Is the Rufous Hummingbird a Mexican monsoonal molt migrant?

RUHUTimHopwood

Rufous Hummingbird, Photo credit: T Hopwood

Many species of North American hummingbirds are highly migratory, with some traveling over 2,000 km. These migratory birds risk more feather wear due increased solar exposure on an annual basis compared to non-migratory species. Hummingbirds, like all birds, need their feathers to be in good condition to survive and therefore have to undergo molt.

All North American migratory hummingbirds undergo complete molts on wintering grounds in Mexico before they migrate north. In first-year birds, this complete molt has been referred to as the performative molt and in all subsequent years, it is called the definitive prebasic molt. First-year males were also known to replace some throat feathers during the fall, replaced again during the complete overwinter molt.

However, a recent study by Dittmann and Cardiff suggested that molting in migratory hummingbirds may be more complex than simply the annual molt (2009, Birding 41: 32–35). They examined molt of Ruby-throated Hummingbirds during their fall migration from June to September on the Gulf Coast of Louisiana and found evidence of molting at this time in both adults and young. They called this molt an inserted molt, separate from the overwinter complete molt.

Curious whether inserted molts might occur in other migratory hummingbird species, we examined molt in the Rufous Hummingbird, a Mexican species that migrates to the northern United States and Canada to breed. We examined 346 specimens of Rufous Hummingbirds from three different specimen collections in California. We looked at each specimen for signs of molt in four feather regions: the crown, the back, the throat (including the gorget in adult males), and the underparts. We considered a bird as showing evidence of molt if we found any replaced feathers contrasting with worn feathers, indicating two generations of feathers in a single region, or if we found pin feathers present in any of the regions.

As expected, the majority of the specimens collected in the winter from southern Mexico showed signs of the complete molt that was known to take place on the wintering grounds. However, we also found signs of molting in both young birds and adults during southbound migration in the fall. These inserted molts occurred in all four feather regions but were more limited in extent than those of Ruby-throated Hummingbirds. Many (but not all) molting specimens were collected in the Mexican monsoonal region of northwestern Mexico. The Sierra Madre Occidental, a mountain range that lies within the monsoonal region attracts Rufous Hummingbirds because of the high flower abundance that occurs there, and we infer that the hummingbirds take advantage of this food resource to molt in this area, as do many western North American species of passerines.

Surprisingly, we found that the inserted throat feather molt in Rufous Hummingbirds varied greatly among ages and sexes. Adult males showed no evidence that their iridescent throat feathers were replaced in fall, whereas some adult females and all young birds of both sexes replaced at least some throat feathers during the inserted molt. New iridescent throat feathers might give young birds an advantage when defending territories by showing dominance. While it is still unclear why the highly migratory Ruby-throated and Rufous Hummingbirds undergo inserted fall molts in other body regions, one reason could be that their feathers become more worn compared to non-migratory hummingbirds.

We compared molts of Rufous Hummingbirds to the molts of basal Apodiform taxa, including Vaux’s Swift and Rivoli’s Hummingbird. We propose a terminology that would consider the inserted molts as the definitive prealternate molt in adults and the preformative molt in young birds. Therefore, we propose that Rufous Hummingbirds undergo a prealternate and preformative molt migration to the Mexican monsoonal region.

Our work on Rufous Hummingbirds is part of The Institute for Bird Populations’ Avian Molt Research Program, aimed at understanding molt strategies of all species of birds, standardizing molt and plumage terminologies worldwide, and applying findings to age-related demographic studies, habitat requirements for molt, and, ultimately, the conservation of birds.

~Post written by authors: Desmond Sieburth and Peter Pyle

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Evidence for a prealternate molt-migration in the Rufous Hummingbird and its implications for the evolution of molts in Apodiformes is available, at http://www.bioone.org/doi/full/10.1642/AUK-17-231.1 (issue URL http://www.bioone.org/toc/tauk/135/3).

About the journal: The Auk: Ornithological Advances is a peer-reviewed, international journal of ornithology that began in 1884 as the official publication of the American Ornithologists’ Union, which merged with the Cooper Ornithological Society in 2016 to become the American Ornithological Society. In 2009, The Auk was honored as one of the 100 most influential journals of biology and medicine over the past 100 years.

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