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Full hemisphere abundance models

The latest eBird modeling is available on the eBird Status & Trends page. Please see those pages for more details.

Those models cover Canada, Alaska, and the lower 48 United States, the Caribbean, and Middle America. However, earlier versions of these models, produced in the summer of 2017, extended to South America.

Below are four example species, their full annual life cycle animations, and some text explaining their movements.

eBird Abundance Models provide an unparalleled window into the full annual cycle of bird populations in the Western Hemisphere. These species distribution models have been specifically developed for eBird data by statisticians and researchers at the Cornell Lab of Ornithology. These models have been used in a number of papers and reports including three State of the Birds Reports, most recently in 2016. eBird Abundance Models take eBird data from birdwatchers in the Western Hemisphere, add a suite of environmental variables, and generate predictions of where birds are on the landscape. The power of these models is the ability to standardize the variety of data that people collect in their eBirding, and also extrapolate beyond the data we have to make predictions in places where nobody has ever eBirded. Each 8km pixel in these models gives a specific prediction for the expected number of individuals you could expect to encounter in that region if you went birding for one hour at 7am covering one kilometer of distance. Notable in these models is that they’re actually predicting the number of birds you’d encounter, which is why counting birds matters! Understanding abundances of birds is immensely more valuable for conservation, since it lets you know where the bulk of the population is at any given time, instead of just understanding whether any individuals of that species is present (which is all that you can know from an “X” on your checklist).
 

Barn Swallow (Hirundo rustica) may be the most widespread and well-known passerine, with highly migratory populations that span the Americas as well as Europe, Asia, and Africa. Related species, and some Barn Swallow subspecies, are resident in Africa, the Middle East, and Australia. This remarkable variability makes Barn Swallow a great case study in migratory behavior. The varied migratory behaviors in this species globally are also seen on a smaller scale in the Americas, as this new eBird Abundance Model helps illustrate.

Barn Swallow breeds in pairs or small colonies widely across the United States and southern Canada—absent or scarce only in the driest desert, highest mountains, and colder Arctic regions. It mostly requires human structures for nesting so does not extend too far into Canada as settled areas become more rare, and while it breeds in central Mexico, it does not breed in the hotter and more humid coastal regions, as well as Central America and the Caribbean. Like almost all aerial insectivores (birds that eat flying insects), it has shown marked population declines in recent years, possibly related to widespread pesticide use among other potential causes.

Like several other swallows that move to South America, fall movements already are evident by early July, with many birds spreading out in dispersive movements to prospect future breeding sites. Others are already migrating south in July, and by 25 July the animation shows their arrival in Central America, with northern South America lighting up just after. Barn Swallows continue to flow southward until late October, with many birds staging before migration in large communal roosts in wetlands and agricultural areas. These roosts tend to occur in tall grass and are located in areas with abundant flying insects that allow these birds to fatten up for a long migration. Watching at dusk as a vortex of thousands of swallows descend into tall grass for their night roost is one of the joys of birding in late summer and fall. 

Barn Swallows molt in winter, after their migration, unlike certain other swallows (e.g., Tree Swallow) that have much shorter migration distances. During the winter they may occur almost anywhere south of the U.S. border as far south as northern Chile and central Argentina (Buenos Aires province). Just a few highly localized areas in the southern U.S. get wintering Barn Swallows, such as the Salton Sea area of California, which is highlighted on the winter maps (e.g., 26 December) in the animation. Wintering Barn Swallows mostly occur in large flocks, centered again on areas with abundant flying insect life such as marshes, river edges, and moist agricultural regions. Large flocks spread out across the landscape by day and gather in large roosting flocks at night, all the while replacing their body feathers and flight feathers so they will have a fresh set of feathers for their spring migration and upcoming breeding season.

Northward migration is underway early, by late January, with the first hints of arrival in southern California and Texas appearing on the 8 February map. By early March, this vanguard may already be beginning to nest in Texas and California, even as a flood of migrants from farther south begins to arrive. From March to May, Barn Swallows push into the country on a broad from from the West Coast to the Eastern Seaboard, with an especially strong push through the Midwest and Great Plains where the species can be especially common. It is not until mid to late May that the most northern breeding areas are filled in.

This map is awash with color, which obscures a lot of migratory complexity, although it may be possible to get some hints of the movements of some subpopulations. Watch to see if you can see the following, which has been determined from stable isotope analysis by Hobson et al. (2015): West Coast birds moving to-and-from Mexico; Prairie Provinces birds moving to-and-from Central America; eastern birds moving to-and-from eastern and southern South America. Within the east, southeastern birds make a comparatively short jump to northern South America while it is the more northerly birds that are the champion migrants, making a longer jump that "leapfrogs" over northern South America, crosses the equator, and winters mostly south of the Amazon even as far as Argentina. These birds likely employ an over-water route in fall and a much more westerly spring route through South America and the Yucatan, jumping across the Gulf of Mexico from there. This looped migration is common in many species (e.g., Veery, Blackpoll Warbler, Hudsonian Godwit) and takes advantage of favorable tailwinds from an ocean crossing from North America to South America, but takes care to migrate over land on the return voyage since westerlies would be a headwind that would blow them far into the Atlantic if they attempted the same route in spring.

A final interesting wrinkle in Barn Swallows is playing out in Argentina. Watch for the strong signal for the species in the Buenos Aires area from November through January. While many Barn Swallows overwinter here, much of the strength of that signal applies to breeding birds. Since the early 1980s, a few Barn Swallows have successful bred in this area of Argentina and in the past decade the population has exploded, with many thousands now breeding across the entirety of Buenos Aires province and expanding into surrounding areas, where fairly recently-built bridges and culverts seem to be providing ideal breeding areas. It was only this year that we conclusively learned (through the use of light-level geolocators; Winkler et al. 2017) the "wintering" grounds of these birds, which apparently head to northeastern South America for the austral winter. Interestingly, Barn Swallows on the other side of the Atlantic have also bred in South Africa, giving some insight as to how new migratory strategies develop.

To learn more about the life history of Barn Swallow, please consider a subscription to Birds of North America (just $42/year), where experts in the species have written a full-length species account covering all aspects of the species biology, from migration to diet to conservation status. 
 

Indigo Bunting (Passerina cyanea) is a familiar summer bird in the eastern United States and southern Canada, with males sporting an almost unnatural hue of bright blue that is richer on the face than on the breast and back. In the fall and winter males lose their blue plumage and resemble the brown females.

In the summer, Indigo Buntings breed in brushlands, open forests, powerline cuts, and woodland edges, and are often seen singing from conspicuous treetop perches or even roadside wires. Like many shrubland species, they are declining as shrublands regenerate to forest or are lost to suburbia or large-scale agriculture. Indigos avoid northern forest, so they don’t penetrate far into Maine or Canada, are largely absent from the Adirondack Mountains and ne. Minnesota, and only breed in southernmost Ontario and Quebec where more southerly forest types predominate. These areas of depressed abundance are clearly on the summer maps. It is rare to see Indigo Buntings in urban or suburban areas, so the summer maps show conspicuous “holes” around large urban centers. Notice how the June and July maps (e.g., June 27) show conscious “holes” on the map at St. Louis, Atlanta, Washington D.C., Kansas City, Detroit, Shreveport, etc. At the western margin of the range you can see the species drop out as the prairies give way to the high plains, which are drier and have fewer trees. However, this species does turn up in small numbers in wetter river valleys through Arizona, Utah and other western states, and this is shown on the summer maps as very low predictions in areas where those habitats occur (e.g., the upper Rio Grande Valley in New Mexico). In this portion of the range, Indigo Buntings may hybridize with Lazuli Bunting, which is more a bird of brushy western mountain slopes.

In both spring and fall, Indigo Buntings show a fairly expected pattern of movement, gradually withdrawing from their summer range from September to November and gradually arriving from March to May. Not all species show such a gradual arrival and departure, as you will see in future eBird Abundance Model maps. During migration periods, watch for Indigo Buntings to “stack up” along the Gulf coast. This is obvious in the fall, when the Gulf coast starts lighting up in mid-October and remains bright even after most have left the country. These are birds presumably using coastal lowlands to fatten up prior to making their southward overwater flight, along with some migrants that may "fall out" on the coast. A similar effect is seen in spring, as new arrivals refuel after a Gulf coast crossing but before moving inland to breed: for example, notice the strong Gulf coast signal on the 21 March map. Indigos that winter the Yucatan and eastern Central America are trans-Gulf migrants, crossing the Gulf of Mexico on their spring and fall migrations.

In winter, Indigo Buntings are found in brushy areas and forest clearings on both slopes of Mexico. Indigos may mix with other Passerina buntings in mixed flocks where one can find Indigo, Painted, and Blue Grosbeak together in on the Caribbean slope of Mexico and Central America and can have those species plus Lazuli, Varied, and Orange-breasted Buntings together on the Pacific slope. A few Indigos winter in the southern United States, where they are often found at bird feeding stations, especially in Florida and south Texas. In spring, feederwatchers can hope to see the bright males farther north when they first arrive, but their diet usually shifts to insects by late spring and summer, which makes them always a treat to see at feeders.

 To learn more about the life history of Indigo Bunting, please consider a subscription to Birds of North America (just $42/year), where experts in the species have written a full-length species account covering all aspects of the species biology, from migration to diet to conservation status.
 


Wood Thrush (Hylocichla mustelina) is a well-known forest species, especially so in the eastern United States where its melodic song rings through eastern forests from May to July. It migrates to Central America for the winter, where it also prefers dark, close-canopy broadleaf forests, very similar to its breeding grounds. During the winter it is much less conspicuous, largely detected by its chattering call notes and occasionally flushed from forest trails.

Wood Thrushes really depend on the dense tropical broadleaf forests of the Yucatan Peninsula, eastern Guatemala, and Belize: the Maya Forest. The winter map (e.g,, 4 January) shows a hotspot of highest abundance right in the central southern Yucatan Peninsula and northern Guatemala. Lower abundance extends into Veracruz and south to Costa Rica and Panama, primarily on the wetter Atlantic Slope. Wood Thrushes maintain territory in winter, and are not known to move around at all during this period.

Once late March arrives, one can clearly see the first Wood Thrushes making landfall on the Gulf Coast. Wood Thrushes are trans-Gulf migrants, jumping directly across the Gulf of Mexico with a suite of other migrants that risk a long flight over open water instead of taking a longer route around the Gulf. Once in the US, Wood Thrushes rapidly fill in eastern forests east of the Great Plains.

On their breeding grounds Wood Thrushes have been imperiled, with factory emissions from the Great Lakes and Midwest of the US being implicated in their declines. As those emissions drift eastward with the wind, precipitates affect forest floor invertebrates, with several studies tying acid rain to reduced food resources for Wood Thrushes and their young. More recently, heavy metals have been a concern.

Like many thrushes, Wood Thrushes get very secretive and difficult to detect during the late summer. They largely stop singing, get very secretive, and their detection rate on eBird lists drops drastically. This is evident on the maps as the relative abundance drops continually from early August to mid-September without any indication of southward movement. These birds are not migrating during this period, and it is not until the 19 September map that migration is evident. Just a week later the first signal becomes evident in the Yucatan.

This is a pattern that is shown by many species and it is important to understand that this does not represent a drop true abundance, but rather a drop in the detection rate. Accounting for changing detectability is one of the holy grails of relative abundance modeling and is something Cornell Lab researchers are trying to account for better in these models.

As the fall migration wraps up in November, this species makes its return to Middle America, with most heading to the Maya Forest of the Yucatan Peninsula. Watch the sequence from July to November to see just how many Wood Thrushes pack into a relatively small region. Clearly the food resources of the super-productive tropical forest here allow for a denser wintering population, since Wood Thrushes from across the entire eastern U.S. and southern Canada all end up packing into a region of Middle America that is a tiny fraction of the size of their breeding range. Consider the conservation implications: loss of wintering habitat will affect many more Wood Thrushes in Central America. The Maya Forest here is under pressure from deforestation, so the future of Wood Thrushes and other Maya Forest species is far from secure.

To learn more about the life history of Wood Thrush, please consider a subscription to Birds of North America (just $42/year), where experts in the species have written a full-length species account covering all aspects of the species biology, from migration to diet to conservation status.
 
Greater Yellowlegs (Tringa melanoleuca) is a migratory shorebird that occurs from southernmost South America to the northern boreal forests. Small groups overwinter and migrate through wetlands, but in summer males scold intruders from the peaks of spruce trees, possibly to protect their nest at the base of that same spruce!

See photos | Listen to audio

One of the chief challenges of eBird Abundance Models to date has been to accurately portray the range and movements of waterbirds in addition to landbirds. Thanks to recent advancements in the models, Greater Yellowlegs is one of a suite of shorebirds now being modeled from eBird data.

Season-specific information

Winter
Greater Yellowlegs is one of the more widespread shorebird species and, along with Sanderling, it may have one of the most extensive winter ranges. It is one of the shorebirds most likely to be found of Christmas Bird Counts in the United States, with regular wintering as far north as Massachusetts and Maine on the East coast and British Columbia on the West coast. Although most winter in coastal estuaries, in warmer regions they can be quite common at inland marshes and freshwater wetlands. Even small water bodies may host a Greater Yellowlegs: often it will be the only shorebird species at a small inland pond.

They occur throughout South America in winter, more commonly on the coasts, as far south as central Argentina and the southern portions of Chile. As seen on the map, the pampas--extensive seasonally wet grasslands south and west of Buenos Aires--is a really important wintering area for this species in Argentina. Unfortunately one of the areas with large concentrations of the species is northeastern South America, where thousands spend the winter (and hundreds oversummer), especially in the Brazilian states of Pará, Maranhão, and Amapá as well as French Guiana. eBird Abundance Models don't have enough data to work with to make predictions in this area; these areas badly need more eBird data so that we can produce more accurate models in these regions. Enter your records please if you have them!

Spring
Greater Yellowlegs are one of the earlier spring migrants, with early hints of northward movement visible along the Mississippi River Valley in the last week of February. During March they stream northward, showing up in coastal areas where they did not winter as well as inland marshes and pond edges as winter frost begins to lose its grip. A wet flooded field with puddles can be perfect habitat and full of yellowlegs.

The smaller cousin of Greater Yellowlegs, the Lesser Yellowlegs (Tringa flavipes) often occurs together in the same areas but tends to be a later migrant by two weeks or more. This migration timing can be a good aid to identification in spring.

Summer
Greater Yellowlegs is a forest bird in summer, occurring in bogs, muskeg, and clearings in spruce forest. They aggressively guard their territories with their loud calls. As with many species of central and northern Canada, eBird data are sparse so the eBird Abundance Models have less information to work from, which is apparent from the fairly distinct "no prediction" edge to the range in the north.

Even though the model limits itself from making predictions far to the north, the range shown is quite accurate, showing the main breeding area in the northern Prairie Provinces (Manitoba, Saskatchewan, and Alberta) extending east to northern Ontario, Quebec, and Labrador and west to parts of Alaska. It does not breed in the mountains much, so British Columbia is relatively devoid of yellowlegs.

As with many shorebirds, a number of birds (especially immatures) oversummer on the winter range, and this is evident on the animation and accounts for the obvious signal along the coasts of southern Brazil, Uruguay, and Argentina during June. Some of these oversummering birds may have significant parasite loads that prevent migration: see the BNA account for more information on this.

Autumn
Fall migration begins very early for Greater Yellowlegs, with the first southbound birds arriving in late June (note the 27 June map, which shows obvious arrival in the Lower 48 United States). They also have one of the longest migration windows of any American bird, as migrants can be found anytime from late June to mid-November. As with all shorebirds, the southward migration of adults precedes the migration of juveniles. In the field, you'll see the southward migration of adults peaking late June to mid-September and juveniles from mid-August to mid-November.

Although the fall movement overlaps broadly with Lesser Yellowlegs, the migration timing still differs, with Greater Yellowlegs still on the move into November while most Lessers have reached their wintering range by late October.  

To learn more about the life history of Greater Yellowlegs, please consider a subscription to Birds of North America (just $42/year), where experts in the species have written a full-length species account covering all aspects of the species biology, from migration to diet to conservation status. 
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