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Jeff Mitton: Natural Selections: Broad-tailed hummingbirds need to return to Colorado sooner

A female broad-tailed hummingbird sits on the nest she has used for two years. (Jeff Mitton / For the Camera)
A female broad-tailed hummingbird sits on the nest she has used for two years. (Jeff Mitton / For the Camera)

Male broad-tailed hummingbirds (hereafter BTH), Selasphorus platycercus, arrived first, and they immediately got down to business. Each male’s first objective was to find and defend a breeding territory that would enable him to mate with multiple females.

Females came shortly thereafter, and each female had to either repair the nest she used last year or construct a new nest. She uses spider threads and gossamer to form an insulating inner cup and then camouflages the outside with bits of lichen, moss and bark held together with spider silk.

Jeff Mitton. Natural Selections

Everything seemed normal, but scientists have documented a growing asynchrony that will require BTH to change either their times of migration or shift to other food sources.

BTH have a polygamous mating system, with males advertising their territories with spectacular dives while using their wing feathers to produce a loud, metallic trill. Males mate with receptive females, but mating is the only contribution that females can expect from males. Females must build the nest, incubate one or two eggs for 16 to 19 days and feed the offspring for 21 to 26 days until they fledge.

When the BTH arrive in the Colorado Front Range in spring, they have accomplished a migration of 1,500 miles from their wintering area in southern Mexico, between the Pacific Ocean and the Trans-Mexican Volcanic Belt. Average migration flight speed is 30 miles per hour and whether BTH are hovering or migrating, they beat their wings about 50 times per second. The oldest BTH, as judged by banding records, lived 12 years. That bird made 12 round trips between Mexico and Colorado, an accumulated distance of 36,000 miles. But not all BTH migrate.

Two BTH subspecies are recognized: Selasphorus platycercus platycercus and S. p. guatemalae.  S. p. platycercus has two populations in Mexico, one composed of birds that migrate and another that stay year-round in Mexico. S. p. guatemalae is found only in Guatemala, where it resides year-round. Genetic data indicate that these two subspecies are no longer in contact, no longer exchanging genes. These subspecies and the migrating and year-round residents of S. p. platycercus are being used to study the evolution of bird migration.

David Inouye and Amy McKinney and their colleagues at Rocky Mountain Laboratory in Gothic have been studying the phenology or timing of plant blooming and animal migrations to look for patterns through time. Recently, they reported on changes of plant blooming relative to the arrival of BTH. They collected data from two sites, the Santa Catalina Mountains in southern Arizona and Gothic Valley in western Colorado. In both sites, they had over 30 years of data on the first arrival of BTH and first flowering of BTH nectar resources. In Colorado, BTH rely first on glacier lilies, Erythronium grandiflorum, and then dwarf larkspur, Delphinium nuttallianum.

The flowering data showed a pattern that is easily interpreted in the context of climate change. The date of first flowering in plant species examined in the Santa Catalina Mountains did not change over time. But the glacier lilies and dwarf larkspur in Gothic showed a regular and significant change over time, both species blooming earlier in spring. Studies of climate change have shown repeatedly and convincingly that climate change impacts are minimal at lower latitudes but increasingly apparent with increasing latitude.

In contrast to blooming dates, BTH arrival dates did not change over time in either the Santa Catalina Mountains or Gothic Valley. Climate change has had minimal impact in southern Mexico, so the migrating hummingbirds have not changed the time that they leave for northern breeding grounds.

Historically, BTH arrived in Gothic Valley shortly before glacier lilies began to bloom, and they relied on glacier lily nectar for energy to secure and defend breeding territories and to begin their trilling displays. Females began repairing or building nests as dwarf larkspur began to bloom. These are two of the earliest blooming wildflowers in Gothic Valley.

Because glacier lilies and dwarf larkspur are now blooming 17 days earlier than in the 1970s while the hummingbirds are arriving at about the same time, the earliest BTH have missed the earliest nectar in recent years. If the trends continue for two more decades, BTH will miss glacier lily nectar altogether. Given that the increase of atmospheric CO2 shows no sign of abating, this dire prediction will probably come true.

What will BTH do? They could learn to start spring migration earlier, or they might be able to rely on other nectar sources. Because billions of birds migrate in North America, this asynchrony of needs and availability of resources could become a widespread problem.

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