Ruby-throated Hummingbird (Archilochus colubris) are sexually dimorphic; adult males and females are different in appearance AND size. Females can be 15-25% larger than adult males.

All text & photos © Bill Hilton Jr.

On average, female (left) Ruby-throated Hummingbirds are about 15-25% larger than males with regard to mass, wing chord, and length of culmen (upper bill). It is speculated that the larger size makes the female better able to incubate eggs and brood the young; the male's smaller size may facilitate his escaping predators or performing more aerobatic courtship displays.

Typical adult female Ruby-throated Hummingbirds (RTHUs) at Hilton Pond Center for Piedmont Natural History show a wing chord between 40mm and 46mm, while adult males range from 37mm to 39mm. All RTHUs molt while they are on the wintering grounds in the tropics, losing and replacing plumage--including their wing feathers. On average, young males banded at Hilton Pond have wing chords 2-3mm shorter after they molt during their first winter, while young females have wings about a millimeter shorter after that first molt. Overall body length (tip of bill to tip of tail) in RTHUs ranges from about 70mm in males to about 90mm in females.

During the breeding season, typical male RTHUs have masses from 2.8g to 3.5g, while females average about half a gram heavier. Prior to migration, the birds goes into hyperphagia--feeding frequently and heavily and building up fat reserves; they may increase their masses by 35-70%. The heaviest female at Hilton Pond Center had a mass of 6.1g (8 Oct 1996); the heaviest male weighed 5.2g (9 Oct 1993); both birds were preparing for autumn migration.

The exposed upper bill (culmen) in a typical male RTHU averages 15mm-18mm, while females measure 17mm-21mm. Bill length is much shorter in nestlings, but the bill--and the rest of the body--usually have all reached full length by about the time a young bird fledges.

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