Effects of Livestock Grazing on Birds in the West
Carl E. Bock, Ph.D.
Because livestock grazing is widespread, it has the potential to affect all bird species in western grassland, shrub/steppe, and riparian habitats. However, bird responses to livestock grazing are species--and habitat--specific, so that some populations benefit while others are harmed. Large, permanent livestock exclosures are needed throughout the West to provide critical habitat for the assortment of species that are negatively affected by livestock grazing.
Carl E. Bock is a professor of biology at the University of Colorado at Boulder. His research interests include ecology and conservation biology, especially as related to grasslands, birds, livestock grazing, and fire. He is the author of over one hundred scientific articles and book chapters and recently published a book, The View from Bald Hill: Thirty Years in an Arizona Grassland, with his wife, Dr. Jane Bock, about their work at the Audubon Society's Research Ranch Sanctuary - ungrazed by cows since 1968.
Livestock grazing is the most widespread economic use of both public and private lands in western North America, 1 and it potentially affects all birds associated with grassland, shrub/steppe, and riparian habitats. 2 Birds may be especially responsive to livestock grazing, compared with other native plant and animal populations. 3 North American Breeding Bird Survey data indicate that grassland species are experiencing national declines that are more widespread and dramatic than in any other bird group. 4 Therefore, we have reason to be concerned about continuing negative effects of livestock grazing on birds, and we ought to plan and implement conservation strategies designed to offset these effects.
|Hawk caught in a barbed wire fence.|
There is little doubt that domestic livestock frequently
have a controlling influence over the structure and function of the ecosystems
where they are grazed. Nevertheless, it is essential that we not overgeneralize
negatively or positively about effects of grazing on bird populations.
There are three reasons for being careful and case-specific in making our conclusions. First, a variety of factors, in addition to direct effects of livestock grazing, have changed western ecosystems. 5 It is important that bird population changes be properly attributed among these possible causes. Agricultural cultivation of former grasslands, dewatering of wetlands and riparian habitats, fire suppression, introduction and spread of exotic vegetation, extirpation of native herbivores such as bison (Bison bison) and prairie dogs (Cynomys spp.), predator and pest control, and habitat fragmentation due to agriculture and urban sprawl all have impacted western bird populations. Some, but not all, of these are indirectly related to livestock production, as, for example, dewatering of riparian habitats for irrigation of livestock forage.
Second, ecological consequences of livestock grazing depend very much on the historical association of particular sorts of ecosystems with large herds of native grazers, especially bison. Rangelands of the Intermountain West and Southwest apparently supported few if any bison, 6 and so large herds of domestic grazers represent an alien ecological force that has drastically changed these ecosystems from what they once were. Birds doubtless have responded accordingly. By contrast, grasslands of the central and western Great Plains had a long evolutionary association with large herds of bison, and here livestock may hold the grassland ecosystems in something more like their prehistoric condition than would an absence of domestic grazers. 7
Third, individual bird populations will respond differently to livestock grazing, depending on their particular habitat and resource needs. Some birds almost always respond negatively to effects of livestock, others usually respond positively, and still others vary in their responses, depending on the particular plant community and the type and intensity of grazing.
My objectives in this essay are four: to describe briefly the various ways that livestock can affect birds, either positively or negatively; to synthesize evidence from the literature about the direct effects of livestock grazing on bird populations in different western ecosystems; to review some case studies illustrating the range of responses of individual species, in particular habitats, to livestock grazing; and to make recommendations regarding livestock grazing and the conservation of bird populations in the American West in general, and on public rangelands in particular.
Livestock Grazing Impacts on Bird Populations
The brown-headed cowbird (Molothrus ater) is an obligate brood parasite - a bird that never builds its own nest and always lays its eggs in the nests of other bird species. 8 The host parents then incubate the cowbird eggs and feed the cowbird nestlings, often to the detriment of their own young. Cowbirds forage in shortgrass and edge habitats, usually following herds of grazing mammals that flush their insect foods. Brown-headed cowbirds originally followed bison herds and apparently were restricted to areas where bison were common. Cowbirds switched from bison to livestock when that option became available, greatly expanding cowbird numbers and range. They have since negatively impacted a wide variety of birds in places where avian nest parasites once were scarce or absent - for example, plumbeous vireos (Vireo plumbeus) in Colorado 9 and willow flycatchers (Empidonax traillii ) in California. 10
The principal means by which livestock grazing affects bird populations is by altering habitat structure and food availability. Grazing invariably reduces the height and ground cover of grasses, at least temporarily, thus depriving many birds of the cover they need, both as a refuge from predators and as a favorable thermal environment for roosting and nesting. Grazing-related loss of grass cover also decreases the frequency and intensity of fire in grassland and shrub/steppe ecosystems, thereby facilitating the spread of woody vegetation that otherwise might be outcompeted by grasses or killed by fire. These changes in habitat structure can significantly alter the avifauna by favoring species that use woody vegetation as cover, and by reducing species whose predator escape strategies require relatively open habitats. 11 Grazing also can reduce grass seed production, significantly affecting winter bird densities. 12
In riparian habitats, livestock can widen stream channels by trampling the banks, and grazing then reduces or eliminates recruitment of trees and shrubs. 13 These changes in riparian plant community structure can significantly change avian diversity, abundance, community composition, and reproductive success. 14
In 1993, three colleagues and I summarized the results from forty-two different studies published through the early 1990s that compared bird populations in grassland, shrub/steppe, and riparian habitats in the West that differed in terms of grazing history. 15 Individual birds species were categorized as responding positively, negatively, or in uncertain or mixed ways to the direct impacts of livestock grazing (see Table 1 below). An additional category that emerged just for grasslands was a group of nine species that appear to benefit from grazing in relatively lush mixed and tallgrass prairies of the central and eastern Great Plains while being negatively impacted in arid shortgrass, desert, and intermountain grasslands.
Relationships between birds and livestock grazing clearly are more complex than this simple summary suggests, since the comparisons involved study areas that differed in such variables as grazing intensity, season, and duration. In many cases there were no ungrazed areas available, and the comparisons involved lightly versus heavily grazed sites. Despite these confounding factors, the data summarized in Table 1 serve to make one point clear - that livestock grazing is neither universally beneficial nor harmful to bird populations, but that the responses are species- and habitat-specific. The data also suggest that livestock grazing has negatively impacted a higher proportion of species in riparian habitats than in grasslands, with shrub/steppe intermediate.
A few examples will serve to illustrate the species-specific nature of avian responses to livestock grazing and some of the complexities involved in considering such aspects as habitat type, direct versus indirect impacts, and historical versus contemporary effects.
The mountain plover (Charadrius montanus) breeds primarily in shortgrass prairies of the western Great Plains, where it requires disturbed and open patches of bare ground and very low stature vegetation, both of which are essential for foraging and for early visual detection of approaching predators. 16 Historically, this plover occurred in areas where bison and black-tailed prairie dogs (Cynomys ludovicianus) created these habitat conditions. The mountain plover has declined significantly in recent times, and the species has been proposed for federal listing as threatened. It has been argued that livestock are beneficial to this species because of its dependence on heavily grazed habitats, 17 and the plover is included in Table 1 as a species that usually benefits from livestock grazing. However, the situation is more complex, because of two negative impacts indirectly related to livestock production. First, livestock growers have eliminated prairie dogs from much of the historic range of the mountain plover. Second, one of the most significant negative effects on plovers is tilled agriculture, 18 and much of the product of that tilled agriculture is grain for livestock.
The grasshopper sparrow (Ammodramus savannarum) is a North American grassland specialist that has experienced widespread recent declines. 19 This species generally relies on intermediate levels of grass cover for nesting and foraging. 20 Grasshopper sparrows can virtually disappear from moderately to heavily grazed western grasslands 21 but may be dependent on grazing or other disturbances to create openings in tallgrass prairie.
The Botteri's sparrow (Aimophila botterii) is a southwestern tallgrass specialist that occurs north of Mexico only along the coastal plain of south Texas and in grasslands of southeastern Arizona. 22 So dependent is this species on heavy grass cover that it disappeared from Arizona between 1903 and 1932, following drought and overgrazing in the late 1890s. It remains dependent on stands of thick and tall grass cover, and today it occurs in Arizona only in such habitats. The related Cassin's sparrow (Aimophila cassinii) is another species that reaches highest densities only in those southwestern arid shrub/grasslands that are ungrazed or lightly grazed. 23 At the same time, this species requires some woody vegetation on its territories to serve as song perches. Historical grazing-related invasions of shrubs and mesquite into formerly pure grasslands may benefit the Cassin's sparrow, as long as adequate grass cover remains.
The Brewer's sparrow (Spizella breweri) likely has had a similar relationship to livestock grazing in shrub/steppe habitats of the Great Basin, profiting from early shrub increases but then suffering from continued grazing-related depletion of herbaceous ground cover. 24
Relatively little is known about the impacts of livestock grazing on birds of prey, perhaps because their large home ranges frequently exceed the sizes of habitat patches with different grazing histories, especially the sizes of most livestock exclosures. Species such as the short-eared owl (Asio flammeus) and northern harrier (Circus cyaneus), which both hunt and nest in relatively heavy ground cover, are likely to do best in areas with moderate to light or no grazing. 25 Species such as the ferruginous hawk (Buteo regalis) may benefit from a mosaic of habitat patches, including protected areas in which to place their ground nests, and grazed sites where prey may be more conspicuous and abundant. 26 At the same time, control of ground squirrels and prairie dogs in the name of enhancing livestock production will negatively affect this bird, as it will any raptor with a rodent prey base.
In riparian ecosystems with intact woodland canopy, the sorts of birds most negatively impacted by livestock grazing include many relatively uncommon ground and understory nesters, 27 such as the willow flycatcher (Empidonax traillii), veery (Catharus fuscescens), and yellow-breasted chat (Icteria virens). Other common riparian species that forage on open ground, such as the killdeer (Charadrius vociferus) and American robin (Turdus migratorius), may be more abundant in grazed sites. Canopy-nesting riparian birds may be little affected as long as the mature trees survive.
The effects of livestock grazing on wildlife in the American West are substantially scale-dependent. Moderate levels of grazing may increase avian diversity at a local scale, because the habitat needs of many species will be met, as, for example, in a mixture of shrubs and grasses in the desert Southwest. 28 However, at a larger scale, such a uniform management strategy would depress avian diversity, because the most grazing-sensitive species would have no refuge. Without data from replicated long-term livestock exclosures, we almost certainly have a distorted view of what the avifauna of many western North American ecosystems once was and might again become. There are scarcely any such exclosures around that are large enough to support self-sustaining bird populations.
Given the prevalence of livestock grazing in grassland, shrub/steppe, and riparian habitats, especially on western public lands, it is those bird species least tolerant of livestock activities that have the fewest secure places in which to live. It is on this group that our most concerted conservation efforts must be focused. I renew my call for establishment of a system of large, permanent livestock exclosures throughout the American West, to provide critical habitat for that assortment of species negatively impacted by livestock grazing. 29
Table 1. Numbers of Bird Species Responding in Various Ways to Livestock Grazing in Grassland, Shrub/Steppe, and Riparian Habitats in the American West
|Species usually responding positively to grazing||
|Species usually responding negatively to grazing||
|Species usually responding positively in taller, more mesic grasslands but negatively in shorter, more arid grasslands||
|Species unresponsive, or showing mixed or uncertain responses||
Source: V.A. Saab, et al. 1995. "Livestock Grazing Effects in Western North America," in ECOLOGY AND MANAGEMENT OF NEOTROPICAL MIGRATORY BIRDS, T.E. Martin and D.M. Finch (eds.). Oxford University Press. New York, NY.
1. T. L. Fleischner, "Ecological Costs of
Livestock Grazing in Western North America," Conservation Biology 8 (1994):
2. C. E. Bock et al., "Effects of Livestock Grazing on Neotropical Migratory Landbirds in Western North America," in Status and Management of Neotropical Migratory Birds, edited by D. M. Finch and P. W. Stangel, USDA Forest Service General Technical Report RM-229 (Fort Collins, Colo.: USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, 1993).
3. D. G. Milchunas, W. K. Lauenroth, and I. C. Burke, "Livestock Grazing: Animal and Plant Biodiversity of Shortgrass Steppe and the Relationship to Ecosystem Function," Oikos 83 (1998): 65-74.
4. B. G. Peterjohn and J. R. Sauer, "Population Status of North American Grassland Birds from the North American Breeding Bird Survey, 1966-1996," Studies in Avian Biology 19 (1999): 27-44.
5. F. L. Knopf, "Avian Assemblages on Altered Grasslands," Studies in Avian Biology 15 (1994): 247-257.
6. R. N. Mack and J. N. Thompson, "Evolution in Steppe with Few Large, Hooved Mammals," American Naturalist 119 (1982): 757-773.
7. Milchunas, Lauenroth, and Burke, "Livestock Grazing"; D. G. Milchunas, O. E. Sala, and W. K. Lauenroth, "A Generalized Model of the Effects of Grazing by Large Herbivores on Grassland Community Structure," American Naturalist 132 (1988): 87-106.
8. C. P. Ortega, COWBIRDS AND OTHER BROOD PARASITES (Tucson: University of Arizona Press, 1998).
9. R. E. Marvil and A. Cruz, "Impact of Brown-headed Cowbird Parasitism on the Reproductive Success of the Solitary Vireo," Auk 106 (1989): 476-480.
10. J. C. Uyehara and P. M. Narins, "Nest Defense by Willow Flycatchers to Brood-Parasitic Intruders," Condor 97 (1995): 361-368.
11. S. L. Lima and T. J. Valone, "Predators and Avian Community Organization: An Experiment in a Semi-Desert Grassland," Oecologia 86 (1991): 105-112.
12. C. E. Bock and J. H. Bock, "Response of Winter Birds to Drought and Short-Duration Grazing in Southeastern Arizona," Conservation Biology 13 (1999): 1117-1123; H. R. Pulliam and J. B. Dunning, "The Influence of Food Supply on Local Density and Diversity of Sparrows," Ecology 68 (1987): 1009-1014.
13. R. D. Ohmart, "The Effects of Human-Induced Changes on the Avifauna of Western Riparian Habitats," Studies in Avian Biology 15 (1994): 273-285.
14. E. M. Ammon and P. B. Stacey, "Avian Nest Success in Relation to Past Grazing Regimes in a Montane Riparian System," Condor 99 (1997): 7-13; D. S. Dobkin, A. C. Rich, and W. H. Pyle, "Habitat and Avifaunal Recovery from Livestock Grazing in a Riparian Meadow System of the Northwestern Great Basin," Conservation Biology 12 (1998): 209-221; Ohmart, "Effects of Human-Induced Changes."
15. V. A. Saab et al., "Livestock Grazing Effects in Western North America," in Ecology and Management of Neotropical Migratory Birds, edited by T. E. Martin and D. M. Finch (New York: Oxford University Press, 1995).
16. F. L. Knopf, "Mountain Plover (Charadrius montanus)," in The Birds of North America, No. 211, edited by A. Poole and F. Gill (Philadelphia: Academy of Natural Sciences, and Washington, D.C.: American Ornithologists' Union, 1996).
17. Milchunas, Lauenroth, and Burke, "Livestock Grazing."
18. Knopf, "Mountain Plover."
19. Peterjohn and Sauer, "Population Status."
20. P. D. Vickery, "Grasshopper Sparrow (Ammodramus savannarum)," in The Birds of North America, No. 239, edited by A. Poole and F. Gill (Philadelphia: Academy of Natural Sciences, and Washington, D. C.: American Ornithologists' Union, 1996).
21. Bock and Bock, "Response of Winter Birds to Drought."
22. E. A. Webb and C. E. Bock, "Botteri's Sparrow (Aimophila botterii)," in The Birds of North America, No. 216, edited by A. Poole and F. Gill (Philadelphia: Academy of Natural Sciences, and Washington, D.C.: American Ornithologists' Union, 1996).
23. J. B. Dunning Jr. et al., "Cassin's Sparrow (Aimophila cassinii)," in The Birds of North America, No. 471, edited by A. Poole and F. Gill (Philadelphia: The Birds of North America, Inc., 1999).
24. D. F. Bradford et al., "Bird Species Assemblages as Indicators of Biological Integrity in Great Basin Rangelands," Environmental Monitoring and Assessment 49 (1998): 1-22; Saab, "Livestock Grazing Effects."
25. D. L. Johnson et al., Effects of Management Practices on Grassland Birds: Northern Harrier (Jamestown, N.D.: Northern Prairie Wildlife Research Center, 1998); id., Effects of Management Practices on Grassland Birds: Short-eared Owl (Jamestown, N.D.: Northern Prairie Wildlife Research Center, 1998).
26. J. A. Dechant et al., Effects of Management Practices on Grassland Birds: Ferruginous Hawk (Jamestown, N.D.: Northern Prairie Wildlife Research Center, 1999).
27. Saab, "Livestock Grazing Effects."
28. T. Nelson et al., "Wildlife Numbers on Late and Mid Seral Chihuahuan Desert Rangelands," Journal of Range Management 50 (1997): 593-599; G. Smith, J. L. Holechek, and M. Cardenas, "Wildlife Numbers on Excellent and Good Chihuahuan Desert Rangelands: An Observation," Journal of Range Management 49 (1996): 489-493.
29. C. E. Bock, J. H. Bock, and H. M. Smith, "Proposal for a System of Federal Livestock Exclosures on Public Rangelands in the Western United States," Conservation Biology 7 (1993): 731-733; D. L. Donahue, THE WESTERN RANGE REVISITED: REMOVING LIVESTOCK FROM PUBLIC LANDS TO CONSERVE NATIVE BIODIVERSITY (Norman: University of Oklahoma Press, 1999).