Importance of North America's Grasslands

IMPORTANCE OF NORTH AMERICA’S GRASSLANDS TO BIRDS

PETER BLANCHER

for the

COMMISSION FOR ENVIRONMENTAL COOPERATION

MAY 2003 PREFACE

Considered the largest of the vegetation formations found in North America, the grassland ecosystem is the only continuous terrestrial ecosystem linking Canada, the United States and Mexico. Yet within the last 150 years relentless exploitation has turned this into the most threatened ecosystem of the region. Furthermore, the challenge of conserving this ecosystem is not only regional but global, since grasslands are also considered worldwide among the most imperiled ecosystems.1

The severe degree of loss of this ecosystem throughout North America is reflected in numerous ways at the species level. In the 19th century with the almost total annihilation of the tens of millions of bisons, heralding the end of an era of Native American prairie cultures whose livelihood depended on this species. The decline of grasslands species is still present–as expressed by the steady reduction, in the last 35 years, of the population of most grassland bird species. Recent studies, including this one prepared by Birds Studies Canada’s Pete Blancher, have shown that, as a group, avian grassland species have been more subject to population declines than any other group in any other ecosystem in North America.

In this context, the central grasslands of North America offer a unique opportunity for cooperation to conserve our shared natural endowment from the species to the ecosystem level. What started in the year 2000 as an initiative prompted by the Commission for Environmental Cooperation (CEC) www.cec.org aimed at the recovery and conservation of a portfolio of species of common concern2, soon turned into a comprehensive effort looking into the major drivers that threaten these species and their habitat, highlighting the need for an ecosystem approach. The latter is manifest in the recently published “Grasslands: Toward a North American Conservation Strategy.” 3

The present study “Importance of North America’s Grasslands to Birds” forms part of an overall CEC effort to help develop a cogent ecosystem approach related to species´ conservation by assisting individuals, institutions and organizations to become aware of their role in “the big picture” and to better realize their potential to collaborate. All in all, this approach aims to facilitate coordination among regional multi-stakeholder efforts and to allocate their resources in a more effective way pursuant their conservation and sustainable use goals.

Through this report, the CEC is honoured to support Birds Studies Canada’s series of landmark continental bird assessments. The seminal idea of this work benefited greatly from the insights provided by Dr. Art Martell.

Hans Herrmann, Head, Biodiversity Program Jürgen Hoth, Program Manger, Biodiversity Program North American Commission for Environmental Cooperation (CEC)

1 WRI, 2000. Pilot Analysis of Global Ecosystems: Grassland Ecosystems. World Resources Institute. 2 The complete list of species of common conservation concern (SCCC) can be found in www.cec.org/files/PDF/BIODIVERSITY/SCCC-Web-e_EN.PDF 3 See www.cec.org/pubs_docs/documents/index.cfm?varlan=english&ID=1246

1 INTRODUCTION

North America’s grasslands are a special place for birds and other wildlife. Our intent with this project is to provide information to substantiate this view, for birds.

Our specific objectives in this report are:

• to use bird survey data and range maps to measure and illustrate the importance of the central grasslands to the North American avifauna; and

• to demonstrate the interdependency of Canada, Mexico and the United States for the yearly life cycle of migratory birds

SCOPE

The project focuses on the contiguous central short-grass/mixed-grass prairie ecosystem ranging from Canada to Mexico (Fig. 1), defined by the following 5 Bird Conservation Regions (BCRs): BCR 11 - the Prairie Potholes, BCR 17 - Badlands & Prairies, BCR 18 - the Shortgrass Prairie, BCR 19 - the Central Mixed Grass Prairie, and BCR 35 - the Chihuahuan Desert (Fig. 2).

Figure 1: Extent of the central grasslands in North America

Figure 2: Bird Conservation Regions making up the central grasslands

GRASSLAND ASSOCIATED BIRDS

Which species of birds are of most closely associated with the central grasslands, during breeding and/or winter seasons?

To answer this question we relied on a combination of range maps and bird survey data.

NatureServe digital range maps provide the simplest way to identify species using the central grasslands. For example, it is clear from range maps that Worthen’s Sparrow is heavily reliant on grasslands in northern Mexico year-round (Fig. 3).

Figure 3: Range maps of 2 grassland associated species (adapted from NatureServe maps).

We calculated the proportion of breeding, wintering and year-round range in the central grasslands, for all species with any range in the grasslands. For example, Spraque’s Pipit has a high proportion of both breeding and winter range in the central grasslands, though it is a migrant (Fig. 3).

But range maps can be a misleading indication of distribution. For example, Lapland Longspur winter range is very broad (Fig. 4), with only 27% of Western Hemisphere range in the central grasslands. Christmas Bird Counts show (Fig.5), however, that this species tends to be much more concentrated in winter, with an estimated 90% of western hemisphere birds wintering in the central grasslands (averaged across 8 years of winter counts in the 1990s).

Figure 4: Lapland Longspur range map (from NatureServe)

Figure 5: Lapland Longspur abundance on Christmas Bird Counts (from BirdSource)

So it is useful to use abundance data where they are available.

We used Breeding Bird Survey (BBS) data to estimate proportions of breeding population in the Canadian and U.S. parts of the grasslands. There is very good BBS coverage in the U.S. and Canadian grasslands (Fig. 6). Range maps were used to estimate proportion of breeding population in Mexico.

Figure 6: Grasslands coverage by Breeding Bird Survey routes

Christmas Bird Counts were used to estimate proportion of winter populations in the U.S. and Canadian parts of the grasslands. There is reasonably good coverage of grasslands in U.S. and Canada (Fig. 7). Again, winter range maps were used for Mexico.

Figure 7: Grasslands coverage by Christmas Bird Counts

5 NUMBER OF GRASSLAND BIRD SPECIES

A large number of species make use of the grasslands - 536 species have at least some population in the grasslands (Table 1).

Number of Grassland Species

% of Any Year- Population Season Breeding Winter Round

any 536 484 457 405

>1% 394 330 323 261

>25% 128 105 62 39

>50% 42 39 14 11

Table 1: Numbers of bird species in the grasslands by season

Numbers of species are spread realtively evenly across the grasslands during the breeding season, with a tendency for lower numbers of species in the central portions (Fig. 8). In winter, species richness shifts to the south, so that there is a strong north-south gradient in species richness across the grasslands.

Figure 8: Distribution of grassland species richness, by season

Over 100 species are estimated to have 25% or more of their population in the grasslands in at least one season, with almost a third of those making significant use of the grasslands year- round. Fourty-two species are estimated to be highly dependant on the grasslands for at least 50% of their population (Table 1).

6 Birds that are highly dependant on the central grasslands throughout the year are listed in Table 2; these are mainly sparrows and upland game birds. Several are resident species, but a few are also migratory. All are landbirds.

Grassland-Dependant: Year-Round

Species Resident Breeding Winter Lesser Prairie-Chicken Tympanuchus pallidicinctus 100% Greater Prairie-Chicken Tympanuchus cupido 90% Sharp-tailed Grouse Tympanuchus phasianellus 83% Worthen's Sparrow Spizella wortheni 71% Scaled Quail Callipepla squamata 67% Maroon-fronted Parrot Rhynchopsitta terrisi 54% Baird's Sparrow Ammodramus bairdii >99% 65% Chestnut-collared Longspur Calcarius ornatus 99% 55% Lark Bunting Calamospiza melanocorys 94% 56% McCown's Longspur Calcarius mccownii 78% 85% Cave Swallow Petrochelidon fulva 68% 86%

Table 2: Species dependant on the grasslands year-round

The Greater Prairie-Chicken is an example of a resident species that is largely confined to the central grasslands (Fig. 9). While the Chestnut-collared Longspur migrates between the northern and southern portions of the grasslands.

Figure 9: Range maps of two species dependant on the grasslands year-round (see also Figure 3)

A larger number of grassland-dependant birds are mainly reliant on the central grasslands for breeding (Table 3). These birds are a much more diverse taxonomic group, and it is clear that during the breeding season, the grasslands is important to several shorebirds, waterfowl and waterbirds in addition to migratory landbirds.

7 Grassland-Dependant: Breeding Season

Species Breeding Winter Sprague's Pipit Anthus spragueii 96% 34% Colima Warbler Vermivora crissalis 94% 0% Marbled Godwit Limosa fedoa 84% 0% Upland Sandpiper Bartramia longicauda 83% 0% Mountain Plover Charadrius montanus 79% 29% Blue-winged Teal Anas discors 77% 11% Swainson's Hawk Buteo swainsoni 76% 0% Mississippi Kite Ictinia mississippiensis 74% 0% Cassin's Sparrow Aimophila cassinii 72% 37% Western Meadowlark Sturnella neglecta 72% 26% Grasshopper Sparrow Ammodramus savannarum 69% 13% Piping Plover Charadrius melodus 66% 0% Chihuahuan Raven Corvus cryptoleucus 66% 35% Yellow-headed Blackbird Xanthocephalus xanthocephalus 65% 35% Western Kingbird Tyrannus verticalis 65% 3% Ferruginous Hawk Buteo regalis 65% 49% Canvasback Aythya valisineria 65% 10% Black Tern Chlidonias niger 64% 0% Wilson's Phalarope Phalaropus tricolor 58% 0% Black-capped Vireo Vireo atricapillus 56% 0% Lark Sparrow Chondestes grammacus 55% 19% Gadwall Anas strepera 55% 12% Nelson's Sharp-tailed Sparrow Ammodramus nelsoni 52% 0% American Coot Fulica americana 52% 9% Northern Shoveler Anas clypeata 51% 12% Willet Catoptrophorus semipalmatus 51% 0% Marsh Wren Cistothorus palustris 51% 15% Long-billed Curlew Numenius americanus 50% 21%

Table 3: Species dependant on the grasslands during the breeding season

Figure 10: Range maps of 2 species dependant on the grasslands in the breeding season

Some like the Swainson’s Hawk are long-distance migrants, in this case wintering mainly in South America (Fig. 10). While others like the Colima Warbler migrate shorter distances.

Finally, a few grassland-dependant birds are mainly reliant on the grasslands outside of the breeding season (Table 4). The Sandhill Crane is an example of a bird most reliant on the central grasslands in winter (Fig. 11). Lapland Longspur is heavily dependant on the central grasslands for the wintering range of its western hemisphere population.

Grassland-Dependant: Non-breeding Season

Species Winter Breeding Smith's Longspur Calcarius pictus 74% 0% Sandhill Crane Grus canadensis 63% 5% Common Merganser Mergus merganser 55% 1%

Table 4: Species dependant on the grasslands during the non-breeding season

Figure 11: A species dependant on the grasslands during the non-breeding season

ABUNDANCE OF GRASSLAND BIRDS

We examined broad patterns in abundance and distribution of grassland birds, again using range maps and bird surveys.

A variety of adjustments are used to convert survey data into population estimates – these are described in more detail in Rosenberg & Blancher (2003).

A time of day correction is calculated for each species, as shown for Western Meadowlark and Common Nighthawk (Fig. 12). This factor adjusts BBS average abundance across full routes to

9 the time of maximum detectability (about 30 minutes after sunrise for the meadlowlark, pre- dawn for the nighthawk). Time of day correction for Scaled Quail (1.73) is intermediate between those of meadowlark and nighthawk.

Figure 12: Time of day corrections, based on stop by stop BBS data

Birds are categorized according to estimated detection distance, in order to estimate density from BBS relative abundance. Density is then extrapolated to area of each Region in order to obtain a rough estimate of population size by region. An example of the calculations involved is shown for Scaled Quail in the U.S. portion of the Chihuahuan Desert (Table 5). Note that these are

10 rough estimates only, without statistically-defined confidence limits, but validation with independent estimates suggests they are typically well within the correct order of magnitude.

Scaled Quail Population Estimate BCR35 - Chihuahuan Desert (U.S. Part)

A. Quail per BBS route in BCR 35 12.7 pairs B. Quail time of day adjustment 1.73 C. Quail per BBS at peak time of day (= A x B) 22.0 pairs D. Quail detection distance on BBS (approx.) 400 m E. Quail BBS detection area per route (50 stops) 25.1 km2 (= π x 50 x D x D) F. Quail density in BCR 35 (= C / E) 0.87 pairs / km2 G. Area of BCR 35 (U.S. Part) 188,222 km2 H. Quail population, BCR 35 U.S. (= F x G) 164,689 pairs I. Quail population, BCR 35 U.S. (= H x 2) 329,379 birds

Table 5: Example of a regional population estimate from BBS data

Estimates are then added across BCRs for the total grassland population estimate (as in Table 6). Estimates for the Mexican portion of BCR 35 are based on range in this region multiplied by density in the U.S. portion of BCR 35.

Scaled Quail Population Estimate Central Grasslands BBS Breeding average Birds BCR 11 - Prairie Potholes 0.0 0 BCR 17 - Badlands & Prairies 0.0 0 BCR 18 - Shortgrass Prairie 2.8 148,882 BCR 19 - Central Mixed Grass Prairie 0.1 5,732 BCR 35 - Chihuahuan Desert (U.S.) 12.7 329,379 BCR 35 - Chihuahuan Desert (Mexico) * 675,525 Total 1,159,518 Estimated Grasslands Population = 1,200,000 Birds

* based on range in Mexico, density in U.S.

Table 6: Example of grasslands population estimate

11 Putting these estimates together for all species, about 3/4 of a billion breeding birds live in the central grasslands, based on extrapolations from the Breeding Bird Survey (Table 7).

Estimated Grassland Bird Population

Species Breeding Birds Red-winged Blackbird 57,000,000 Horned Lark 49,000,000 Mourning Dove 44,000,000 Cliff Swallow 34,000,000 Lark Bunting 26,000,000 Common Grackle 26,000,000 Brown-headed Cowbird 24,000,000 House Sparrow 23,000,000 Western Meadowlark 22,000,000 American Robin 21,000,000

All Species (365 with estimates) 735,000,000

Table 7: Population estimates for most common grassland birds

This estimate is conservative, as it does not include estimates for some species breeding only in the Mexican part of the grasslands. And in late summer, the addition of juveniles raises the estimated total population to well over a billion birds. The most common birds tend to be wide- ranging landbird species not strongly dependant on the grasslands (Table 7).

Winter counts are dominated by flocking birds (Table 8).

Grassland Bird abundance in Winter

Species CBC Birds/Count Red-winged Blackbird 2,730 Canada Goose 2,105 Mallard 1,091 Common Merganser 268 Horned Lark 257 Brown-headed Cowbird 244 Snow Goose 226 Sandhill Crane 218 American Crow 172 Lapland Longspur 166

All Landbirds 7,316 All Birds 11,842

Table 8: Most common birds on grassland Christmas Bird Counts

12 These counts are likely biased against native grassland habitats, so extrapolation to total population estimates of birds in the grasslands is not attempted here. However this does not diminish the value of using CBC counts to show winter distribution within species.

SEASONAL DISTRIBUTION OF GRASSLAND BIRDS

A map of the breeding distribution of grassland birds can be created by stacking the range maps of all species in the grasslands, weighting each species by percent of population breeding in the central grasslands (Fig. 13). Such a map shows the relatively even distribution of species across the central grassland BCRs, as seen previously in the species richness map (Fig. 8).

Figure 13: Weighted distribution of breeding grassland birds in summer

In winter, these species become more concentrated in their distribution, typically in southwest United States and Mexico (Fig. 14). Much of the winter concentration remains in the central grasslands - because many grassland species are short-distance migrants or residents.

Figure 14: Weighted distribution of breeding grassland birds in winter

Winter grassland birds (those with highest % of winter population in the central grasslands, based on estimates from Christmas Bird Counts) show a very similar distribution, concentrated in the southern half of the grasslands, though more birds are distributed in northern grasslands than is true of the wintering breeders (Fig. 15).

Winter grassland birds show a strong tendency to come from breeding grounds in the central grasslands and from nearby intermountain areas to the west (Fig. 16). There are are also a significant number of birds and species that come from breeding grounds further north.

Again the bulk of birds reliant on the grasslands in winter are short-distance migrants.

Figure 15: Weighted distribution of winter grassland birds in winter

Figure 16: Weighted distribution of winter grassland birds in summer

15 CONSERVATION STATUS OF GRASSLAND BIRDS

The current conservation status of grassland birds is of particular interest, since most of the original prairie has been converted to agriculture and other land uses since the time of European settlement. What is the status of the grassland-dependant species shown above?

Some grassland species, like the Lesser Prairie-Chicken and Black-capped Vireo have been widely extirpated from their former ranges (Fig. 17).

Figure 17: Examples of species extirpated from parts of their grassland range

According to Breeding Bird Survey trend information, MOST grassland species have undergone significant population declines over the past 35 years. As a group, grassland species have been more subject to population declines than any other habitat guild (Fig. 18).

70%

60%

50%

40%

30%

20%

10%

0% % Significant Declines (P < 0.1) Grassland Urban Scrub Woodland Wetland All Species Guilds, by Habitat

Figure 18: Percent declining BBS trends, by habitat guild

The geographic pattern of population declines highlights the central grasslands as THE area of most pervasive declines (Fig. 19). Here the geographic pattern is illustrated using by Partners in Flight scores for declining trends (dark colour and high score reflects more severe declines). The pattern of declines for wintering birds also highlights the central grasslands, though shifted further south (Fig. 20).

Figure 19: Distribution of average PIF population trend scores, breeding season

Figure 20: Distribution of average PIF population trend scores in winter

Note that the pattern from the U.S. and Canada is extrapolated into Mexico assuming that shared species show similar trends into Mexico - the patterns in Mexico are therefore hypothetical, and do not include trend data from Mexican endemic species. However, comprehensive species assessment is currently being conducted in Mexico and trend scores based on expert opinion will be available in the near future.

Population declines are prominent among species earlier identified as most heavily dependant on the central grasslands. But population declines are not the only indicator of conservation concern for grassland birds. Average Partners in Flight (PIF) assessment scores are all high for grassland-dependant species, i.e. these birds often have small populations, restricted ranges, and high perceived threats to future population health (Fig. 21).

3.8 3.6 3.4 Grasslands 3.2 3.0 Species 2.8 PIF - All 2.6 Landbirds Average PIF Score 2.4 2.2 2.0 Pop'n Range Range Threats Threats Pop'n Size Size Size (Br) (Wi) Trend (Br) (Wi)

Figure 21: PIF assessment scores for grassland-dependant species vs all landbirds

Grassland birds are clearly a group in need of conservation attention.

GRASSLAND SPECIES TARGETED AS PRIORITIES

The Prairie Potholes (BCR 11) portion of the central grasslands has been identified as highest continental priority for breeding ducks, according to the North America Waterfowl Management Plan (draft 2003 Update). For non-breeding ducks, the Shortgrass Prairies (BCR 18) is a moderate to high priority region.

18 Several grassland species have been identified in conservation plans as high priorities for attention at the continental level. Of the 42 species that are most heavily dependant on the central grasslands (Tables 2-4), 37 have been assessed for priority/concern, and 23 of these have been identified as moderate to high continental priority or conservation concern (Table 9).

North American Waterfowl Management Plan - draft 2003 Update Blue-winged Teal Anas discors Moderate to High Priority Gadwall Anas strepera Moderate Priority Northern Shoveler Anas clypeata Moderate Priority Canvasback Aythya valisineria Moderate Priority North American Waterbird Conservation Plan - Version 1 2002 Black Tern Chlidonias niger Moderate Concern Canadian Shorebird Conservation Plan 2000 / U.S. Shorebird Conservation Plan 2001 Long-billed Curlew Numenius americanus Highly Imperiled Piping Plover Charadrius melodus Highly Imperiled Mountain Plover Charadrius montanus Highly Imperiled Marbled Godwit Limosa fedoa High Concern Wilson's Phalarope Phalaropus tricolor High Concern Upland Sandpiper Bartramia longicauda High Concern (U.S.) Willet Catoptrophorus semipalmatus Moderate Concern Partners in Flight North American Landbird Conservation Plan - draft 2003 Lesser Prairie-Chicken Tympanuchus pallidicinctus Highest Concern Black-capped Vireo Vireo atricapillus Highest Concern Swainson's Hawk Buteo swainsoni High Concern Greater Prairie-Chicken Tympanuchus cupido High Concern Scaled Quail Callipepla squamata High Concern Sprague's Pipit Anthus spragueii High Concern Baird's Sparrow Ammodramus bairdii High Concern Colima Warbler Vermivora crissalis Biome-restricted McCown's Longspur Calcarius mccownii Biome-restricted Nelson's Sharp-tailed Sparrow Ammodramus nelsoni Biome-restricted Smith's Longspur Calcarius pictus Biome-restricted Table 9: Grassland dependant birds identified as moderate to high continental priority / concern

It is apparent that many shorebirds and landbirds in particular are of very high continental concern in the grasslands, due to a combination of population declines, threats and small remaining populations. When Mexican species assessment has completed, there may be additions to this list, since some grassland dependant species such as Maroon-fronted Parrot and Worthen’s Sparrow are endemic to Mexico with quite restricted distributions.

19 INTERDEPENDENCE OF CANADA, MEXICO AND THE UNITED STATES FOR GRASSLAND BIRD LIFE CYCLES

It is obvious from the shift in breeding to winter distribution of grassland birds that many short- distance migrants link the three countries (see earlier section on seasonal distribution of grassland birds, especially Fig. 13-16).

Bird Banding data provide direct evidence of links among countries by individual birds. Band recovery links between birds captured or recovered in the Canadian Prairies (in red) and the same birds captured and recovered elsewhere (in blue) are illustrated in Fig. 22. There is a strong north-south pattern of recoveries through grasslands of the United States to Mexico.

Figure 22: Band recovery links to the Canadian prairie portion of the central grasslands

The proportion of total species population that migrates between countries can be estimated using shifts in population and range between breeding and winter. Of the 42 species of highly- dependant grassland birds (Tables 2 to 4), 36 are migratory. Most (23) of these species are highly reliant on migration between Canada, the United States and Mexico, with an estimated 25% to 97% of their total population migrating between these 3 countries each year (Table 10). These include species that rely on the grasslands in either or both breeding and winter seasons. Exchange among countries is a mix of species that move between all three countries, or mainly between Canada and the United States, or between the United States and Mexico, or between Canada and Mexico.

20 Migratory Exchange between Canada, United States & Mexico Grassland-Dependant Birds

Net Movement Breeding to Winter Species % Population Exchanged (< 5% population in parentheses) Grassland-Dependent Year-round Baird's Sparrow 81% Canada + USA to Mexico Lark Bunting 56% (Canada) + USA to Mexico Cave Swallow 42% USA to Mexico McCown's Longspur 42% Canada + USA to Mexico Chestnut-collared Longspur 35% Canada + (USA) to Mexico Grassland-Dependent Breeding Sprague's Pipit 86% Canada to USA + Mexico Nelson's Sharp-tailed Sparrow 80% Canada to USA Long-billed Curlew 77% Canada + USA to Mexico Yellow-headed Blackbird 75% Canada + USA to Mexico Mountain Plover 68% USA to Mexico Marbled Godwit 67% Canada to USA + Mexico Canvasback 62% Canada to USA + Mexico Black-capped Vireo 60% USA to Mexico Lark Sparrow 56% (Canada) + USA to Mexico Grasshopper Sparrow 52% (Canada) + USA to Mexico Western Kingbird 44% (Canada) + USA to Mexico Northern Shoveler 43% Canada to (USA) + Mexico Marsh Wren 37% Canada + USA to Mexico Gadwall 29% Canada to (USA) + Mexico Ferruginous Hawk 29% Canada + USA to Mexico Grassland-Dependent Winter Smith's Longspur 97% Canada to USA Common Merganser 61% Canada to USA + (Mexico) Sandhill Crane 48% Canada to USA + Mexico Table 10: Percent Migratory Exchange of Grassland-Dependant Birds

In addition, there are several grassland-dependant species that winter south of Mexico but still rely on an exchange of birds among countries during migration. For example, Swainson’s Hawk (Fig. 10) and Upland Sandpiper are species that breed mainly in grasslands of Canada and the United States, with large numbers of migrants moving through Mexico on their way to South American wintering grounds.

As a group, sparrows and other members of the Sparrow Family (Emberizidae) highlight well the interdependence of the three countries. Most sparrows are typical of grassland habitats, are short-distance migrants, and many are closely associated with the central grasslands. Many have also shown important population declines.

21 SUMMARY

In summary, many findings illustrate the importance of the central grasslands to birds:

• Many species are largely reliant on the grasslands during breeding and/or non-breeding seasons, including a mix of landbirds, shorebirds, waterfowl and a few waterbirds.

• The central grasslands are home to about ¾ of a billion breeding birds, and more than a billion birds at the end of the breeding season

• Most grassland birds are short-distance migrants or residents, wintering in or near the southern end of the grasslands in winter

• Grassland birds have shown the most consistent and pervasive declines of all birds monitored by the Breeding Bird Survey, and are in need of conservation attention

• Most grassland birds are strongly dependent on grasslands in a combination of the 3 countries, with a high percent of total population migrating among countries.

ACKNOWLEDGEMENTS

Digital range map files were an invaluable starting point for several of the analyses and maps. The range files used in this report were provided by NatureServe in collaboration with The Nature Conservancy/Migratory Bird Program, Conservation International/CABS, World Wildlife Fund-US, and Environment Canada/WILDSPACE. Bruce Young (NatureServe) and Don McNicol (Environment Canada) provided additional information about use of the files. Andrew Couturier (Bird Studies Canada) converted the maps to areas and assigned ranges to degree blocks, in order that the these could be combined with other data by the author, and used to create the stacked maps shown in this report.

Many additional sources of data were used here and the author wishes to thank the thousands of volunteers and scientists that collected these data, and the individuals and institutions that made these data available, including Breeding Bird Survey data and Bird Banding Files (US Geological Survey and Canadian Wildlife Service), Christmas Bird Count data (National Audubon and Cornell Lab of Ornithology), and Partners in Flight data (Rocky Mountain Bird Observatory and the PIF Technical Committee).

The methods for estimating population sizes from BBS data were developed with Ken Rosenberg (Cornell Lab of Ornithology). Art Martell (NABCI-Canada) provided the idea for this work, and valuable input was received from Art, Jurgen Hoth (Commission for Environmental Cooperation), Michael Bradstreet (Bird Studies Canada), and Bob McCready (The Nature Conservancy).

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