientifically credible.

Documentation of the arrival of Ruby-throated Hummingbirds in spring in North America, and their subsequent progression northward, has been well described over a long period of time. The hummingbirds.net map has them arriving in many places a month or more ahead of schedule. Fact, or fiction? And if the data is fiction, what could account for such a spate of misidentifications?

Birds are not normally confused with insects, but in the case of certain big sphinx moths, it happens quite frequently. This is a white-lined sphinx, Hyles lineata, which appears astonishingly hummingbirdlike. Some refer to the so-called hummingbird moths as “hummingbird mimics”, but their resemblance may actually be an example of convergent evolution.

Photo courtesy Wikimedia Commons

An amazing photo of a snowberry clearwing, Hemaris diffinis. The sphinx moths in the genus Hemaris resemble hummingbirds to an incredible degree. Many a person has been fooled by these day-flying moths, and upon seeing one, some might swear that a hummer just shot by.

Photo: Wikimedia Commons

This is a hummingbird clearwing, Hemaris thysbe, and it is probably the champion of hummingbird lookalikes. These moths hover before flowers, extracting nectar with their long tongues. Their wings beat so rapidly as to be just a blur, and they aren’t much smaller than a Ruby-throated Hummingbird. To top off the illusion of a hummer, Hemaris moths even produce an audible buzzing with their wings, as do the birds.

Both the hummingbird clearwing and snowberry clearwing are common, widely distributed species throughout eastern North America, as are a few other sphinx moth species that resemble hummingbirds, albeit to a lesser degree. If one of these moths buzzes by, and good looks are not had, and the observer is not all that experienced, they can and do get reported as hummingbirds.

Members of the honeysuckle family, such as this arrowwood viburnum, Viburnum dentatum, are the host plants for Hemaris hummingbird moths.

This is the caterpillar of a snowberry clearwing. The adult moths lay eggs on suitable honeysuckle family hosts, which eventually hatch these cool-looking caterpillars. The cats then feed and grow, molting through several instars before reaching maturity.

Photo: Patrick Coin

Once a hummingbird moth caterpillar is fully mature, it drops from its host plant into the leaf litter below. Ensconced in the leaves, it transforms to its pupal stage, as seen above. The Hemaris hummingbird moths pass the winter in the pupal stage, and warming spring weather triggers their transformation from this penultimate stage to the beautiful winged adults.

Ruby-throated Hummingbirds should not be influenced to arrive in the United States any earlier than normal because of our unseasonably warm weather. Most of the hummers are 1,500 - 2,000 miles away – how are they going to know that it’s been balmy and spring is ahead of schedule up here? The impulse for Neotropical birds to begin migration is triggered by photoperiod - changes in daylight length - and possibly other environmental cues.

On the other hand, moth pupae overwintering in leaf litter could, and probably would, mature earlier than normal in the presence of unusually warm temperatures. Hemaris moths normally emerge in March in the southern U.S., and can be on the wing in April in the northern U.S. The well above average temperatures of late winter and early spring have likely prodded the moths to rise early from their winter slumber, and eager hummingbird seekers may be mistaking them for their wished-for feathered harbingers-of-spring.

At least that’s one theory to explain the hummingbird.net map. Another might be that most or all of those hyper-early hummingbird reports pertain to vagrant western species, such as Rufous Hummingbirds, which do have a penchant for appearing outside the windows of normal Ruby-throated Hummingbird passages. An ever-increasing number of non-Ruby-throated Hummingbirds winter in the Gulf coastal states. I can’t buy that, though – there is no precedent for a sweeping vernal invasion of scores of non-Ruby-throated hummingbirds in the eastern U.S.

Yet a third theory might be that these reports represent Ruby-throated Hummingbirds that overwintered in the southernmost U.S., as a small number of birds do. Perhaps these hummers might be compelled to begin migration earlier, as they could possibly somehow be better tuned in to climatic factors in the northern reaches of their breeding range. That would mean that they are either overriding the built-in photoperiod response that stimulates physiological changes and induces migration, or somehow the mild winter altered their brains' interpretation of the normal cues that instigate migratory behavior. As far as I know, there would be no precedent for this.

This is hummingbirds.net’s map from 2011. Everything looks just about spot-on in regards to timing, although I would question the northernmost red dots, which indicate March sightings.

We may indeed set some legitimate early records for returning Ruby-throated Hummingbirds in Ohio and other northern states. The abundance of prematurely early flowering plants may speed them along their way as they advance northward from the Gulf States. But I don’t believe that we’ve had hummers up here yet, and in the absence of concrete evidence, I certainly can’t accept the map at the beginning of this blog. It seems more likely it charts the emergence of clearwing hummingbird moths than the northward sweep of Ruby-throated Hummingbirds.