Allen's Hummingbird: Why One Population Is Vanishing While Another Booms
Allen's Hummingbird is currently doing two almost opposite things, depending on which population you're watching. Along its true breeding range, the tightly banded strip of California and southern Oregon coastline where most of the species nests, numbers have fallen far enough that the 2025 U.S. State of the Birds report lists it as a Red Alert Tipping Point bird, a category reserved for species that have lost more than half their population in the past 50 years. Cornell Lab's own tracking of Breeding Bird Survey data puts a number on that decline: an estimated 2.6 percent lost every year between 1968 and 2023, a cumulative drop of 77 percent. At almost the same time, a separate, non-migratory population of the exact same species has expanded its Southern California range by an estimated 9,200 percent since 1970, moving block by block into the backyards of Los Angeles, Orange County, and San Diego. Both of those facts describe Allen's Hummingbird. Neither one is the whole story, and figuring out how they fit together says more about this bird than any single field mark does.
How to Identify an Allen's Hummingbird
Size is a useful starting point. Allen's Hummingbird measures about 3.5 inches long and weighs just 2 to 4 grams; Cornell's identification guide describes it as larger than a Calliope Hummingbird, the smallest breeding bird in the US and Canada, but slightly smaller than an Anna's Hummingbird, the species most Southern Californians already know from their own backyard. Wingspan runs about 4.3 inches per the same guide, though Audubon's field guide lists a slightly wider range of 4.3 to 4.7 inches.
Adult males carry a bronze-green back, a coppery tail, a coppery belly, and a deep reddish-orange throat patch, or gorget, that can look nearly black until it catches direct light, per Cornell's field guide. That green back is worth flagging on its own: it's genuinely useful for separating an adult male Allen's from an adult male Rufous Hummingbird, which typically shows a solid copper back instead, according to Audubon. Females and immatures are considerably harder to place. Cornell describes them as bronze-green above with paler, coppery sides, showing throat spotting rather than a solid gorget, with adult females carrying a small patch of reddish orange in the center of an otherwise pale throat.
Because Allen's and Rufous Hummingbirds are close enough in appearance to challenge experienced birders, and because that particular comparison deserves its own careful treatment rather than a quick aside, we've covered the full identification challenge, including the one feature that comes closest to a definitive answer, in a dedicated guide: Rufous vs. Allen's Hummingbird: How to Actually Tell Them Apart. This profile sticks to Allen's Hummingbird on its own terms.
Two Subspecies, Two Very Different Ranges
Almost everything else about this bird, its range, its migration timing, even how endangered it is, depends on which of its two subspecies you're talking about. The nominate subspecies, Selasphorus sasin sasin, breeds in a narrow strip of coastal scrub and chaparral running from California up into southern Oregon and winters in central Mexico, per Cornell's overview. It's also an unusually early migrant among North American birds: Cornell notes that northbound birds can depart Mexico as early as December, arriving back on the California coast by January, timed to the winter rains that trigger the season's first flower blooms.
The second subspecies, Selasphorus sasin sedentarius, does not migrate at all. It was historically confined to the Channel Islands off the Southern California coast and a small patch of the adjacent mainland, but Cornell's Birds of the World species account now describes it as "currently expanding its breeding range in southern California," a description that undersells just how much that expansion has actually changed the picture.
The Population Paradox: A "Tipping Point" Species That's Also Booming
Every wide-lens conservation number about Allen's Hummingbird looks bad. Cornell's overview lists the species as a Red Alert Tipping Point bird, and its life history page attributes that status to Breeding Bird Survey data showing a 2.6 percent annual decline between 1968 and 2023, a 77 percent cumulative loss over 55 years. Audubon's own field guide lists the species as IUCN Least Concern globally but still flags that "surveys still show decreasing populations in recent decades." Read only these sources and the conclusion seems straightforward: a small coastal specialist is disappearing.
That conclusion turns out to be only half right, and the half that's wrong comes down to where, and when, the surveys actually look. In a 2017 study published in The Condor: Ornithological Applications, UC Riverside ornithologist Christopher Clark analyzed more than two decades of eBird citizen-science checklists and found that the non-migratory sedentarius subspecies expanded its mainland Southern California range from roughly 70 square kilometers in 1970 to about 13,000 square kilometers by 2016, a roughly 9,200 percent increase. The spread followed a documented path across the region: birds were first reported in Orange County in 1996, then Pasadena in 1998, San Diego in 2001, Santa Barbara in 2005, Riverside in 2008, and Temecula by 2012. By Clark's count, the share of eBird checklists reporting the species in the region rose from under 5 percent in 1990 to well over 20 percent by 2016, with a current Southern California population estimated somewhere between 46,000 and 350,000 birds.
The reason the decline numbers and the growth numbers can both be accurate comes down to a straightforward methodology gap. The Breeding Bird Survey that generates the official trend data runs its counts in late spring, roughly mid-May through June, which lines up well with when migratory populations farther north are still actively breeding and visible. But Southern California's resident sedentarius population breeds much earlier in the year and becomes noticeably scarcer by the time standard survey routes run, and those routes were never designed to sample the urban backyards, parks, and gardens where this subspecies has actually settled, per the Condor study's own analysis. Clark put it plainly to PBS SoCal: "the surveys are not measuring where the birds actually are."
What's actually driving the urban boom seems to be a mix of opportunity and biology. Coverage of the same research from the American Ornithological Society notes that the non-migratory subspecies appears to do better in parks and backyards than its migratory relative, and that it produces more offspring during the breeding season, with suburban landscaping, nonnative flowering plants, and feeders all supplying food across more of the year than the coastal scrub habitat sasin depends on ever could. None of this erases the real, documented decline in the migratory coastal population, and it doesn't mean Allen's Hummingbird as a whole is doing fine. It means the two subspecies are having genuinely different experiences of the same changing landscape, and a single population trend line was never going to capture both.
Diet, Courtship, and Nesting
Allen's Hummingbird feeds on nectar from a wide range of native and non-native flowering plants. Cornell's life history page lists bush monkeyflower, Indian paintbrush, columbine, currant, gooseberry, twinflower, penstemon, ceanothus, sage, eucalyptus, and manzanita among its regular nectar sources, and notes the birds supplement that diet with small insects, caught mid-air or picked directly off vegetation, for the protein that nectar alone doesn't supply.
Males announce themselves with one of the more elaborate courtship displays among US hummingbirds. Per Cornell's overview, a male performs a side-to-side shuttle display with his gorget flared, followed by a pendulum display that produces a stuttering, bumblebee-like sound, before climbing as high as roughly 100 feet and diving straight down, his tail feathers producing a sharp, metallic squeal on the way. It's a genuinely showy performance for a bird that weighs about as much as a dime.
Nest building falls entirely to females. Cornell's life history page describes a tiny cup nest, about 1.25 inches across on the inside, built from spiderweb and downy plant material and camouflaged on the outside with lichen and moss, typically placed 2 to 50 feet up in a tree or shrub near a stream. Construction takes 7 to 13 days. The eggs, usually two per clutch and white, measure roughly a half inch long and take 17 to 22 days to hatch, with chicks fledging 22 to 25 days later. A female can raise one to three broods in a single season, and males play no role in incubation, feeding, or any other part of raising the young; the species mates polygamously, and a male's involvement ends when mating does.
Naming and Natural History: Who Was "Allen"?
The bird's scientific name and its common name come from two different people who never worked together. According to Cornell's Birds of the World species account, the species was first formally described in 1829 by the French naturalist René Lesson, based on specimens collected by Paolo Botta, who used a Nuu-chah-nulth word for hummingbird, "sasin," to name it Ornismya sasin. Under the rules of taxonomic priority, that's why the bird is still classified as Selasphorus sasin today, even though almost nobody calls it that in conversation.
The common name honors someone else entirely: Charles Allen, a 19th-century collector based in Marin County, California, who was among the first to recognize this bird as a species distinct from the very similar Rufous Hummingbird. In 1877, per the same source, Allen sent specimens to the naturalist William Brewster in Massachusetts, who forwarded them to Henry Henshaw for a formal description under Allen's name, unaware that Lesson had already described the same species almost 50 years earlier on the other side of the naming process. The result is a bird named for the person who first told it apart from its look-alike, carrying a scientific name that predates him by decades.
What This Means If You Want to Attract One
Whether you have a realistic shot at hosting an Allen's Hummingbird depends heavily on where you live and which subspecies is nearby. Along the immediate coast from central California into southern Oregon, expect the migratory sasin population only, present mainly in spring and summer as birds move through to breed, per Cornell's range description. In and around Los Angeles, Orange County, San Diego, and the wider Southern California basin, the resident sedentarius population means an Allen's Hummingbird at a feeder is plausible in any month of the year, not just during a narrow migration window.
Either way, the basics that help any hummingbird species also help this one. Cornell's own conservation notes point to hummingbird feeders and nonnative flowering plants like eucalyptus as resources that may be partly offsetting habitat loss for the declining coastal population, alongside the native nectar plants listed earlier in this guide. Our guide to attracting hummingbirds covers the fundamentals of feeder placement and planting in more depth, and applies just as well here as it does for any other species passing through a backyard.
Frequently Asked Questions
Are Allen's Hummingbirds rare?
It depends on the population. The migratory coastal population is genuinely declining, listed as a Red Alert Tipping Point species after losing an estimated 77 percent of its numbers since 1968. But the non-migratory Southern California population has expanded its range by roughly 9,200 percent since 1970 and now numbers somewhere between 46,000 and 350,000 birds, so an Allen's Hummingbird in a Los Angeles or San Diego backyard is not rare at all.
Where do Allen's Hummingbirds live?
The migratory subspecies breeds in a narrow strip of coastal scrub and chaparral from California into southern Oregon and winters in central Mexico. A separate, non-migratory subspecies lives year-round on California's Channel Islands and has spread widely across mainland Southern California, including Los Angeles, Orange County, San Diego, Santa Barbara, and Riverside.
What is the difference between a Rufous and an Allen's Hummingbird?
The two species are close enough in appearance that even researchers who handle the birds directly describe identifying some individuals as genuinely difficult. Adult male Allen's Hummingbirds typically show a green back rather than the solid copper back common on male Rufous Hummingbirds, but that mark isn't fully reliable on its own, and females of both species look nearly identical. For the full comparison, including the field mark that comes closest to definitive, see our dedicated Rufous vs. Allen's Hummingbird guide.
How can you tell a male from a female Allen's Hummingbird?
Males show a bronze-green back, a coppery tail and belly, and an iridescent reddish-orange throat that can look almost black without direct light. Females and immatures are duller, bronze-green above with paler coppery sides, and show only throat spotting with a small patch of orange in the center rather than a full iridescent gorget.
What do Allen's Hummingbirds eat?
Mostly flower nectar, drawn from plants including bush monkeyflower, Indian paintbrush, penstemon, sage, ceanothus, gooseberry, and eucalyptus, supplemented with small insects caught in flight or picked off vegetation for the protein that nectar doesn't provide.