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WHERE ARE THE HUMMINGBIRDS?
(Part 2)

A month ago we posted herein a treatise dealing with this spring's apparent low numbers of Ruby-throated Hummingbirds (RTHUs) in parts of their range. We described how our total of five RTHUs banded through the end of May 2005 was well short of our 22-year average of almost 17, and was even further below our mean of more than 20 during the past eight years. We also designed an on-line poll that allowed hummingbird watchers in the U.S. and Canada to vote by region on whether their RTHU numbers this spring were "above," "below," or "about the same" as in previous springs. Across North America, and despite the general feeling folks have about lower numbers of RTHUs this spring, our poll showed hummer populations were about the same as normal--until we factored in the South Atlantic states. Virginia (including Washington DC), North Carolina, South Carolina, and Georgia made up the only region that reported significantly fewer RTHUs than in past springs, and they did it with such a resounding vote that it brought down the final tally for all regions combined to "below normal."

All text, charts & photos © Hilton Pond Center
Photo above of juvenile male Ruby-throated Hummingbird © Sharon Osterholt

In the four weeks since the end of May we've heard from dozens of hummer enthusiasts who've relayed lots of anecdotal information. Most if them concurred their RTHU numbers seem to be low this spring--drastically down in some cases--but a few said they had more hummers than ever. We also received reports of successfully breeding RTHUs; a photo (above) taken 12 June by Sharon Osterholt of Dora FL (about 20 miles northwest of Orlando) shows a hatch-year male RTHU that--based on numerous red feathers in its gorget--must have hatched out quite early this spring. And then there were Wayne and Marilyn Osmond who reported an adult male ruby-throat frequenting their feeder at Cape Ray, Newfoundland--near the absolute northeastern limit of the species' breeding range! The Osmonds tell us this is only the second year they've seen hummers at their place. (Global warming, anyone?)

In the 30 days since we reported capturing only five new RTHUs in 2005 at Hilton Pond Center, things have picked up a little, and we now have 18 banded through the end of June (see Table 1 below). Like last month, this number is considerably lower than the average number of 26.7 RTHUs banded through the period, but oddly enough 17 is one more than we had through June in 1999 and 2002--both of which ended up being pretty good hummer years with 191 and 186 RTHUs banded. The bottom line is that we really won't know how successful RTHU migration and breeding have been in 2005 until we band our last hummer of the year--probably in early to mid-October.

* = Bander was absent from Hilton Pond for part of field season (1984 banding began 27 July)
** = Averages do not include 2005 or the five earlier years with partial field seasons

We find it quite interesting that after getting our second- and third-earliest RTHUs ever on 30 and 31 March this year, we banded only two in April and one in May--the latter on the 5th. Then our ruby-throats almost disappeared--each day in May we saw only one or two color-marked birds we'd already banded--until things began to pick up in early June. On 8-11 June we banded seven RTHUs and then there was another lull, with no new birds again until one each on 25 and 29 June.

Click HERE for an enlarged view of the chart above

Despite these lower-than-average bandings of RTHUs in 2005, the general timing of captures has been simlar to the norm, as depicted on the chart above. By mid-April we typically get an early rush of new hummers that continues until late in the month and then drops off. Toward the end of May things pick up again and then decline gradually but significantly until the beginning of July when numbers begin to skyrocket--primarily because young-of-the-year are fledging. Numbers swell throughout July and August and then the bottom falls out after mid-September when ruby-throats are making a bee-line for their winter homes.

These trends are also reflected in anecdotal reports and banding records from many southern RTHU sites. Most serious hummer watchers contend the drop in numbers in early May reflects two things: 1) The departure of early migrants just passing through on their way northward; and 2) female RTHUs sitting on their first egg clutches and making many fewer trips to feeders and flowers.

The second increase of RTHUs around 1 June and the eventual drop in numbers by the end of that month are harder to understand. It's been suggested the second peak reflects increased activity by food-seeking females whose first clutch has hatched, but our 22 years of banding data at Hilton Pond Center show we also have an increase in adult males at that time--and male RTHUs don't feed the chicks. After many hours of contemplating these twin peaks for ruby-throat captures in mid-April and early June, we're not sure just what is going on, but we do have a working hypothesis.

We speculate that these two peaks could reflect two separate migrational waves among Ruby-throated Hummingbirds. The first wave would be composed of "early" migrants, some moving through and pushing northward as far as southern Canada before stopping on the species' breeding grounds (red area on map below left). A second wave of "late" migrants--perhaps relatively young birds making their first northward migration--either depart later from the tropics or merely take their time moving north. In any case, RTHUs that leave too early from the wintering grounds (blue area on map) run the risk of perishing in late spring storms or of having their first nesting attempts aborted by bad weather. Those that arrive late in the U.S. and Canada may find prime breeding territories already taken and/or end up having insufficient time to nest or re-nest before having to fly back south. Every unpredictable spring season renews this tension between weather and optimal migration time, such that in some years the "early" birds win while in others the nesting prize goes to "late" migrants. Over time, it is the "average" migrants whose progeny have the greatest rate of survival--even though the optimal date for migration may move forward or backward as "average" environmental conditions fluctuate. (Did we mention global warming?)

We continue to analyze our Ruby-throated Hummingbird data from Hilton Pond Center in the hope of solving the "twin peak" mystery illustrated on our chart above. In reality, however, what ornithology needs is many more active, collaborative ruby-throat banders working and sharing data across Central and North America. Maybe then we'll have a more accurate answer when people ask, "Where ARE the hummingbirds?"

All text, charts & photos © Hilton Pond Center