Allen’s hummingbirds (Selasphorus sasin) are a small species of hummingbird found along the Pacific Coast of North America. Determining the exact population size of Allen’s hummingbirds is challenging, but estimates put the total number of mature individuals at around 1.5 million. In this article, we’ll take a closer look at the challenges in estimating Allen’s hummingbird populations, geographic range, habitat and threats to help understand population trends for this tiny bird.
Estimating Allen’s Hummingbird Populations
Allen’s hummingbirds are found along the Pacific Coast, ranging from southern Oregon to southern California. Within this range, they occur in appropriate habitat including coastal scrub, chaparral, woodland edges, gardens and parks. Their small size, camouflage plumage and tendency to occur in low densities makes getting accurate population surveys difficult.
Researchers estimate the population based on extrapolating from surveys done in small study areas to the birds’ overall breeding range along the Pacific Coast. However, habitat varies considerably across this span, so coming up with a single population estimate is challenging. The estimated global population is around 1.5 million mature individuals, but the true number may be higher or lower.
More extensive surveys and population monitoring will help refine population estimates for Allen’s hummingbirds. Mark-recapture banding studies at migratory sites like bird banding stations can provide valuable population data. New technologies like surveying with drones may also help cover more habitat.
Breeding Bird Survey
One of the main sources of information on breeding bird populations in North America is the Breeding Bird Survey (BBS) organized by the USGS Patuxent Wildlife Research Center and Environment Canada. The BBS involves over 2,000 skilled volunteer birdwatchers conducting roadside bird counts across North America during the breeding season. Routes are randomly located along secondary roads and volunteers conduct point counts of all birds seen or heard at half mile intervals along the route.
Allen’s hummingbirds are among the hundreds of species documented on BBS routes in their western breeding range. BBS data indicates an average decline of -0.81% per year for Allen’s hummingbirds from 1966 to 2019. However, trends showed strong regional variation. California showed a decline of -1.25% annually, while Oregon and Washington trended stable with 0% change. Habitat loss in coastal California may explain the downward trend there.
Christmas Bird Counts
Christmas Bird Counts conducted by volunteer birders provide another source of long-term population data on hummingbirds, including Allen’s. These counts take place in late December and early January during the non-breeding season when many hummingbirds have migrated to wintering grounds. Though they don’t directly measure breeding populations, declines during winter may reflect issues on nesting grounds. Data from 119 counts from 1959-1988 showed a small increasing population trend for Allen’s hummingbirds.
Difficulties in Estimating Populations
Even with survey data, accurately determining populations for a small, fast-moving species like the Allen’s hummingbird has many challenges:
- Their small size and ability to fly long distances makes detection difficult.
- They can be hard to distinguish from similar looking Anna’s and Rufous hummingbirds.
- Abundance fluctuates seasonally with migration and post-breeding dispersal.
- Populations are sparse even in suitable habitat.
- Much of their range has not been thoroughly surveyed.
Despite these difficulties, existing surveys and estimates give a good baseline to monitor long-term population trends for Allen’s hummingbirds.
Geographic Range
Allen’s hummingbirds breed along the Pacific Coast from southern Oregon to southern California. Their breeding range extends approximately 1,000 miles north to south. Eastward from the coast, they breed in the Coast Ranges and western slopes of the Sierra Nevada up to around 5,900 feet elevation.
The northernmost breeding populations occur in the Rogue River Valley of southern Oregon. Nesting range extends south throughout coastal and inland California. Breeding gets patchier in southern California south of Los Angeles County and tapering off near the Mexico border.
During the non-breeding season, most Allen’s hummingbirds migrate south to wintering grounds in Mexico. A smaller number spend winters in mild coastal and southern California locations. They winter from southern California south to the state of Guerrero, Mexico on the Pacific coast.
Vagrant birds are sometimes spotted well outside the normal range in eastern and midwestern states. But these records likely represent lost migrants and do not indicate range expansion. The core breeding and wintering ranges have remained largely consistent in recent decades according to banding and survey data.
Breeding Range Map
The breeding range map below shows the distribution of Allen’s hummingbird nesting habitat along the west coast:
Source: Birds of the World, Cornell Lab of Ornithology.
Habitat
Allen’s hummingbirds breed in a variety of relatively open habitats where flowering plants provide nectar. They are found in both natural and human-modified environments. Typical breeding habitats include:
- Coastal scrub
- Chaparral
- Woodland and forest openings
- Suburban parks and gardens
- Brushy creeksides
Coastal scrub habitat along the Pacific Ocean provides ideal nesting habitat in northern and central parts of its range. Further inland, drier chaparral scrublands with manzanita, ceanothus and other flowering shrubs are favored.
At higher elevations in the Sierra Nevada, open ponderosa pine or mixed evergreen forests with meadow openings provide habitat. Riparian areas and zones between forest and scrub support high densities.
In human-modified environments, they readily inhabit parks, gardens and backyards if sufficient flowers and nest sites are available. They often occupy habitat mosaics combiningnative vegetation and landscaping.
Plant Communities
Preferred plant communities feature a mix of forest, shrub and herbaceous layers with abundant tubular flowers. Important native plants include:
- Manzanitas (Arctostaphylos spp.)
- California fuchsia (Zauschneria californica)
- Sages (Salvia spp.)
- Columbine (Aquilegia spp.)
- California poppy (Eschscholzia californica)
Cultivated flowering plants like fuchsia, coral honeysuckle, columbine and trumpet vine attract Allen’s hummingbirds to yards and gardens. They also forage on introduced weeds such as tree tobacco (Nicotiana glauca).
Nesting Habitat Requirements
Suitable nesting areas include trees, shrubs, vines and structures that provide dense cover. Preferred plants for nest placement include cypress, pine, eucalyptus, oak, sycamore and cottonwood. They often nest along riparian corridors where dense vegetation overhangs the water.
In human habitats Allen’s hummingbirds nest in trees, shrubs, potted plants or suspended from awnings and lampposts. They may reuse nests from previous years or build new ones each season. Males defend nesting territories, but home ranges often overlap between neighboring males and with foraging areas.
Migration and Wintering Habitat
The majority of Allen’s hummingbirds migrate over 500 miles south to wintering grounds on the Pacific coast of Mexico. Banding records document birds banded in California or Oregon recovered later in states such as Sinaloa, Jalisco, Guerrero and Oaxaca.
A smaller number spend winters in California. Winter populations are concentrated along the coast from Marin County south and in areas like the Channel Islands and Santa Monica Mountains. Scattered winter records occur inland to the Mexican border.
Winter habitats are similar to breeding areas – scrublands, forest edges, parks and gardens with flowering plants. Higher elevation breeding grounds are abandoned in winter. The increasing popularity of backyard feeders allows more Allen’s hummingbirds to overwinter in urban and suburban areas.
Migrants begin arriving on the wintering grounds in Mexico in August. Numbers peak in November and December before northbound migration starts in January and February. Males appear to migrate north first with females and immatures following later.
Population Threats
The majority of Allen’s hummingbirds breed in California, which has undergone extensive habitat loss and alteration. Threats that may impact hummingbird populations include:
Habitat Loss
Coastal development in California has eliminated much former nesting habitat. Fire suppression has enabled forest maturation and expansion in some areas, reducing openings. Riparian areas have suffered losses to agriculture, grazing and development.
Counterbalancing this, urban sprawl and landscaping have created habitat in previously unoccupied areas. But habitat fragmentation remains an issue for native plant-dependent wildlife.
Invasive Species
Non-native plants have displaced native flora important to breeding hummingbirds in some regions. For example, invasive Cape ivy (Delairea odorata) has overgrown coastal scrub in areas of southern California.
Diseases and parasites from domestic birds may also pose risks. Nest predation from expanding urban-adapted species like house cats is likely increasing.
Climate Change
A warming, drying climate may negatively impact Allen’s hummingbird populations. Hotter droughts could desiccate vegetation and reduce flower availability during the breeding season. At higher elevations, upward contraction of breeding ranges may occur as formerly suitable habitat becomes unsuitable.
However, Allen’s hummingbirds may fare better than some species due to habitat flexibility and ability to utilize human-supplemented resources. Backyard feeders and gardens can provide food and water during hot, dry conditions.
Pesticides
Pesticide application in agricultural and developed areas poses risks through both direct toxicity and reducing insect prey populations. However, impacts specific to Allen’s hummingbird are not well studied. Maintaining organic gardens and landscapes may help provide cleaner habitat.
Conservation Status
Allen’s hummingbirds remain a common and widespread species along the Pacific Coast. They are considered a species of Least Concern on the IUCN Red List due to the large population size and range.
Partners in Flight estimates a breeding population of 1.5 million and gives a PIF score of 5 out of 20, indicating lowest conservation concern. Christmas Bird Count data shows a positive population trend overall since the 1960’s.
However, Breeding Bird Survey data indicates declines in southern California that bear monitoring. Habitat preservation efforts focused in coastal California will benefit Allen’s hummingbird populations. Promoting organic gardening practices may also help provide cleaner foraging habitat in urban areas.
Conclusion
Allen’s hummingbirds are a common species along the west coast of North America, with a total population estimated at around 1.5 million mature individuals. However, their small size, camouflage plumage and sparse densities make population estimates challenging. Breeding Bird Survey data shows declines in California that may reflect habitat loss, invasive species and other threats. Preserving coastal scrub, chaparral and riparian habitats will benefit Allen’s hummingbird populations, along with organic gardening and landscaping practices. Despite some regional declines and threats, Allen’s hummingbirds remain widespread along their migration route from Oregon to Mexico and overall appear to have stable, healthy populations.