HOME: www.hiltonpond.org

GENERAL INFO

CONTENTS

RESEARCH

EDUCATION

PUBLICATIONS

MISCELLANY

UPCOMING OPPORTUNITY:
Join us for another
Winter Hummingbird Expedition to Costa Rica
in February 2006

SLOW START, FAST ENDING FOR
RUBY-THROATED HUMMINGBIRDS

Now that the third week in October is past, we feel safe in saying the 2005 season for Ruby-throated Hummingbirds (RTHUs) has likewise come to a close. This spring we expressed concerns the hummer population at Hilton Pond Center--and elsewhere--might have crashed, perhaps due to those back-to-back-to-back hurricanes in the Gulf of Mexico during fall migration a year ago. The 2005 spring hummingbird season actually started quite early at the Center--adult male RTHUs on 30 and 31 March were the second- and third-earliest arrivals we've banded since 1984--but things waned quickly and we caught only two new birds in all of April and just one in May. By 31 May we were scarcely at 30% of our 22-year average for that date. June showed slight improvements with 13 RTHUs banded--raising us to 67% of the norm--but only five more hummers through 14 July indicated our 2005 breeding season was not going well, even though a young male banded on 4 June was five days earlier than our previous early date for a fledgling.

All text, charts & photos © Hilton Pond Center
(Click on chart above for larger view)

It wasn't until mid-August that our numbers began to improve, but they did so with a bang; the rate of increase after that was the steepest we have experienced at Hilton Pond (see red line on chart above), with 30 birds or more per week from 21 August through 14 September. Past mid-September, things tapered off considerably, with just 23 RTHUs banded in the second half of the month. October saw two more hummers on the first and third days, with the final Ruby-throated Hummingbird of the year hitting our nets on 10 October--our third latest date. This brought the tally for 2005 to a record-breaking 226 RTHUs banded and surpassed last year's previous all-time high by a whopping 16 birds (a 7% increase).

An informal Internet survey we conducted earlier this year suggested much of the U.S. and Canada had an average or above-average spring as far as ruby-throats were concerned, but most respondents from the southeastern U.S. reported LOWER hummingbird counts through the end of May. This trend was reflected at Hilton Pond by small numbers of new ruby-throats AND by a relatively low return rate of only 17 RTHUs banded locally in previous years (see table below); normally we recapture about 23 "old" birds, with the average from the ten years immediately preceding actually at 30. What was particularly disappointing was that only 5.2% of last year's 210 hummers came back in 2005--the lowest next-year return rate we've ever had; by comparison, 11.3% of the 141 RTHUs from 2003 showed up again in 2004, and on average we get 10.3% next-year returns.

From this we conclude that an unusually high percentage of our local breeders (and locally produced fledglings) either perished in fall/spring migration or on the wintering grounds. It's likely our record-setting 226 new captures in 2005 were the result of successful breeding north of South Carolina rather than at or around Hilton Pond itself; most or all of this year's 149 birds (66%) caught after 15 August probably were migrants just passing through.

Our new high of 226 RTHUs banded in 2005 came because we caught exceptional numbers of hatch-year birds. In fact, we banded 108 young males--obliterating the previous record of 89--plus 68 young females that exceeded the old record by eight (see table above). Adults were relatively scarce, with only 20 after-hatch year males and 30 older females. As always, young males outnumbered the other age/sex classes, while adult males were in short supply.

Just as we followed the usual pattern of capturing more young birds than adults in 2005, the age/sex ratios also were remarkably similar to what we have found in the past (see chart above). Young males outnumbered young females by a 3:2 margin, while the ratio reversed in adults--implying that young males are an "expendable commodity." Since each male Ruby-throated Hummingbird likely doesn't breed with just one female--some researchers even report a "harem" system in which the male has several mates--this loss of young males isn't as much of a problem as it might at first appear.

(Click on chart above for larger view)

As indicated by the upward-sloping red trend line on the chart above, there has been a steady increase in Ruby-throated Hummingbirds captured annually at Hilton Pond Center. Although some observers have suggested we are simply getting better at catching these speedy little birds, we prefer to think the numbers reflect an improvement in local habitat. Over the past two decades it's been impossible to keep an exact tally of our net-hours and "time spent trapping," but we believe our efforts are about the same year-to-year. Since Ruby-throated Hummingbirds prefer an "edge" rather than deep forest or open prairie, they likely benefit from some vegetational disturbance. And, since they seem adaptable to human development that includes lots of edges and a plethora of flowering plants--to say nothing of a massive increase in numbers of available hummingbird feeders--the species is probably faring well across its vast eastern North American breeding range.

For some reason, however, Ruby-throated Hummingbirds seemed more "trap-shy" than usual around Hilton Pond in 2005. Even though we banded a record number of hummers, 58% of them were captured in mist nets around the feeders rather than in our pull-string traps baited with sugar water. This is significantly different from the norm, since through the years a mere 22.5% of our RTHUs have been caught in nets. It's possible a better-than-average crop of natural nectar sources meant the birds were feeding on flowers rather than seeking our artificial 4:1 water:sugar mix, so there may be a better explanation for this unexpected phenomenon. (Any thoughts?)

In summary, low numbers of returning Ruby-throated Hummingbirds--coupled with precious few captures through mid-July--suggest that many RTHUs from Hilton Pond Center fared poorly in migration or on their wintering grounds, perhaps because of the "unusual" juxtaposition of three Gulf of Mexico hurricanes at the peak of the 2004 migration season. Since tropical storms and hurricanes have been even MORE prevalent in the autumn of 2005 (see composite satellite photo above), it will be interesting to see what happens with our RTHU numbers in 2006. Even though we've been banding hummers at the Center for more than two decades, we still don't have a handle on all the variables that can affect hummingbird populations; some things about hummers seem relatively constant, but each year brings an unexpected surprise or two. We're always learning something new about these tiny balls of fluff that migrate a thousand miles or more one-way and return to our 11-acre preserve--a more-than-ample reason for us to continue our long-term hummingbird study for another 22 years.

All text, charts & photos © Hilton Pond Center
2005 hurricane graphic courtesy Univ. Of Wisconsin