PROLOGUE: On the morning of 11 February 2008, four people piled into a car at Buena Vista Lodge (below)--halfway up the Rincon de la Vieja volcano in Guanacaste Province, Costa Rica--and drove downhill, eventually turning right on the Pan American Highway. Two of them--naturalist/guide Ernesto Carman Jr. and trip leader/hummingbird bander Bill Hilton Jr.--had just spent 16 days with two groups of citizen scientists who came from the U.S. and Canada to help us capture, band, and observe Ruby-throated Hummingbirds, Archilochus colubris, in Aloe Vera fields near Canas Dulces and Liberia. The other two passengers--Ernesto's girlfriend (Elaida Villanueva Mayorga) and his dad (Ernie Carman)--had come over from the San Jose area of eastern Costa Rica the day before to join us. The following account explains why this group of four--known collectively as Krazy '08s Delta--were heading north toward the Nicaraguan border on a hummingbird scouting mission.

All maps, text & photos © Hilton Pond Center

DELTA GROUP ON THE ROAD:
Whither Ruby-throated Hummingbirds?

In autumn, when the vast majority of Ruby-throated Hummingbirds depart North American breeding sites such as Hilton Pond Center, we always think about the long journey lying ahead for these little three-gram dynamos. Based on sightings from oil platforms and ships at sea there's evidence at least some RTHU fly up to 500 non-stop miles across the Gulf of Mexico before making landfall on or near the Yucatan Peninsula; other ruby-throats appear to take an overland route along the Texas Gulf Coast into Mexico. In either case, one BIG question is exactly where they go from there. Although most sources say the majority of RTHU overwinter in central or southern Mexico, they can be found in any of the seven Central American countries from Belize to northern Panama. Within Costa Rica, ruby-throats are essentially absent from the Caribbean provinces, but our four years of work in Guanacaste shows they are common a few miles inland from the northwest Pacific coast--and even ABUNDANT in Aloe Vera plantations. So, we ask, how do all those RTHU get into Guanacaste?

The above map of Central America is modified from a composite photo produced during NASA's Shuttle Radar Topography Mission (Feb 2000). It illustrates why we're perplexed over how ruby-throats disperse into Guanacaste--especially when we take note of the extent and height of mountain ranges throughout Central America. Hummers that cross the Gulf of Mexico and land on the Yucatan Peninsula have no significant topographic barriers moving southward until they reach central Guatemala, where extensive volcanic ridges would require they migrate at high altitudes. However, if they simply follow the Caribbean coastline southward through Mexico and Belize and east past Honduras, RTHU eventually would end up in eastern and southern Nicaragua and be able to move unimpeded into eastern Costa Rica. The map shows from that point they'd still face having to cross a central volcanic ridge to get to Guanacaste, but that seems to be a moot point; experts such as Ernesto Carman Jr.--our in-country guide who looks for ruby-throats throughout Costa Rica--report the species is rare to absent on the eastern side of the central ridge.

Of course, our whole premise that Ruby-throated Hummingbirds enter Guanacaste via Pacific lowlands because volcanic ridges interfere with migration may not be valid. After all, we have seen RTHU at Buena Vista Lodge in Costa Rica halfway up the Rincon de la Vieja at 2,400 feet, and we know of an American expatriate in southern Guatemala who has ruby-throats visiting her feeders at 6,000 feet. We're not aware of large concentrations of wintering RTHU at high elevations anywhere in Mexico or Central America; it may be they pass over mountains in migration before descending to lower elevations. Incidentally, conventional wisdom is that when ruby-throats migrate over land they do so by day--flying just above treetop height so they can see patchy sources of nectar--but the relationship between RTHU migration and topographical elevation is not known.

All maps, text & photos © Hilton Pond Center

About the only way to get a handle on this question of how RTHU get into Costa Rica is to return to the Rio Sapoa valley in November 2008, i.e., prior to January and February 2009 when we will again visit Guanacaste to band ruby-throats. So that's our plan. Ernesto has agreed to meet us at the Liberia airport and to drive around the hinterlands of northwestern Costa Rica in early November--right about when ruby-throats should be arriving en masse at Guanacaste. Assuming we can scrape together funds for lodging and a plane ticket from Charlotte to Liberia, we expect to be there at that very same time to further test our hypothesis that Ruby-throated Hummingbirds enter Costa Rica via the west coast of Nicaragua. If we're right, a mist net across the Rio Sapoa just might capture a whole bunch of migrant ruby-throats--maybe even some that show up a few months later a little further south in the aloe fields at Canas Dulces.

"This Week at Hilton Pond" is written & photographed
by Bill Hilton Jr., executive director of
Hilton Pond Center for Piedmont Natural History.

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