Some folks lament the transition into cold weather seasons, as days grow shorter and nights dip toward frigid temperatures, but for many birders, fall and winter provide an opportunity to observe a fascinating and sometimes unpredictable phenomenon: dispersal and migration following the breeding season. This is an especially exciting consideration for nomadic species which do not move from discrete breeding areas to established wintering areas; resident species; and those which are facultative or opportunistic migrants, as these species may move unpredictably. Across North America, birders eagerly await to see which, if any of these species may be showing large local or long-distance flights each fall and offer conjecture about what may be driving these movements in a given season. Some years, some species move and others don’t; some years no species move; and some years, seemingly everyone is on the move. When enough individuals of a given species begin to wander out of their primary habitat(s) and/or into the same place at the same time, it may be called an “irruption”. Species’ irruptions can be incredibly exciting for birders; they also provide us with an opportunity to document and better understand the ecological causes of these movements.

Though we’ve recently experienced huge irruption years for some species—Snowy Owl (2021–2022) and Evening Grosbeak (2020–2021) come to mind—species irruptions are not a new occurrence. they are part of the standard operating procedure for numerous bird species. These movements are believed to be in response to a number of factors including lack of food in primary habitats; abundance of food in secondary or tertiary habitats; habitat loss or destruction (natural and human-caused); extreme weather patterns; and even high breeding output (exceptionally good breeding success). What may trigger an irruption in one species or population may not affect another species or population. Evolutionarily speaking, it makes sense to have this built in flexibility to a species’s annual cycle, as being able to move among patches of habitat allows populations to better weather stochastic events, endure food shortages, take advantage of food abundances, and ensure increased gene flow among different populations.

In eastern North America, species such as Common and Hoary redpolls, Evening Grosbeak, Purple Finch, Red-breasted Nuthatch may irrupt in any given year. The Finch Research Network (FiRN)assesses the likelihood of species irruption annually. It publishes the much anticipated “Finch Forecast”, which lays out these analyses for eastern North America, each fall. And you can see FiRN’s report on the 2021–2022 irruption in Vo. 72, No. 3 of North American Birds. However, finches aren’t the only birds which may move en masse as a result of environmental or population factors. Numerous songbird species can be irruptive, and even the winter movements of Snowy Owls and other raptors may constitute irruptions in years when species push relatively far south and/or move in relatively large numbers. Individuals of these irruptive species may fly many hundreds of miles to lower latitudes.

In western North America, mountainous geography places potentially irruptive montane species in close geographic proximity to lowland habitats. Biologists have long envisioned mountain ecosystems as islands that rise above the lowland “sea” floor. Such birds may have to fly only a few miles from high elevations to low in order to access alternate habitats, whereas equivalent ecological changes may require many hundreds of miles of travel for birds in much of central and eastern North America. In the mountainous West, annual elevational migration typically occurs each August through October, with movements starting in summer for some species with early breeding cycles (e.g., Pinyon Jay, Red Crossbill, Clark’s Nutcracker). These movements often bring birds lower than their typical habitat preferences, though some species may move up in elevation during portions of the non-breeding season.

Red-breasted Nuthatch, Pine Siskin, Evening Grosbeak, Cassin’s Finch, and Townsend’s Solitaire are classic winter invaders into western North America and may irrupt in any given year. Over the last five years, we have seen irruptions of each of these species in western North America, with 2020 being a particularly interesting year. In fall 2022, there were already movements of all of these species and more, particularly along the Rocky Mountain front from western South Dakota to Texas.

In central New Mexico, three species made prominent appearances across the lowlands in fall 2022: Pinyon Jay, Juniper Titmouse, and Evening Grosbeak. Some years there are few records for these species outside of their montane habitats, but eBird maps comparing September through October sightings for the past few years show how prominent the fall’s irruption of these species has been.

Pinyon Jays were well documented in fall 2022 throughout the Albuquerque area in New Mexico. They showed up in backyards and parks both above and below the pinyon habitats which typically hold this species in place. But these movements were not isolated to central New Mexico by any means. Pinyon Jays were moving to the fringes of their range into areas where they are seldom recorded. There were notable influxes or sightings of single birds as well as large flocks in the Black Hills of South Dakota; several locations in western Nebraska; the sky islands of Southeastern Arizona; and in west Texas between the Guadalupe and Davis mountains. In particular, a large flock of at least 200 Pinyon Jays was reliably observed from mid-Nov into 2023. Clearly, something was happening on the species level in the eastern and southern portions of Pinyon Jay’s range. It’s not yet clear whether this was due to failed pinyon cone crop, high breeding productivity, neither, or both, but because this species is one of conservation concern across its range, birders should be encouraged to report to eBird all sightings and information surrounding they note regarding Pinyon Jays.

eBird map showing records of Pinyon Jay across urban Albuquerque Sep–Oct 2023. Map image provided by eBird and created 1 Jan 2023.

In 2020, significant numbers of Evening Grosbeaks were on the move in eastern North America, from fall through spring migration, but fall 2022 brought this species down to backyards in lowlands across the West. In fall 2021, there were no reports of Evening Grosbeak in central New Mexico, even including the montane habitats of the Sandia Mountains where these birds are expected annually. In fall 2022, there were dozens of records in the Sandias, as well as throughout Albuquerque, where birds are descending into marginal habitats to forage on novel food items such as Russian olives and backyard birdseed. As with Pinyon Jays, this irruption was not limited to central New Mexico, and Evening Grosbeaks showed up in interesting locations, including Lubbock, Texas.

Juniper Titmouse is an interesting case of a resident species that typically seldom leaves its range in a normal year. In fall 2022, the species performed a huge descent from the nearby mountains and high plateaus into Albuquerque—a phenomenon that rarely, if ever, has been documented on such a scale. Early indications were that this may have been in response to a poor juniper “berry” crop in pinyon-juniper habitats, and research into this hypothesis is underway in central New Mexico. Comparing eBird maps for September–October of 2021 and 2021 show how dramatic this lowland invasion has been in the Albuquerque area.

eBird map showing records of Juniper Titmouse across urban Albuquerque Sep–Oct 2023. Map image provided by eBird and created 1 Jan 2023.

Montane bird movements were interesting along the Front Range in Colorado in 2022 as well. As early as summer 2022, I observed a personal record number of Red Crossbills during my (typically) annual week as an instructor at the ABA’s Camp Colorado in Rocky Mountain National Park. It felt like an irruption year may have already been underway at that point. eBird maps from 2021 and 2022 for September and October look very different. In fall 2022 there was a huge influx of reports in primary habitats, as well as reports of birds moving down and out of the mountains. Summer 2022 also saw the first summer records of White-winged Crossbill in central Colorado in several years—perhaps an indicator that this species may make a showing in the western U.S. in 2022–2023.

All along the Front Range, Townsend’s Solitaire showed strong movement downslope in fall 2022. As the name suggests, solitaires move singly down or out of the mountains in search of food, especially juniper “berries”, over the winter months. This species often becomes truly territorial in winter. Once an individual has found adequate food, it will sing, call, and physically defend its territory from other frugivorous birds. Townsend’s Solitaires typically move into the western half of Nebraska during fall, but the species was especially numerous in fall 2022. They regularly wander into eastern North America as fall moves toward winter, and a superb somewhat early record from New Brunswick (10/11/2022) combined with several other sightings in the eastern U.S. were indications 2022–2023 may be an especially good year for eastern birders hoping to see a solitaire.

Clark’s Nutcracker showed some indications that it may be on the move this year with numerous sightings in the mountains of central New Mexico, central Colorado, and in the Black Hills of South Dakota. Blue Jay and Bohemian Waxwing also showed early signs of movement, and indeed, birders are finding Bohemian Waxwings across much of the United States this winter, including one in Arizona, which has very few previous records.

Elevational migration in western North America has largely been overlooked in ornithological studies, which have tended to focus on obligate latitudinal migrants, especially neotropical–nearctic migrants. This is likely in part due to the difficulty of surveying far and wide for nomadic species, plus the difficulty of identifying and isolating driving factors for movement in facultative migrants and irruptive species. An incredible opportunity therefore lies with birders who are increasing in numbers and in willingness to report sightings to eBird. Learning flight calls for irruptive species and frequently checking areas with acorn mast, winter berries, crabapples, and conifer cones may yield the chance to document irruptive species presence and behavior. Winter 2022–2023 seems a particularly good winter for birders to take advantage of this opportunity and help track the exciting movements of these irruptive species. Who knows what might show up in your backyard or local patch!

Asher Gorbet is a wildlife biologist and ornithologist based in Albuquerque, New Mexico. Their work primarily focuses on songbirds: in particular, utilizing bird banding as a research tool to study songbird ecology. They are a North American Banding Council (NABC) certified bird banding trainer and a Director-at-Large for NABC.