Truett, J. C. 2002.Aplomado Falcons and Grazing: Invoking History To

Southwestern Association of Naturalists

Aplomado Falcons and Grazing: Invoking History to Plan Restoration Author(s): Joe C. Truett Source: The Southwestern Naturalist, Vol. 47, No. 3 (Sep., 2002), pp. 379-400 Published by: Southwestern Association of Naturalists Stable URL: http://www.jstor.org/stable/3672496 . Accessed: 01/08/2011 17:13

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http://www.jstor.org SEPTEMBER 2002 THE SOUTHWESTERN NATURALIST 47(3):379-40047(3) :379-400 SEPTEMBER2002

APLOMADO FALCONS AND GRAZING: INVOKING HISTORY TO PLAN RESTORATION

JOE C. TRUETT

TurnerEndangered SpeciesFund, PO. Box 211, Glenwood,NM 88039 Correspondent:jtruett@gilanet. com

ABSTRACT-The federally endangered northern aplomado falcon (Falco femoralis septentrionalis) disappeared as a breeder from its historic nesting range in the southwestern United States in the early to middle 1900s. Since 1995, a small breeding population has been restored to former range in South Texas grasslands, and interest has escalated in restoring the bird to northern Chihuahuan Desert grasslands in southeastern Arizona and southwestern New Mexico. In these latter areas, intensive livestock grazing and associated shrub (brush) encroachment have been theorized to have contributed importantly to the bird's demise, and thus grazing management has been iden- tified as an important restoration issue. A review of the bird's abundance in the context of the grazing history of these areas suggests it was common when grazing, both by livestock and black- tailed prairie dogs (Cynomys ludovicianus), was intense and widespread (1880s-1920s) in the bird's habitat. It declined in abundance and disappeared coincident with declines in livestock abundance and the extirpation of prairie dogs (1930s-1940s). Most locations where observers historically encountered the bird had little brush at the time of its demise. In the Chihuahuan Desert grasslands of Arizona, New Mexico, and West Texas where the bird occurred historically, the avian prey base is presently an order of magnitude or more lower than that in higher-rainfall habitats of the bird in eastern Mexico and South Texas. The avian prey base is similarly depauperate where 2 small populations of aplomado falcon exist in Mexico's northern Chihuahuan Desert, and the bird's reproductive success there is lower than that in eastern Mexico. The historic extirpation of prairie dogs caused a reduction in the prey biomass available to diurnally feeding raptors such as the aplomado falcon. Efforts to restore the falcon to Chihuahuan Desert grasslands in the United States ideally should include monitoring the responses of released birds to levels of grazing, to shrub abundance, to prey abundance, and to black-tailed prairie dog colonies should any exist in release areas.

RESUMEN-El halc6n aplomado (Falco femoralis septentrionalis), puesto en la lista federal de es- pecies en peligro de extinci6n, desapareci6 de su rango hist6rico de anidaci6n en el suroeste de los Estado Unidos de principios a mediados de los 1900. Desde 1995, han reestablecido una poblaci6n pequefia de crianza en el rango hist6rico en los pastizales del sur de Texas, y el interes se ha incrementado en reestablecer el halc6n aplomado a los pastizales nortenos del desierto Chihuahuense en el sureste de Arizona y el suroeste de Nuevo Mexico. En estas uiltimas areas, el pastoreo intensivo de ganado, y la invasi6n asociada del arbusto se sospechan haber contribuido importantemente a la desaparici6n de este pajaro. Entonces, el manejo de pastoreo se ha identi- ficado como importante para la restauraci6n. Una revisi6n de la abundancia del pajaro en el contexto de la historia del pastoreo en estas areas sugiere que era comun cuando el pastoreo, tanto por el ganado como por los perritos de la pradera (Cynomys ludovicianus), fue intenso y extenso (decada de 1880 a decada de 1920) en el habitat del pajaro. Declin6 en abundancia y desapareci6 coincidiendo con declinaciones en abundancia del ganado y la extirpaci6n de los perritos de la pradera (1930-1940). La mayoria de las localidades donde los observadores ubica- ron hist6ricamente el halc6n aplomado tenian pocos arbustos cuando desapareci6. En los pastizales del desierto Chihuahuense de Arizona, Nuevo Mexico, y oeste de Texas donde el pajaro estaba hist6ricamente, la base de aves de presa es actualmente un orden de magnitud o mas baja que en habitat con alta precipitaci6n en el este de Mexico y sur de Texas. La base de aves de presa esta semejantemente pobre donde dos poblaciones pequefas de halc6n aplomado existen en la parte norte de Mexico en el desierto Chihuahuense y el 6xito reproductivo de los pajaros alli es mas bajo que en el este de Mexico. La extirpaci6n hist6rica de los perritos de la pradera 380 The SouthwesternNaturalist vol.47, no. 3

caus6 una reducci6n en la biomasa de la presa disponible para alimentaci6n de los rapacesdiurnos tales como el halc6n aplomado. Los esfuerzos para reestablecer el halc6n a los pastizales del desierto Chihuahuense en los Estados Unidos deben incluir idealmente vigilar la reacci6n a pajaros liberados a los niveles del pastoreo, a la abundancia de arbustos, la abundancia de presa, y a las colonias de perritos de la pradera si existen en las areas de liberaci6n.

In 1986, the northern aplomado falcon (Fal- Meine (1999) pointed out the fundamental co femoralis septentrionalis) was designated en- role of historical inquiry in the practice of con- dangered (U.S. Fish and Wildlife Service, servation biology. In this paper I invoke histor- 1990) in response to its demise in the United ical analysis as a precursor to restoration. I ex- States (Hector, 1987) and evidence of popula- amine the decline of the aplomado falcon in tion declines in eastern Mexico (Kiff et al., southeastern Arizona and southwestern New 1980). Historical records (Hector, 1987) sug- Mexico as these relate to past patterns of graz- gest that in the late 1800s and the early 1900s ing and associated habitat changes. On the ba- it was rather common in the United States por- sis of this examination, I assess the implications tion of its range (Fig. 1). Falcons seen, nests of grazing and related phenomena on attempts found, and birds and eggs collected indicate to restore the falcon to historic habitats in these the birds commonly were encountered within regions. the range in the United States until about HABITAT RELATIONSHIPSOF APLOMADO 1930, after which encounters precipitously de- FALCONS-The falcon's clined. Between the early 1950s and the 1990s aplomado preferred habitats are savanna-like with in the United States, few birds were seen and landscapes widely trees or tall shrubs none were documented to nest after 1952. spaced (Hector 1985, 1986; Perez, 1995). The size and Recently in South Texas breeding pairs have acceptable spacing of the can as been reestablished a reintroduction woody plants vary considerably, through studies made in 3 locations. In effort the U.S. Fish and Wildlife Service and suggested by by eastern found The Fund that commenced in 1985 Mexico, Keddy-Hector (1988) Peregrine the of in falcon (Perez, 1995;Jenny, 1999). This and the recent height woody plants aplomado nesting territories to average about 9 m and discovery of 2 small breeding populations in the density of those >2 cm in diameter at northern Chihuahua, Mexico (Montoya, breast height to be about 2/ha. In South Tex- 1995), has prompted investigations (e.g., as, Perez et al. (1996) found that few woody et al., to assess the of Young 1999) feasibility in areas exceeded 2 m in the falcon's restoration in extreme northwest- plants foraging height and that the density of those >0.5 m ern parts of its historic range, i.e., northern tall averaged 2.6/ha. In nesting territories in Chihuahuan Desert grasslands in southeastern northern Chihuahua, Montoya (1995) and Arizona and southwestern New Mexico. Montoya et al. (1997) found that only yuccas livestock is a conten- Grazing by potentially (Yucca spp.) were taller than 2 m and that tious land use related to the falcon's restora- woody plants >0.5 m tall averaged 72.6/ha. tion to Arizona and New Mexico. The U.S. Fish Observations of foraging aplomado falcons and Wildlife Service brush (1990) implicated suggest that the height and cover of the her- encroachment caused as a by overgrazing po- baceous vegetation also are important. Hector tential cause for the northern fal- aplomado (1986) found that nesting territories in eastern con's historic decline. Direct effects of grazing Mexico had shorter herbaceous vegetation, on the prey base also have been speculated to with less cover between 50-100 cm, than did have adverse effects on habitat potentially qual- areas where falcons were not nesting. Perez ity for the birds in northern Chihuahuan De- (1995) found that foraging aplomado falcons sert regions (U.S. Fish and Wildlife Service, in South Texas frequented sites dominated by 1997a, 1997b). Recent investigations of the naturally low-growing herbaceous vegetation. populations of aplomado falcons in Chihua- Both authors also observed falcons foraging in hua, Mexico, led to speculations that livestock areas, such as fallow fields, that were almost grazing might adversely affect the species denuded of vegetation; they hypothesized that (Montoya, 1995). the perch-foraging strategy of aplomado fal- September 2002 Truett-Aplomado falcons and grazing 381

0 1 ? Unvalidated

FIG. 1-Distribution of the aplomado falcon in the United States about 1900 (from U.S. Fish and Wildlife Service, 1990).

cons made prey detection and capture easier huahua, Mexico (Montoya, 1995; Montoya et in locations where ground cover was relatively al., 1997; A. Montoya, The Peregrine Fund, short or sparse. Thiollay (1980, cited in Perez, pers. comm.). 1995:37) found that patches of short grass were Aplomado falcons are known to feed on a preferentially used by aplomado falcons (and wide variety of prey types, including birds, bats, in other grassland raptors) eastern Mexico. rodents, lizards, frogs, and insects (Hector, Under some circumstances grazing appar- 1985). Recent investigations in eastern Mexico enhances the of habitat ently quality foraging (Hector, 1985, 1986) and northern Chihuahua the stature of the Perez by reducing grass. (Montoya et al., 1997) have found a prepon- (1995) noted the for tendency foraging aplo- derance of birds in the diet. Some early anec- mado falcons in South Texas to select areas dotal reports from Arizona and New Mexico where had been grazed by cattle to <60 grasses (e.g., Bendire, 1892; Ligon, 1961) indicated cm in All areas where Hector height. (1986) that lizards, rodents, and insects were more found aplomado falcons in eastern Mexico commonly eaten than birds. showed effects of, or were created and main- Aplomado falcons usually nest in shrubs or tained by, human agricultural activities, mainly low trees in abandoned nests of other raptors cattle ranching. Biologists collecting aplomado or ravens (Bendire, 1892; 1988; falcon nestlings in eastern Mexico (Tabasco, Keddy-Hector, U.S. Fish and Wildlife Service, 1990; Chiapas, and Veracruz) in 1988 and 1989 to Montoya, also will a of other build a captive breeding stock in the United 1995). They accept variety elevated structures that offer a stick-like sub- States reported that the birds selectively nested bromeliads in eastern Mexico in areas where livestock grazed and avoided ar- strate, including eas not being grazed (P. Jenny, The Peregrine (U.S. Fish and Wildlife Service, 1990) and hu- Fund, pers. comm.). Grazing is ubiquitous but man-constructed platforms (Jenny, 1998). its effects have not been measured in the only Nests have been found from a few to several presently occupied habitats resembling those meters above the ground (U.S. Fish and Wild- in Arizona or New Mexico, i.e., northern Chi- life Service, 1990). 382 The SouthwesternNaturalist vol. 47, no. 3

APLOMADO FALCON DECLINE IN ARIZONA AND 1992). No reliable encounters were reported NEW MEXICO-Between the mid-1800s and the in southwestern New Mexico between 1952 mid-1900s, reported occurrences of aplomado and 1971 (Williams, 1998). falcons in the United States outside Texas clus- Only after the species was listed in 1986 as tered in southeastern Arizona and southwest- endangered did the rate of reported encoun- ern New Mexico (Table 1, Fig. 1). The scarcity ters appreciably increase. At least 12 encoun- of winter encounters with the birds in this re- ters (some of multiple birds) considered by gion suggests that the birds bred but seldom Williams (1998) to be reliable were reported overwintered (Bailey, 1928). The first bird col- from southwestern New Mexico during 1987- lected in the United States (in 1854) came 1998. Raymond Meyer (La Tierra Environmen- from somewhere in southeastern Arizona or tal Consulting, pers. comm.) recently made 2 southwestern New Mexico; the last known nest observations in southwestern New Mexico: a of the species in the United States was south- juvenile in summer 1999 that was banded that west of Deming, New Mexico, in 1952. spring about 200 km south and a pair in spring The chronological distribution of observa- 2001 that hatched (but failed to fledge) young, tions and collections made in this region dur- the first known nesting in the state since 1952. ing this period (Table 2) shows that the fre- Given the enhanced interest in the bird follow- quency of encounters was greatest during the ing its listing, this post-1986 upsurge in en- period 1870-1940. Within this time span, 3 pe- counters may have resulted at least partly from riods exist of >10 years within which no birds increased observer effort. were encountered: 1855-1872, 1888-1907, and 1929-1938. These absences of encounters HISTORYOF GRAZING-The history of live- could have resulted from low intensities of stock grazing in southeastern Arizona and search, few birds present, chance, or combi- southwestern New Mexico has been reasonably nations thereof. Some of the observations sug- well documented (Buffington and Herbel, gest a clumped spatial distribution, whether 1965; Bahre, 1991; Dick-Peddie, 1993). Graz- from clumping of birds or search is not clear. ing by the black-tailed prairie dog (Cynomyslu- For example, C. Benson found 5 nests in 1 lo- dovicianus) also occurred in historic aplomado cation in 1887, and J. S. Ligon found several falcon range in these states but has been less in 1 location in 1908-1909. The last nest in the commonly appreciated and discussed. United States, found by A. Bayne in 1952, was Livestock-The late 1600s saw the first live- the result of substantial search at the request stock in appreciable numbers come to south- of J. S. Ligon (J. P. Hubbard, New Mexico De- eastern Arizona (Wagoner, 1952) and south- partment of Game and Fish [retired], pers. western New Mexico (Buffington and Herbel, comm.). 1965). At that time herds of cattle and sheep The general increase in rate of encounter were brought from the south by Spaniards, and up to about the mid-1920s can be explained these herds began to multiply along rivers, on the basis of increased search intensity, if such as the Santa Cruz in Arizona and the Rio one assumes that the effort of those afield who Grande in New Mexico. Grazing remained lo- would report falcons increased in concert with calized near these and a few other riverine set- the general human population increase. The tlements for 200 years thereafter, and not until decline in encounters thereafter probably oc- the late 1800s did large numbers of stock begin curred despite increasing search. Hector to gain a permanent foothold in historic aplo- (1987) believed that the aplomado falcon in its mado falcon habitat. United States range did not become rare until An exponential increase in the intensity and after 1930, and Table 2 suggests this general the extensiveness of grazing within the historic assessment may hold true for its occurrence in range of the aplomado falcon in New Mexico that portion of its range in southwestern New and Arizona commenced in the early 1880s, Mexico and southeastern Arizona. For nearly a when American cattlemen began to import half century following the decline of the spe- large herds of cattle and to develop water in cies, few were encountered in this Arizona-New previously waterless areas. The number of cat- Mexico region. No confirmed sightings were tle in Cochise County, Arizona, which encom- reported in Arizona after 1940 (Corman, passes essentially all the historic aplomado fal- cn

e~ TABLE 1-Aplomado falcon records from southwestern New Mexico and southeastern Arizona (1850-1960).a ?s

Date Location Data Encounter Observer

1854 (Mar) SW New Mexico and/or SE Arizona At least 3 birds 1 collected Heerman 1873-1874 Near Ft. Bowie, AZ Several birds Observed Henshaw 1875 (Jul) Near Ft. Bayard, NM Adult male Collected Stephens 1875 (Aug) Ft. Bayard, NM Adult female Collected Stephens 1886 (Jun) A few miles S Hachita, NM Pair Observed Anthony 1887 (May) Ft. Huachuca, AZ Imm. male, imm. female Collected Benson 1887 (Apr-May) Ft. Huachuca, AZ 5 nests Observed Benson 1908 (Jul) Hachita, NM Subadult male Collected Goldman '-3 1908 (Aug) Playas Valley, NM (Old Hatchet Ranch) 1 bird Observed Goldman n 1908-1909 (Spring) Jornada [del Muerto] adjacent to Rincon, NM Several nests Observed Ligon ". 1909 (Jun) 10 mi E [N?] Rincon, NM Nest with feathered young Observed Ligon AZ 1 bird Before 1910 Near Tucson, Collected Lusk o 1910 (Feb) Near Fairbanks, AZ 1 bird Observed Willard 1917 (Jun) 45 mi S Alamogordo, NM (5,500 ft elev.) Subadult female Collected Ligon 0 1917 25 mi N NM Adult Observed (Aug) Engle, pair Ligon cs 1917 (Sep) 3 mi SW Nutt, NM (4,600 ft elev.) Female, imm. Male Collected Ligon o 1918 (Nov) N of Engle, NM Pair Observed Ligon 1918 10 mi N NM Pair Observed (Dec) [NE?] Engle, Ligon 0- 1919 (May) 30 mi SE Silver City, NM (Canaigre Place) Adult male Collected Kellogg 1924 (May) 15 mi SE Cutter, NM Adult female Collected Ligon C/q 1924 (Jun) 4 mi N Separ, NM (Tadpole Ranch) 2 males, 1 female (adults) Collected Kellogg 1928 (Sep) Warm springs (probably Faywood), 20 miles 1 female Collected Kellogg SE Silver City, NM 1939 (Oct) Near St. David, AZ 1 bird Observed Huey 1939 (Oct) 4 mi S Animas, NM Imm. Female Collected Toumey 1939 (Nov) Near McNeal, AZ 1 bird Observed Monson 1952 (May) Near (E of) road about halfway between Deming 1 nest Observed Bayne, Campbell and Hermanas, NM 1952 (May?) Playas Valley W of Antelope Wells, NM, 1 nest Observed Bayne S side US-Mexico border

aSources: Anthony, 1892; Bailey, 1928; Hector, 1987; Williams, 1998; J. Hubbard, New Mexico Department of Game and Fish (retired), pers. comm.; H. Campbell, New Mexico Department of Game and Fish (retired), pers. comm. 384 The SouthwesternNaturalist vol. 47, no. 3

TABLE 2-Number of aplomado falcons and nests recognition that ranges were being heavily encountered and number of falcons collected per overgrazed (Buffington and Herbel, 1965). interval in southwestern New Mexico and 10-year The passage of the Taylor Grazing Act in 1934 southeastern Arizona see Table 1 for (1850-1960); about further reductions in livestock sources. brought numbers (Bahre, 1995). Prairie black-tailed Time Birds Dogs-Grazing by prairie exerts control on the interval Number of birdsa collected dogs commonly greater structure and composition of the herbaceous 1851-1860 3+b 1 plant community than does livestock grazing 1861-1870 0 0 (Koford, 1958; 1998). Colonies of 1871-1880 5+ 2 Detling, these rodents maintain the 1881-1890 14+ (5 nests)c 2 continuously vege- 1891-1900 0 0 tation at a height of 5-10 cm (Whicker and 1901-1910 12+ (4+ nests)c 2 Detling, 1988) or shorter (pers. obser.) by graz- 1911-1920 10 4 ing and clipping. Additionally, they sometimes 1921-1930 5 5 create extensive unvegetated areas (Fig. 2) by 1931-1940 3 1 burying vegetation near burrow entrances and 0 1941-1950 0 by digging up below-ground parts of grasses 1951-1960 2 (1 0 nest)c and other plants (Whicker and Detling, 1988). a Includes birds collected. The historic range of the black-tailed prairie b If "several" reported, 3+ added to number. dog in southeastern Arizona and southwestern c Two added to number for each nest reported. New Mexico closely matched that of the aplomado falcon (Fig. 3). The specific locations within this range from which records exist for con range in that state, grew from substantially the falcons (Table 1) all supported substantial fewer than 10,000 in 1880 to 60,000 in 1886 to populations of prairie dogs in the late 1800s 84,000 in 1890 (Wagoner, 1952). The number and early 1900s. These included, for example of cattle in Socorro County, New Mexico, in- (Fig. 4), the Sulphur Springs Valley in Arizona creased from 9,000 to 70,000 between 1882 (Bailey, 1889; Mearns, 1907); the Animas, Pla- and 1884 and Herbel, mir- (Buffington 1965), and Hachita and Nutt Plain in New the situation elsewhere in the south- yas, valleys roring Mexico the western of the state. of (Bailey, 1908; 1931); vicinity (main- part Development south and of Silver New Mexico and of water wells in waterless ly east) City, springs drilling and the Jornada del Muerto ba- basins commenced in the late 1870s and esca- (Bailey, 1931); sin in New Mexico (Oakes, 1998a, 1998;, E. lated dramatically in the 1880s (Wagoner, Fredrickson, Jornada Range, 1951; Buffington and Herbel, 1965). By 1910 Experimental comm.). Given that intensive essentially all the southern New Mexico range pers. grazing by livestock is known to benefit black-tailed was in use by livestock (Jardine and Forsling, prairie (Koford, 1958; Knowles, 1986), the 1922). Indeed, most of the range in both states dogs increase in cattle stimulat- had been stocked so heavily with cattle since post-1880 probably ed an increase in the distribution and abun- the 1880s that, during unusually dry periods, dance of and Schmitt, large numbers died from starvation (Jardine prairie dogs (Hubbard and Forsling, 1922; Wagoner, 1952; Bahre, 1984). 1995). The U.S. Government initiated campaigns to control about 1912. Prairie Livestock grazed ranges year-round, and prairie dogs dog started to decline in numbers of grazing animals between the late populations substantially 1880s and the 1920s far exceeded those in later some areas by 1920, peaked in decline rate in years. For example, in 1906 the estimated num- the 1930s, and had been almost eliminated in ber of grazing livestock-cattle, sheep, goats, southeastern Arizona and southwestern New and horses-in New Mexico was 7,250,000; by Mexico by 1950 (Hubbard and Schmitt, 1984). 1979 the number had declined to 2,201,000 Grazing by prairie dogs, in concert with that (Dick-Peddie, 1993). The high stocking rates by cattle, undoubtedly maintained the grasses began to decline somewhat in some areas as in large portions of some valley habitats at a early as the late 1920s and early 1930s with the low stature and sparse cover during the 1880s September 2002 Truett-Aplomado falcons and grazing 385

FIG. 2-Black-tailed prairie dog colony in Chihuahuan Desert grasslands near Carrizozo, New Mexico, illustrating effects on vegetation of grazing, clipping, and digging activities by these rodents; beyond colony periphery dominant grasses are 10 to 40 cm tall.

to 1920s. Vernon Bailey (1931:123-124) offers PATTERNSOF BRUSH ENCROACHMENT-Causes a dramatic description: of brush invasion in Chihuahuan desert grasslands are variable and Soil influ- "In 1909, on a from to complex. type August, trip Deming ences whether and how brush Hachita and the and Animas rapidly invades; through Playas for areas with soils show the writer found the nu- example, sandy higher Valleys prairie dogs instances and levels of historic brush invasion merous in many localities, especially along than areas with clay soils (Buffington and Her- the elevated and more open margins of the bel, 1965; et al., 1983; McAuliffe, valleys, but extensive colonies were also seen Hennessy 1995). Periodic in the last in the bottoms of these great desert valleys. droughts century have favored the increase of at Animas Valley was an almost continuous woody species the of (Herbel et al., 1972), as prairie-dog town for its whole length [about expense grasses have with drier summers and wetter 160 km] and breadth. In many places where periods winters than usual rain had missed a part of the valley the prai- (Burgess, 1995). Grazing and associated activities of livestock have stim- rie dogs had taken all the season's vegetation and had made barren deserts miles in ex- ulated brush invasion in many areas (Hum- tent." phrey, 1958; York and Dick-Peddie, 1969; Bah- "Ten prairie dogs to an acre were esti- re,1995; Van Devender, 1995) by such mecha- mated as a fair allowance for their numbers nisms as enhancement of the viability and in the colonies, and apparently these colo- spread of mesquite seeds, trampling of the soil nies covered at least a third of Grant County, with consequent reduction of water infiltra- or approximately 1,000 square miles, which tion, and reduction of fire frequency by re- would give 6,400,000 prairie dogs to one moval of the grass cover necessary for carrying county [Grant County at the time included fires. That brush encroached into some areas Hidalgo County, see Fig. 4]." prior to the advent of domestic livestock (E. 386 The Southwestern Naturalist vol. 47. no. 3

/ / \ Socorro/

/ /

f / /, /I I / Engle I W.~-, . J Alamogordo Ft.G.a nt I (" \ Silver 0 I r , City L cC 0

LordsburgOdsburg 0 U

I , Z-, MEXICO Ft.Huachucha I ^ \ 'I( IZONA 0 50 miles I I I l \ MEXICO -07-

----- Limits (W and N) of historic range of black-tailedprairie dog 0 Aplomadofalcon nest o Aplomadofalcon collected \ Aplomadofalcon observed FIG. 3-Aplomado falcon nests, collections, and observations in southeastern Arizona and southwestern New Mexico, 1854-1952, in the context of historic black-tailed prairie dog distribution in the region. Aplo- mado falcon data from Anthony (1892), Bailey (1928), Hector (1987), Williams (1998),J. Hubbard (1999, pers. comm.), and H. Campbell (1999, pers. comm.). Prairie dog data from Mearns (1907), Bailey (1931), Findley et al. (1975), and Hoffmeister (1986).

Fredrickson, Jornada Experimental Range, young plants. Weltzin et al. (1997) and List pers. comm.; Van Devender, 1995) and more (1997) showed that black-tailed prairie dogs, recently increased in some areas protected and the herbivores and granivores associated from grazing (Buffington and Herbel, 1965; with their colonies, probably maintained grass- Hennessy et al., 1983) suggest that grazing was land within these colonies in the Southwest by only sometimes or partly the cause of brush preventing woody species, such as mesquite, encroachment. from establishing or attaining dominance. Prairie dogs may have prevented brush en- Thus, brush invasion may not have begun in croachment within the confines of their colo- prairie dog colonies until the colonies were nies. Horne (1941) and Koford (1958) com- eradicated, which in the Arizona-New Mexico mented on the ability of black-tailed prairie segment of the aplomado falcon's range oc- dogs to maintain their colonies free of mes- curred primarily in the 1930s and early 1940s quite (Prosopis glandulosa) by clipping the (Hubbard and Schmitt, 1984). September 2002 Truett-Aplomado falcons and grazing 387

NEW MEXICO

0 60 miles I I

FIG. 4-Counties and valleys in southeastern Arizona and southwestern New Mexico in which aplomado falcons were encountered and that had large populations of black-tailed prairie dogs in the late 1800s and early 1900s.

The current extent of brush encroachment height and cover by grazing, encroachment of into previous aplomado falcon range is sub- woody plants, and alterations in the potential stantial in some localities. In the southern Jor- prey base for falcons. Did the demise of the nada del Muerto in New Mexico, for example, aplomado falcon track in meaningful ways any mesquite has encroached and proliferated to of these changes? such an extent that the cover and biomass of Regional Trends in Grazing-As discussed pre- grasses in many sites are a small fraction of viously, numerous historical accounts (e.g., what they were a century ago (Buffington and Bryan, 1925; Morrisey, 1952; Wagoner, 1952; Herbel, 1965; Hennessy et al., 1983). York and Buffington and Herbel, 1965) indicate that in- Dick-Peddie (1969) compared grassland cov- tensive grazing of grasslands by livestock in erage of selected townships in southern New southeastern Arizona and southwestern New Mexico between the late 1800s and the late Mexico commenced in the early 1880s. It had 1960s based on surveyors' notes (1800s) and been going on for 40 to 50 years by the time a the wandering quarter method (1960s) de- decline in aplomado falcons became evident. scribed by Catana (1964). They concluded Encounters with aplomado falcons underwent that, in the areas they studied, grass "cover" a clear decline only after livestock numbers had, on average, declined from 75% to 5%. had begun to decline and efforts to eradicate prairie dogs had increased. HABITATCHANGE AND FALCONDECLINE-One Large numbers and wide distributions of would expect the falcon population decline to prairie dogs were reported primarily between coincide temporally with any causative habitat 1880 and 1920, the same period that saw the changes. Historic changes in habitat that were highest numbers of livestock. The decline in related to grazing and that might have influ- prairie dog abundance peaked in rate and spa- enced falcons include reduction of grass tial extent (Hubbard and Schmitt, 1984) dur- 388 The SouthwesternNaturalist vol. 47, no. 3 ing the same decade the Taylor Grazing Act ca grassland. I concluded that most, perhaps passed and the falcons virtually ceased being all, of the sites had little brush at the time the encountered-the 1930s. By 1955, 3 years after falcon population declined. Bayne reported the last documented falcon Two Arizona-New Mexico sites where mul- nest southwest of Deming, black-tailed prairie tiple birds nested are 1) the vicinity of Fort dogs were extinct in southeastern Arizona and Huachuca, Arizona, where Benson found 5 also in southwestern New Mexico, except for a nests in 1887, and 2) the Jornada del Muerto few small colonies (Hubbard and Schmitt, east of Rincon, New Mexico, where Ligon 1984). Thus, the highest rates of encounter found several nests in 1908 and 1909 (Table 1, with aplomado falcons coincided with the Fig. 3). Paragraphs that follow attempt to re- greatest intensities of grazing by cattle and construct grazing conditions at these sites prairie dogs. The diminishment of grazing by when the falcons nested there and to assess cattle and prairie dogs, the latter via intensive whether brush encroachment could have eradication efforts, coincided with the decline made the sites uninhabitable by falcons by the of the aplomado falcon. 1930s. Site-Specific Vegetation Changes-I evaluated Near Fort Huachuca, Lieutenant Benson, ac- historic patterns in brush encroachment and cording to Bendire (1892:307), reported aplo- grazing at 2 kinds of locations: 1) sites where mado falcons to be fairly common in 1887, pre- falcons were encountered and 2) sites where ferring "open plains, covered here and there more than 1 falcon nested. I assessed by on-site with low mesquite trees, yuccas, and cactuses inspection the present status of brush at his- [sic]." All the nests Benson found were in "low toric encounter sites I could locate with some mesquite trees from 7 to 15 feet from the accuracy. Historic documents and photographs ground" in old nests of white-necked (Chihu- were the primary sources used to assess historic ahuan) ravens (Corvus cryptoleucus).The pre- grazing and trends in brush encroachment at cise locations of the nests were not described nesting sites. but almost certainly were west, north, or east In March 1999, I visited 13 sites that were of the fort, which backs against the Huachuca within what I judged to be several kilometers Mountains to the south. of historic encounter sites (Table 3). Four sites North of Fort Hucachua about 8 km, the Ba- showed little or no evidence of woody plant bocomari River (a then-perennial stream) ran encroachment in the surrounding landscape west to east, and 16 to 19 km east of the fort, except near stream beds or at sites of localized the San Pedro River (another perennial disturbance; those were 4.8 km southwest of stream) ran south to north. Springs in Hu- Nutt, 48 km southeast of Silver City, 32 km achuca Canyon south of the fort furnished wa- southeast of Silver City, and 6.5 km north of ter for horses and people (S. Stone, Fort Hu- Separ. Three sites north of Engle had localized achuca, pers. comm.). Compared with most ar- brush invasion, estimated to have affected less eas, the Fort Huachuca area was abundantly than half the surrounding landscape. At 3 oth- watered, which is one reason it had been se- er sites brush seemed to have extensively in- lected in 1877 as a "reserve" to support the vaded coarse soils on valley slopes, but not ap- cavalry's horses (S. Stone, pers. comm.). By preciably the finer soils in valley bottoms, ex- 1887, when Benson found the falcons nesting cept at roadsides and other disturbed areas; in the area, the Fort Huachuca region was these were Hachita Valley west and south of stocked not only with horses but also with large Hachita, 6.5 km south of Animas, and 18 km numbers of cattle. Moreover, it was just emerg- north of Hermanas. Localities near Fort Ba- ing from a drought in which numerous cattle yard varied greatly in cover by woody plants had starved to death (D. Hadley, Arizona State (junipers [uniperus spp.] and oaks [Quercus Museum, pers. comm.). spp.]) depending on topographic conditions; Areas such as Fort Huachuca that were near an extensive plain with little brush character- permanent water were favored grazing spots ized the habitat a few kilometers south of Fort for livestock. Indeed, in May of 1886, the army Bayard. Only 1 locality, 16 km east of Rincon, quartermaster at the fort had ordered 9,450 m appeared to have changed to a totally mes- (31,000 ft) of barbed wire with which to build quite-dominated landscape from an open yuc- a fence to keep range cattle from consuming September 2002 Truett-Aplomado falcons and grazing 389

TABLE 3-Present-day habitat conditions at sites in New Mexico where aplomado falcons historically were encountered and where the sites could be located with some precision (i.e., within several kilometers); see Table 1 for more about encounters.

Date Site Encounter Present-day habitat conditions 1875 Ft. Bayard 2 adults collected Patches of yucca (Yucca spp.) grassland almost free of brush sur- rounded by juniper-oak wood- land (Juniperus spp.-Quercus spp.); grassland and savanna become dominant a few km to the south 1886 Hachita Valley S of Pair seen Grass without brush in valley bot- Hachita tom; mesquite (Prosopis glandulosa) and creosote brush (Larrea tri- dentata) dominant on slopes 1908 Hachita 1 subadult collected Grass without brush in valley bottom; mesquite and creosote brush dominant on slopes 1908-1909 10 mi E Rincon, Jornada Several nests seen Strongly dominated by coppice- del Muerto dune mesquite, little grass present, many dead soaptree yucca (Yucca elata) 1917 3miSWNutt 1 female, 1 imm. Open soaptree yucca grassland with male collected negligible amounts of brush 1917-1918 Engle to 25 mi N (3 3 pairs of adults seen Open soaptree yucca grassland sites) dominated locally and sparingly by mesquite, creosote bush, and tarbush (Flourensia cernua) 1919 30 mi SE Silver City 1 adult male collect- Open soaptree yucca grassland ed dominated locally (near stream- courses) by mesquite 1924 4 mi N Separ 3 adults collected Open soaptree yucca grassland with herbaceous understory dominated by snakeweed (Gutierreziasar- othrae) 1928 20 mi SE Silver City 1 female collected Open soaptree yucca grassland with occasional mesquite dominance near water courses 1939 4 mi S Animas 1 immature female Open soaptree yucca grassland in collected valley bottom with mesquite dom- inating slopes of valley 1952 Halfway between Dem- 1 nest observed Open grassland areas interspersed ing and Hermanas with closed creosote-tarbush- mesquite areas; soaptree yucca sparingly present in both types

the grass reserved for horses (S. Stone, pers. tat containing the nests is a plain with widely comm.). In addition, there were "many [black- scattered patches or individuals of mesquite, tailed] prairie dogs about five miles east" and yucca (probably soapweed yucca, Yucca elata), "a small isolated colony about ten miles south- and cactus (probably prickly pear, Opuntia west," although apparently the prairie dogs spp.); see Bendire (1892) and Walmo (1955). were not widely distributed over the plain near The nests contained young birds and eggs in the fort (Hoffmeister, 1986:195). late April and early May (Bendire, 1892), well The picture that emerges of the likely habi- into the driest period of the year. Most of the 390 The SouthwesternNaturalist vol. 47, no. 3

grasses, being C4 (warm-season) species, would several aplomado falcon nests in this region, have produced their last significant growth the water sources for livestock were generally 10 previous September and probably had been km or less apart (Fig. 5). grazed by cattle and horses throughout the Even before the advent of Detroit Troughs winter, when the animals would have been able and the water wells, some cattle had grazed the to range farthest from water. Consequently, the waterless basin. "Worlds of cattle" reportedly grass likely was cropped short. grazed the west side of the basin by 1884, and During 1949-1951, a decade after the last in 1885 cattle ranged from the Ropes Springs authenticated sighting of an aplomado falcon Ranch west almost to the Rio Grande (Buffing- in Arizona, the grasslands near Fort Huachuca ton and Herbel, 1965:146), an apparent dis- apparently remained largely brush-free. Along tance of 16 to 24 km from permanent water. lower washes northeast of the fort, desert-scrub Then water development came, and by 1915 vegetation dominated by white thorn (Acacia about 1 cow per 17 ha grazed the Jornada Ex- constricta) occurred, and mesquite had spread perimental Range year-round (Buffington and from the higher washes and canyons onto at Herbel, 1965). This level of grazing probably least some overgrazed mesas interspersed be- had been going on more or less since the water tween these habitats (Wallmo, 1955). However, wells had been developed. That it was far too most of the gently sloping terrain, which oc- high to be sustained (Jardine and Hurtt, 1917) cupied the great majority of the landscape, still is indicated by reductions thereafter-numbers had little brush cover (Wallmo, 1950). Today of cattle on the Experimental Range were cut much of the terrain is brush-dominated or has in half after 1916, in half again in 1928, and to been subjected to housing developments and 1 cow per 77 ha after 1941 (Buffington and associated disturbances. Grasslands with little Herbel, 1965). The basin's largely warm-season brush remain in some undeveloped areas near grasses probably would have been heavily the original fort. cropped by cattle during the springs of 1908 In the Jornada del Muerto, the aplomado and 1909 when Stokely Ligon found the vari- falcon nests Ligon found in 1908 and 1909 ous aplomado falcon nests. were described in W.W. Cooke's entry in Bailey Prairie dog colonies occupied probably (1928:191): "A nest containing feathered <10% of the Jornada del Muerto basin. On the young was found June 17, 1909, 10 miles east 77,730-haJornada Experimental Range, for ex- of Rincon, and several other nests in the ad- ample, these colonies occupied an estimated jacent Jornada during this and the preceding 1,420 to 1,620 ha (E. Fredrickson, Jornada Ex- year." For present purposes I assume all nests perimental Range, pers. comm.) to 4,220 ha were within several kilometers of the vicinity of (Oakes, 1998a) at the time Ligon found the 10 miles (16 km) east of Rincon, although falcon nests. Colonies at this time were inter- Cooke's presentation of Ligon's statement is spersed throughout the area that extended somewhat ambiguous. from South Well to Red Lake Well (Fig. 5). Twenty years prior to Ligon's finding those Given the former assessment, grazing inten- first nests in 1908, a water development that sity probably was high by today's standards opened the area to heavy grazing took place when aplomado falcons nested at the Fort Hu- about 16 km east of Rincon-the Detroit and achuca and Jornada del Muerto sites. Cattle Rio Grande Livestock Company constructed a probably were the dominant grazers at both cattle watering site called Detroit Troughs sites, although prairie dog colonies existed (Buffington and Herbel, 1965). Detroit within a few to several kilometers in each case. Troughs, filled by water pumped from the Rio At each of the sites, intensive grazing pressure Grande to a tank on a high point overlooking by livestock apparently had existed for several the river, introduced an era of water provision years at the time nests were encountered. to a previously waterless basin. Development of In terms of brush encroachment, the site at other water in the nearby Jornada del Muerto Detroit Tanks is now dominated by woody followed-at least 7 wells supplied water to the plants to a far greater extent than is the Fort Jornada Experimental Range and the vicinity Huachuca area. But in 1917, after an estimated immediately north and east of Detroit Troughs 30 years of intensive grazing, that site (at a wa- by 1906. Thus, by the time Ligon found the ter source where the grazing would have been September 2002 Truett-Aplomado falcons and grazing 391

FLATLAKE WELL

Nest GOLDENBURGRANCH location *GOLDENBURGSPRING D ROPESSPRINGS RANCH )vDETROIT TROUGHS

RIOGRANDE RIVER BURKESPRINGS SUMMERFORDWELL ?

0 3 6 I i I ! I I MILES

FIG. 5-Pre-1907 water sources on the Jornada Experimental Range and vicinty, New Mexico (after Buf- fington and Herbel, 1965). The aplomado falcon nest location in 1909 described by Ligon (Bailey, 1928) as "10 miles east of Rincon" would have been about 5 km NE of Detroit Troughs. intense) had only local patches of brush (Fig. Grazing, the presence of woody plants, and 6). It seems doubtful that 10 to 20 years later, the presence of prairie dog colonies all may when the aplomado falcon population appar- have affected the ability of aplomado falcons ently waned, brush encroachment would have to secure prey when they nested in Arizona made the site uninhabitable by the birds. Such and New Mexico. In the discussions that follow, probably would have been even less likely at I focus primarily on prey availability during the Fort Huachuca. falcon's breeding season from mid-spring to Changes in the Prey Base-R. Meyer, La Tierra mid-summer, when the birds would require Environmental Consulting, and A. Montoya, more food than usual to produce eggs and The Peregrine Fund, have suggested (pers. rear young. Small birds (20 to 200 g) consti- comm.) that prey scarcity may account for the tuted the major biomass proportion of the low productivity they observed in the 2 popu- aplomado falcon's diet in the falcon popula- lations of aplomado falcons presently known to tions quantitatively studied, although small nest in the northern Chihuahuan Desert. mammals often were taken(Hector, 1985; Ked- These small populations are located in north- dy-Hector, 1988; Montoya, 1995; Rodriguez, ern Chihuahua, Mexico, about 200 km south 1996). of the southeastern limits of the Arizona-New Information from recent studies is equivocal Mexico region addressed herein. Mean brood about the direct effects of livestock grazing on sizes and fledging rates of both these popula- the avian prey base. Some preferred prey spe- tions were lower than those of eastern Mexico cies, such as mourning dove (Zenaida ma- populations. These measures were lower in the croura) and meadowlarks (Sturnella spp.) show population with lower estimated prey availabil- positive or negative responses to livestock graz- ity. These data suggest that small historic ing depending on the geographic location, the changes in prey abundance in such a prey-de- height of the ungrazed grasses, the level of pauperate region (see Table 4) could have af- grazing, and other variables (Hector, 1985; fected the persistence of populations. Bock et al., 1993; Montoya, 1995). Mourning 392 The SouthwesternNaturalist vol. 47, no. 3

t , -1-,4i. i

FIG.6-Cattle at Detroit Troughs, Jornada Basin, New Mexico, September 1917. The grassland in the backgroundhas little sign of woody plants except soapweed yucca (Yuccaelata), although a stringer of brush, probably mesquite (Prosopis glandulosa), appears at left just below the ridgeline. Photo courtesy Rio Grande Historical Collections, New Mexico State UniversityLibrary, Las Cruces. doves often respond positively to reductions in birds and more species typically accompany herbaceous ground cover from grazing and the proliferation of the woody plant compo- other causes (Baker and Guthery, 1990; Reyn- nent. However, the potential benefits to aplo- olds and Krausman, 1998). Henry (1998) mado falcons of this increase in potential avi- found no significant differences in the abun- faunal prey has been assumed to be cancelled dance or biomass of breeding-season birds be- at some level of "brushiness" because the birds tween grazed and ungrazed grassland sites at 2 become more difficult for the falcons to catch locations in the northern Jornada del Muerto (Keddy-Hector, 1988). basin, New Mexico. At a site 16 km west of Fort Historically, the biomass of small birds po- Huachuca, Arizona, and similar to Fort Hu- tentially available for prey probably was greater achuca in elevation, Bock et al. (1984) found inside prairie dog colonies than outside. Ag- bird numbers in summer to be significantly new et al. (1986) found a greater avian biomass greater in a heavily-grazed area than in an ad- inside than outside black-tailed prairie dog col- jacent area ungrazed for 13 to 14 years. onies in the northern Great Plains. Manzano- Brush encroachment may be expected to el- Fischer et al. (1999) found birds to be usually, evate avian prey abundance but to reduce its but not always, more abundant inside colonies availability. It is almost axiomatic that increases than on a grassland site outside colonies in in the structural complexity of vegetation, re- northern Chihuahua, Mexico. The unit-area sulting from the addition of vertical layers or biomass of about 150 g/bird of burrowing owls an increase in horizontal patchiness, enhance (Speotyto cunicularis) in some Chihuahuan De- the richness and abundance, and often the sert prairie dog colonies may alone have ex- biomass, of the associated avifauna (MacAr- ceeded that of all small birds outside prairie thur and MacArthur, 1961; Willson, 1974; Ro- dog colonies. For example, Truett and Savage tenberry and Wiens, 1980). Numerous studies (1998) documented the late-spring density of (e.g., Raitt and Maze, 1968; Davis et al., 1974; burrowing owls (exclusive of young) in 3 new, Raitt and Pimm, 1976) verify this relationship small (<1 ha) prairie dog colonies in historic with respect to brush encroachment into aplomado falcon habitat in southwestern New southwestern grasslands and savanna-more Mexico to be equivalent to 84,000 g/40 ha September 2002 Truett-Aplomado falcons and grazing 393

TABLE 4-Estimated breeding-season (late spring-midsummer) density and biomass of birds in the size range used by aplomado falcons as prey (Hector, 1985) in current (Mexico) and historic (South Texas and New Mexico) habitats.

Mean density Mean (birds/ biomass Location 40 ha) (g/40 ha) Source Eastern Mexico Veracruz, Chiapas, Campeche, Tabasco 290 29,000a Hector, 1985 South Texas Calhoun Co.-near Seadrift (grassland with few shrubs) 294 29,400b Roth, 1977 Northern Mexico NW Chihuahua 18 1,112 Meyers, 1997, citing Montoya, 1995 SW New Mexico Otero Co.-Otero Mesa (grassland; mean of 2 sites) 17 553 Meyer, 1997 Sierra Co.-Northern Jornada del Muerto (grassland) 28 1,540c Henry, 1995 Socorro/Sierra cos.-Northern Jornada del Muerto (grassland; mean of 6 sites) 2,510d Henry, 1998 Sierra Co.-Armendaris Ranch, Jornada del Muerto (swales) 28 1,540c Henry, 1995 Sierra Co.-Armendaris Ranch,Jornada del Muerto (up- lands) 11 605c Henry, 1995 Dona Ana Co.-Southern Jornada del Muerto (grassland) 17 940 Raitt and Pimm, 1976 Dona Ana Co.-Southern Jornada del Muerto (grassland; mean of 2 sites) 1,327d Henry, 1998 Hidalgo Co.-Playas Valley (grassland; mean of 2 sites) 2,833d Henry, 1998 a Assuming a mean weight per prey item of 100 g (estimated from Hector [1985:Table 1]). b Assuming a mean weight per prey item of 100 g, as calculated for South Texas scrub grassland birds by Emlen (1972). c Assuming a mean weight per prey item of 55 g (calculated from data of Raitt and Pimm [1976]). d Average of 5 surveys conducted mid-May to late July.

(compare with Table 4). In the same area a few though yearling and adult prairie dogs may be years later, D. Berardelli (New Mexico State too large (about 400 to 1,000 g; Hoogland, University, unpubl. data) found in 12 ha of a 1995) for aplomado falcons to readily capture prairie dog colony 46 adult burrowing owls and kill. Rough calculations of the biomass of (23,000 g/40 ha) in spring and 107 adults plus juvenile prairie dogs emerging each year in emergent young (54,000 g/40 ha) in summer. May and June can be made using Bailey's Surveys prior to establishment of these colo- (1931) estimate of 10 animals per acre (25/ha, nies had disclosed no burrowing owls (J. a reasonable estimate by today's standards; Ce- Truett, unpubl. data). ballos et al., 1993) and the tendency for the Small mammals, which sometimes are eaten average number of young emerging to equal by aplomado falcons (Ligon, 1961; Hector, at least the number of animals already present 1985; Rodriguez, 1996), would have been (e.g., Knowles, 1987). Twenty-five young per ha much more readily available inside prairie dog would yield about 144,000 g/40 ha. This is sev- colonies than outside. Young-of-the-year prai- eral times more than the rodent biomass that rie dogs (about 144 g/individual; Hoogland, would be expected outside prairie dog colo- 1995) would have been suitably sized prey, al- nies (Wood, 1969), which seldom would be 394 The SouthwesternNaturalist vol. 47, no. 3 available to diurnally hunting raptors. It is the avian prey-base biomass as West Texas hab- about 50 to 280 times the unit-area avian prey itats (Table 4). biomass in northern Chihuahua and south- Both Texas regions reportedly were grazed western New Mexico (Table 4). intensively during the period encompassing The relatively short stature of the vegetation most of the records of aplomado falcons, i.e., in prairie dog colonies probably elevates the 1890-1920 (Hector, 1987). West Texas, like visibility and vulnerability of most prey. New Mexico and Arizona, saw a tremendous Ground-feeding birds, small mammals, rep- influx of cattle and sheep beginning in the late tiles, and insects presumably would be easier 1800s; grazing intensities may have peaked in to detect and capture inside prairie dog colo- the 1880s as suggested by widespread starva- nies than outside, where vegetation usually is tion among cattle (Hollon, 1961). In addition, taller and more dense (Janes, 1985). all West Texas aplomado falcon nesting re- To summarize, grazing perhaps enhanced cords were within the range of the black-tailed prey availability for aplomado falcons when the prairie dog (cf., Schmidly, 1977; U.S. Fish and falcons were common in the northern Chihu- Wildlife Service, 1990). Habitats in South Tex- ahuan Desert. Grazing by livestock may or may as have been grazed near settlements by sheep, not have increased the abundance of avian goats, and cattle since the 1700s (Lehman, prey, but it probably exposed the prey to easier 1969; Weber, 1992) and were densely stocked over areas and capture. Brush encroachment, whether from large (by sheep, horses, cattle) the 1880s livestock grazing or other causes, may have el- by (Johnston, 1963; Jordan, 1981; Prairie never evated bird abundance locally; it probably had Archer, 1989). dogs occupied South Texas. not advanced sufficiently to have reduced prey To what extent declines in catchability in most areas. The presence of, grazing intensity coincided with the demise of the fal- and grazing by, prairie dogs probably substan- aplomado con in Texas is unclear. The 1934 Graz- tially enhanced not only prey abundance and Taylor Act motivated less dramatic stock- biomass but also its catchability. The major ing probably rate reductions in Texas (because the proportion of prey biomass enhancement in ing ranches were than on the prairie dog colonies was caused by increases in private) largely public-lands ranches of New Mexico and Arizona. small mammals, but avian biomass probably Cattle die-offs the late 1800s (Hollon, was also greater inside colonies. during 1961) may have led to reductions in stocking rates, and prairie dogs in West Texas were ex- TEXASPOPULATIONS: A COMPARISON-A brief tensively poisoned under the mandates of a look at the history of the aplomado falcon in 1915 Texas law and with the aid of federal as- Texas shed on reasons for its demise may light sistance during the same approximate period in Arizona and New Mexico. The status present they were poisoned in New Mexico and Ari- of its in South Texas likewise recovery may sug- zona-1920-1940 (Cottam and Caroline, whether it can be recovered farther west gest 1965). However, at least portions of West and in its United States range. South Texas grasslands were grazed heavily by A the in Texas century ago species appears cattle well into the late 1900s (Johnston 1963; to have and nested occupied, in, 2 disjunct Andrew et al., 1988; Telfair, 1999). The only and different habitats. A (Fig. 1) ecologically aplomado falcon nesting attempt reported in West Texas population inhabited a semidesert Texas since 1920 was in Brooks County (South grassland region adjoining southern New Mex- Texas) in 1941 (Oberholser, 1974). ico; the climate (Brown, 1982) and avian prey Grazing apparently stimulated woody plant base biomass (Raitt and Maze, 1968) in this re- proliferation in both West and South Texas. In- gion were probably similar to that in southern creases in mesquite and other shrubs in West New Mexico. Further to the east and south, a Texas, resulting at least partly from grazing, re- South Texas population inhabited a coastal sembled in general timing and character those plain that received approximately 3 times the farther west in southern New Mexico (Brown, average annual precipitation (cf., Schmidly, 1982). Several woody species have proliferated 1977; Brown, 1982; Kingston and Crawford, in South Texas during the past 100 years; graz- 1993) and supported perhaps 15 to 20 times ing and fire cessation are implicated (Archer, September 2002 Truett-Aplomado falcons and grazing 395

1989). Conversion of grassland to shrubland in The information presented herein shows a both West and South Texas did not occur suf- temporal correlation between the historical ficiently early or widely to be implicated in the abundance of grazing animals and the abun- abrupt and region-wide disappearance of the dance of aplomado falcons in Chihuahuan De- falcon. Long after the disappearance of the sert regions of Arizona and New Mexico. Live- species, substantial tracts of grassland with only stock and black-tailed prairie dogs were abun- a minor shrub component remained in both dant and falcons commonly were encountered regions (Johnston, 1963; Schmidly, 1977; during the period 1880-1930. During the 2 de- Perez, 1995). cades following 1930, livestock numbers de- One adverse impact coinciding closely in clined and prairie dogs and falcons virtually time with the bird's apparently rapid demise in disappeared. The similarities in historical dis- Texas during 1890-1910 (Oberholser, 1974) tributions of black-tailed prairie dogs and aplo- was collecting by naturalists (U.S. Fish and mado falcons in this area makes the synchrony Wildlife Service, 1990). The minimum of 124 in the demise of these 2 species particularly sets of eggs and 56 skins taken in the United intriguing. The probable decline in falcon prey States before 1920 came largely from South availability caused by the disappearance of Texas (U.S. Fish and Wildlife Service, 1990). prairie dogs in this prey-poor region (as a re- Wanton killing (e.g., for target practice), sult of eradication measures) suggests possible though undocumented, by other segments of cause and effect. the public may have also contributed to the Correlation cannot in itself prove causation, falcon's demise. Many ornithologists (e.g., Ob- and whether declines in livestock or prairie erholser, 1974; Hector, 1987), while acknowl- dogs contributed to the decline in falcons re- edging the former, have maintained that other mains unknown. Other factors could have factors must also have played a role in the fal- been partly or wholly responsible. Prairie dogs con's demise. were eradicated primarily with strychnine- Historic comparisons indicate that the 2 Tex- laced grain (Hubbard and Schmitt, 1984), as populations of aplomado falcon apparently which undoubtedly killed or disabled a wide declined at about the same time as the New variety of potential mammalian and avian prey Mexico-Arizona population. With only 2 ex- (Apa et al., 1991) and could have affected ceptions, documented nesting attempts termi- aplomado falcons by relay toxicity (Redig et al., nated by 1920 in all 3 states. Records of birds 1982). Historical changes in climate (J. P. Hub- collected and observed became scarce by 1930 bard, New Mexico Department of Game and (Hector, 1987). In Texas, as in New Mexico Fish [retired], pers. comm.) could have been and Arizona, intensive grazing in the bird's involved in the falcon's demise. Livestock graz- habitat commenced a few to several decades ing over time may have reduced the availability prior to any evidence of decline, and apprecia- of perennial wetlands (Brown, 1985), thereby ble woody plant proliferation apparently did reducing habitat quality for aplomado falcons not begin until a few to several decades follow- (U.S. Fish and Wildlife Service, 1990). That ing the decline. Whether reduced grazing ac- the region of interest historically was near the companied the bird's decline is not clear. In- northern distributional limit of this widely-dis- tensive collecting (perhaps to include wanton tributed bird suggests a potential vulnerability killing) immediately preceded the decline in to small changes in habitat quality. South Texas; whether this precipitated the de- The coincidence in timing of the decline of cline or whether this mortality came at a time the aplomado falcon in South Texas, which when populations were already declining in re- had a much greater avian prey biomass and no sponse to other factors is not known. That the prairie dogs and had been grazed heavily for South Texas population occupied habitats at least several decades, adds to the mystery. much different climatically and ecologically Either the synchrony was spurious or the cause than those farther west makes the apparent for the decline was a geographically wide- synchrony in decline across the United States spread phenomenon, such as climate change. range particularly curious. The possibility that reductions in grazing, the disappearance of prairie dogs, or other his- LIMITING FACTORS AND FALCON RESTORATION- toric changes in habitat contributed to the de- 396 The SouthwesternNaturalist vol. 47, no. 3 mise of aplomado falcons in Arizona and New ductions have produced breeding pairs in Mexico has important implications for its res- South Texas. The avian prey base in the re- toration. Because some of these potential lim- cently discovered Chihuahuan Desert nest- iting factors remain operative, the chances for ing areas in northern Chihuahua, where the successfully restoring breeding populations falcons consume primarily birds, apparently may be small. However, experimental releases is similar to that in Arizona and New Mexico. of the falcon, such as those presently under * Research has been equivocal about the pre- way in South Texas, may be the only practical sent-day effects of livestock grazing on the way to assess whether the birds can survive and avian prey potentially available to aplomado breed. Close monitoring of released birds to falcons in southwestern grasslands. Some investigate their responses to grazing, to prai- studies reported no differences in breeding rie dog colonies (if present), to the prey base bird abundance between grazed and ungra- in various habitat contexts, and perhaps to oth- zed areas, some showed that preferred prey er factors seems a desirable component of any species responded variably to grazing, and at release program. least 1 reported an elevated avian biomass in grazed areas. CONCLUSIONSAND RECOMMENDATIONS-This * Occupancy of habitats by black-tailed prairie elevates the available review supports the following conclusions and dogs dramatically prey recommendations: biomass, if juvenile prairie dogs are considered potential prey, and often substantially, * There is little evidence that high intensities even if birds are considered the only poten- of grazing contributed directly to the histor- tial prey. ic decline of the aplomado falcon in Chi- * Declines in prey availability, or relay toxicity huahuan Desert grasslands in southeastern from the strychnine used to kill prairie dogs, Arizona and southwestern New Mexico. In- may help explain the coincidence in timing tensive grazing, both by cattle and prairie of the demise of prairie dogs and aplomado dogs, had occurred in much of the falcon's falcons. habitat in this area for 40 to 50 years by the * Cause-and-effect relationships between his- time of its decline. The falcon's decline ap- toric changes in grazing intensity by cattle peared to coincide with decreases in abun- and prairie dogs and changes in aplomado dances of these grazing animals, attributable falcon abundance cannot at present be dem- in the prairie dog to eradication measures onstrated. that used poison. * Preparations for releasing aplomado falcons * Encroachment of woody plants (brush) into in northern Chihuahuan Desert areas the habitat of the aplomado falcon in Ari- should proceed under the assumption that zona and New Mexico probably had little to the scarcity of available prey could in some do with its decline. The decline occurred locations and circumstances severely limit prior to appreciable levels of brush en- the birds' survival and reproduction. Releas- croachment into places the bird historically es accordingly should be designed to moni- was encountered. Even today, woody plants tor the foraging behavior, survival, and occur sparingly in many of these places. breeding success of the birds in the context However, given the continuing encroach- of the available prey-base and the vegetation ment of brushy species into desert grass- structure in foraging habitats. lands in historic habitat of the bird, habitat degradation by this mechanism may be a EPILOGUE-Beginning in 1985, The Pere- long-term impediment to the bird's success- grine Fund in collaboration with the U.S. Fish ful reestablishment. and Wildlife Service released aplomado fal- * The current avian prey-base biomass in his- cons in South Texas historic range in an at- toric habitat of aplomado falcons in south- tempt to restore a self-sustaining population. eastern Arizona and southwestern New Mex- During the late 1980s, small numbers (<10/ ico appears to be 10% or less of that in areas year) were released annually on the Laguna where the falcon currently breeds abun- Atacosa National Wildlife Refuge and nearby dantly in eastern Mexico and where reintro- ranches (Perez, 1995). None were released September 2002 Truett-Aplomado falcons and grazing 397 during 1990-1992, but 12 to 39/year were re- BAHRE,C. J. 1995. Human impacts on the grasslands of southeastern Arizona. In: M. and leased during 1993-1996. Birds first bred in McClaran, P., T. R. Van Devender, editors. The desert the wild and produced young in 1995 (Jenny, grassland. of Arizona Press, Tucson. 1999). More than 100 captive-bred young were University Pp. 230-264. released each year during 1997-1999. In 1999, BAILEY,F. M. 1928. The birds of New Mexico. New when 115 birds were released at 8 sites, biolo- Mexico Department of Game and Fish, Santa Fe. located 19 8 of which gists free-ranging pairs, BAILEY,V. 1889. Field notes, November 19-26, 1889, to breed 1999). Four of the attempted (Jenny, Dos Cabesas, Arizona. Field Report 21. Smithson- pairs fledged a total of 12 young. It remains to ian Institution Archives. U.S. Fish and Wildlife be seen whether this population can maintain Service, 1860-1961. itself over the longer term once releases of BAILEY,V. 1908. Field notes, August 2-13, 1908, New young are curtailed. Mexico: Grant and Luna Counties. Field Report 72. Smithsonian Institution Archives. U.S. Fish The Turner Endangered Species Fund supported and Wildlife Service, 1860-1961. preparation of this paper. J. Hubbard provided in- BAILEY,V. 1931. Mammals of New Mexico. North formation and extensively reviewed a previous draft, American Fauna Series No. 53. U.S. Bureau of which greatly improved the manuscript. D. Keddy- Biological Survey. Republished in 1971 by Dover Hector shared insights about aplomado falcons and Publications, New York, New York. their habitat relationships during a tour we made of BAKER,D. L., ANDF. S. GUTHERY.1990. Effects of con- their historic habitat in southwestern New Mexico tinuous grazing on habitat and density of ground- and southeastern Arizona. A. Henry provided un- foraging birds in South Texas. Journal of Range published information from prey-base surveys in his- Management 43:205. toric aplomado falcon habitat in southwestern New BENDIRE,C. 1892. Life histories of North American of falcon Mexico. P. Jenny recounted impressions birds, with special reference to their breeding habitat he observed in eastern nesting preferences habits and eggs. Smithsonian Contributions to Mexico. R. discussed various fac- Meyer thoughtfully Knowledge 840. Government Printing Office, tors falcon potentially limiting aplomado popula- Washington, D.C. tions in the Chihuahuan he and A. Desert; Montoya BOCK,C. E., J. H. BOCK,W. R. KENNEY,AND V. M. information about the status of provided breeding HAWTHORNE.1984. Responses of birds, rodents, falcons in Chihuahua, Mexico. E. Fred- aplomado and vegetation to livestock exclosure in a semi- rickson showed me the where S. region J. Ligon desert grassland site. Journal of Range Manage- in 1908 and 1909 found nesting aplomado falcons ment 37:239-242. near New and me obtain Rincon, Mexico, helped BOCK,C. E., V. A. SAAB,T. D. RICH,AND D. S. DOBKIN. historic of in the vicinity. C. photographs landscapes 1993. Effects of livestock grazing on neotropical Oakes provided copies of material from Smithsonian migratory landbirds in western North America. Institution archives. S. Najera translated the abstract In: Finch, D. M., and P. W. Stangel, editors. Status into Spanish. and management of neotropical migratory birds. U.S. Forest Service General Technical Report LITERATURECITED RM-229. Pp. 296-309. BROWN,D. E. 1982. Semidesert grassland. In: Brown, AGNEW,W., D. W. URESK,AND R. M. HANSEN.1986. D. editor. Biotic communities of the American Flora and fauna associated with col- E., prairie dog Southwest-United States and Mexico. Desert onies and adjacent ungrazed mixed-grass prairie Plants 4(14):123-131. in western South Dakota. Journal of Range Man- BROWN,D. E. 1985. Arizona wetlands and waterfowl. agement 39:135-139. of Arizona Tucson. ANTHONY,A. W. 1892. Birds of southwestern New University Press, Mexico. Auk 9:357-369. BRYAN,K. 1925. Date of channel trenching (arroyo in the arid Southwest. Science 62:338- APA, A. D., D. W. URESK,AND R. L. LINDER.1991. cutting) 344. Impacts of black-tailed prairie dog rodenticides L. ANDC. H. HERBEL.1965. on nontarget passerines. Great Basin Naturalist BUFFINGTON, C., Vegeta- 51:301-309. tional changes on a semidesert grassland range ARCHER,S. 1989. Have southern Texas savannas been from 1858 to 1963. Ecological Monographs 35: converted to woodlands in recent history? Amer- 139-164. ican Naturalist 134:545-561. BURGESS,T. L. 1995. Desert grassland, mixed shrub BAHRE,C. J. 1991. A legacy of change: historic im- savanna, shrub steppe, or semidesert scrub? The pact on vegetation in the Arizona borderlands. dilemma of coexisting growth forms. In: Mc- University of Arizona Press, Tucson. Claran, M. P., and T. R. Van Devender, editors. 398 The SouthwesternNaturalist vol. 47, no. 3

The desert grassland. University of Arizona Press, Drought effects on a semidesert grassland range. Tucson. Pp. 31-67. Ecology 53:1084-1093. CATANA,A.J.,JR. 1964. The wandering quarter meth- HOFFMEISTER,D. F. 1986. Mammals of Arizona. The od of estimating population density. Ecology 44: University of Arizona Press, Tucson, and the Ar- 349-360. izona Game and Fish Department, Phoenix. CEBAILOS,G., E. MELLINK,AND L. R. HANEBURY.1993. HOLLON, W. E. 1961. The Southwest: old and new. Distribution and conservation status of prairie University of Nebraska Press, Lincoln. dogs Cynomysmexicanus and Cynomysludovicianus HOOGLAND,J. L. 1995. The black-tailed prairie dog: in Mexico. Biological Conservation 63:105-112. social life of a burrowing mammal. University of CORMAN,T. E. 1992. Evaluation of ten potential Chicago Press, Chicago, Illinois. C. 1941. eradication research at northern aplomado falcon reintroduction areas HORNE, Mesquite in southeastern Arizona. Arizona Game and Fish Texas Tech. Sheep and Goat Raiser 22:24. HUBBARD, ANDC. G. SCHMITT.1984. The black- Department, Nongame and Endangered Wildlife J. P., footed ferret in New Mexico. to U.S. Bu- Program, Technical Report 26. Phoenix. Report COTTAM,C., ANDN. CAROLINE.1965. The black-tailed reau of Land Management, Santa Fe, New Mex- ico, from New Mexico of Game and prairie dog in Texas. TexasJournal of Science 17: Department 294-302. Fish, Santa Fe. HUMPHREY,R. R. 1958. The desert a his- DAVIS,C. A., P. E. SAWYER,J. P. GRIFFING,AND B. D. grassland: of and an of BORDEU. 1974. Bird populations in a shrub-grass- tory vegetational change analysis causes. Botanical Review 24:193-252. land area, southeastern New Mexico. New Mexi- S. W. 1985. Habitat selection in co Agricultural Experiment Station Bulletin 619. JANES, raptorial birds. In: Cody, M. L., editor. Habitat selection in DETLING,J. K. 1998. Mammalian herbivores: ecosys- birds. Academic Press. New New York. tem-level effects in two national York, Pp. grassland parks. 159-188. Wildlife Society Bulletin 26:438-448. JARDINE,J. T., ANDC. L. FORSLING. 1922. Range and DICK-PEDDIE,W. A. 1993. New Mexico vegetation: cattle management during drought. U.S. Depart- past, present, and future. University of New Mex- ment of Agriculture Bulletin No. 1031. ico Press, Albuquerque. JARDINE,J. T., AND L. HURTT.1917. Increased cattle EMLEN, T. 1972. Size and structure of a J. wintering on southwestern U.S. avian in southern Texas. 53: production ranges. Depart- community Ecology ment of Bulletin 588. 317-329. Agriculture JENNY, J. P. 1998. Northern aplomado falcon recov- FINDLEY, S., A. H. HARRIS,R. E. WILSON, AND C. J. The Peregrine Fund, 1975. Mammals of New Mexico. ery. Unpublished report. JONES. University Boise, Idaho. of New Mexico Press, Albuquerque. JENNY,J. P. 1999. Northern aplomado falcon recov- HECTOR,D. P. 1985. The diet of the aplomado falcon ery. Unpublished report. The Peregrine Fund, (Falco in eastern Mexico. Condor 87: femoralis) Boise, Idaho. 336-342. JOHNSTON, M. C. 1963. Past and present grasslands D. P. 1986. and its re- HECTOR, Cooperative hunting of southern Texas and northeastern Mexico. to success and size in an lationship foraging prey Ecology 44:456-466. avian 73:247-257. predator. Ethology JORDAN,T. G. 1981. Trails to Texas: southern roots HECTOR,D. P. 1987. The decline of the aplomado of western cattle ranching. University of Nebras- falcon in the United States. American Birds 41: ka Press, Lincoln. 381-389. KEDDY-HECTOR,D. P. 1988. Vegetative cover, small HENNESSY, T., R. P. M. TROMBLE,AND M. J. GIBBENS,J. bird abundance and patterns of aplomado falcon CARDENAS.1983. from 1935 Vegetation changes habitat quality in eastern Mexico. In: Glinski, R. to 1980 in mesquite dunelands and former grass- L., B. G. Pendleton, M. B. Moss, M. N. LeFranc, lands of southern New Mexico. of Journal Range Jr., B. A. Millsap, and S. W. Hoffman, editors. Pro- 36:370-374. Management ceedings Southwest raptor management sympo- A. L. 1995. of a HENRY, Inventory northernJornada sium and workshop. National Wildlife Federa- del Muerto for grassland the aplomado falcon tion, Washington, D.C. Pp. 157-164. Falco femoralis septentrionalis.Report by New Mex- KIFF, L. F., D. B. PEAKALL,AND D. P. HECTOR. 1980. ico Heritage Program, The Nature Conservancy, Eggshell thinning and organochlorine residues to the Turner Foundation. in the bat and aplomado falcons in Mexico. In- HENRY,A. L. 1998. BLM grassland bird study: draft ternational Ornithological Congress 17:949-952. summary. Report from Arid Lands Project, Santa KINGSTON, M., AND M. G. CRAWFORD.1993 (editors). Fe, New Mexico, to U.S. Bureau of Land Man- Texas almanac, 1994-95. The Dallas Morning agement, Socorro, New Mexico. News, Dallas, Texas. HERBEL,C. H., F. N. ARES,AND R. A. WRIGHT.1972. KNOWLES,C. J. 1986. Some relationships of black- September 2002 Truett-Aplomado falcons and grazing 399

tailed prairie dogs to livestock grazing. Great Ba- on the Pedro Armendaris Ranch. Unpublished sin Naturalist 46:198-203. report to Turner Endangered Species Fund. KNOWLES,C. J. 1987. Reproductive ecology of black- OBERHOLSER,H. B. 1974. The bird life of Texas, Vol. tailed prairie dogs in Montana. Great Basin Nat- 1. Edited by E. B. Kincaid, Jr. University of Texas uralist 47:202-206. Press, Austin. KOFORD,C. B. 1958. Prairie dogs, whitefaces, and PEREZ,C. J. 1995. Movements, habitat use, and sur- blue grama. Wildlife Monographs No. 3. vival of released aplomado falcons at Laguna LEHMAN,V. W. 1969. Forgotten legions: sheep in the Atascosa National Wildlife Refuge, Texas. Unpub- Rio Grande Plains of Texas. Texas Western Press, lished M.S. thesis, New Mexico State University, El Paso. Las Cruces. AND D. SMITH. 1996. Surviv- LIGON,J. S. 1961. New Mexico birds and where to PEREZ,C.J., P.J. ZWANK, movements and habitat use of fal- find them. University of New Mexico Press, Al- al, aplomado cons released in southern Texas. Journal of buquerque, and New Mexico Department of Rap- tor Research Game and Fish, Santa Fe. 30:175-182. RAITr, AND R. L. MAZE.1968. Densities and LIST,R. 1997. Ecology of kit fox (Vulpes macrotis)and R.J., species of birds of a creosote coyote (Canis latrans) and the conservation of the composition breeding bush community in southern New Mexico. Con- prairie dog ecosystem in northern Mexico. Un- dor 70:193-205. published Ph.D. dissertation, University of Ox- RAITT, R. J., ANDS. L. PIMM.1976. Dynamics of bird ford, England. communities in the Chihuahuan Desert, New MACARTHUR, R. H., AND MACARTHUR, 1961. On W.J. Mexico. Condor 78:427-442. bird species diversity. Ecology 42:594-598. REDIG,P. T., C. M. STOWE,T. D. ARENDT,AND D. H. MAZAZANO-FISCHER,P., R. LIST, and G. CEBALLOS. DUNCAN.1982. of in 1999. Grassland birds in towns in Relay toxicity strychnine rap- prairie-dog tors in relation to a eradication northwestern Mexico. Studies in Avi- pigeon program. Chihuahua, and Human 24:335-336. an 19:263-271. Veterinary Toxicology Biology REYNOLDS,M. C., ANDP. R. KRAUSMAN.1998. Effects McAuLIFFE,J.C. 1995. Landscape evolution, soil for- of winter burning on birds in mesquite grassland. mation, and Arizona's desert grasslands. In: Wildlife Society Bulletin 26:859-866. McClaran, M., and T. Van Devender, editors. The RODRIGUEZ,I. Z. F. P. 1996. Ecol6gia halc6n aplo- desert University of Arizona Press, Tuc- grassland. mado (Falcofemoralis) en el estado de Chihuahua. son. Pp. 100-129. Departamento de Recursos Naturales, Universi- MEARNS,E. A. 1907. Mammals of the Mexican dad Autonoma de Chihuahua, Chihuahua, Mex- United States. Smithsonian In- boundary of the ico. stitution. United States National Museum Bulle- ROTENBERRY,J. T., AND J. A. WIENS. 1980. Habitat tin 56. U.S. Government Office, Wash- Printing structure, patchiness, and avian communities in D.C. ington, North American steppe vegetation: a multivariate MEINE,C. 1999. It's about time: conservation biology analysis. Ecology 61:1228-1250. and Conservation 13:1-3. history. Biology ROTH,R. R. 1977. The composition of four bird com- R. 1997. falcon and habitat MEYER, Aplomado survey munities in South Texas brush-grassland. Condor evaluation on Fort Bliss Military Reservation, 79:417-425. 1995-1996. to Conservation Division, Di- Report SCHMIDLY,D. J. 1977. The mammals of Trans-Pecos rectorate of the Fort Bliss. Environment, Texas. Texas A & M University Press, College Sta- MONTOYA,A. B. 1995. Habitat characteristics, prey tion. home of the fal- selection, and ranges aplomado TELFAIR,R. C., II. 1999. Introduction: ecological re- con in Mexico. M.S. Chihuahua, Unpublished gions of Texas: description, land use, and wild- thesis, New Mexico State University, Las Cruces. life. In: Telfair, R. C., II. Texas Wildlife and land MONTOYA,A. B., P.J. ZWANK,AND M. CARDENAS.1997. uses. University of Texas Press, Austin. Pp. 1-39. Breeding biology of aplomado falcons in desert THIOLLAY,J. 1980. Strategies d'exploitation par les grasslands of Chihuahua, Mexico. Journal of repaces d'un ecosystem herbace neotropical. Field Ornithology 68:135-143. Alauda 38:221-253. MORRISEY,R. J. 1952. The early range cattle industry TRUETT,J. C., AND T. SAVAGE.1998. Reintroducing in Arizona. Agricultural History 24:151-156. prairie dogs into desert grasslands. Restoration OAKES,C. L. 1998a. A history of prairie dogs (Cyno- and Management Notes 16:189-195. mys ludovicianus) and vegetation change on the U.S. FISHAND WILDLIFE SERVICE. 1990. Aplomado fal- Jornada Experimental Range. Unpublished re- con recovery plan. U.S. Fish and Wildlife Service, port, Department of Geography, University of Albuquerque, New Mexico. Texas, Austin. U.S. FISH AND WILDLIFESERVICE. 1997a. Biological OAKES,C. L. 1998b. Historic prairie dog distributions opinion on the Socorro Resource Area Resource 400 TheSouthwestern Naturalist vol. 47, no. 3

Management Plan. Memorandum from New WELTZIN,J. F., S. ARCHER,AND R. K. HEITSCHMIDT. Mexico Ecological Services Field Office, Albu- 1997. Small-mammal regulation of vegetation querque, to Area Manager, Socorro Resource structure in a temperate savanna. Ecology 78: Area, Bureau of Land Management, Socorro, 751-763. New Mexico. WHICKER,A. D., AND J. K. DETLING. 1988. Ecological U.S. FISH AND WILDLIFESERVICE. 1997b. Biological consequences of prairie dog disturbances. Bio- opinion on the Caballo Resource Area Resource Science 38:778-785. Management Plan. Memorandum from New WILLIAMS,S. O., III. 1998. Aplomado falcons in New Mexico Ecological Services Field Office, Albu- Mexico: historic and recent. Unpublished report, querque, to Bureau of Land Management, Las New Mexico Department of Game and Fish, San- Cruces District Office, Caballo Resource Area, ta Fe. New Mexico. WILLSON,M. F. 1974. Avian community organization VAN DEVENDER,T. R. 1995. Desert grassland history: and habitat structure. Ecology 55:1017-1029. changing climates, evolution, biogeography, and WOOD,J. E. 1969. Rodent populations and their im- community dynamics. In: McClaran, M. P., and T. pact on desert grasslands. New Mexico State Uni- R. VanDevender, editors. The desert grassland. versity. Agricultural Experiment Station Bulletin University of Arizona Press, Tucson. Pp. 68-99. 555. WAGONER, J. J. 1951. Development of the cattle in- YORK,J. C., AND W. A. DICK-PEDDIE. 1969. Vegetation dustry in southern Arizona, 1870's and 80's. New change in southern New Mexico during the past Mexico Historical Review 26:204-224. hundred years. In: McGinnies, W. G., and B. J. WAGONER,J. J. 1952. History of the cattle industry in Goldman, editors. Arid lands in perspective. Uni- southern Arizona, 1540-1940. University of Ari- versity of Arizona Press, Tucson. Pp. 157-166. zona Social Science Bulletin No. 20. YOUNG,K. E., B. C. THOMPSON,AND R. Valdez. 1999. WALLMO,C. 0. 1950. Vegetation mapping of Fort Determination of habitat suitability for aplomado Huachuca Wildlife Area. Arizona Game and Fish falcons on public lands in southern New Mexico. Commission, Completion Report, Project 46-R-1, Report to Bureau of Land Management, Las Cru- Job 1. ces District, and U.S. Army, White Sands Missile WALLMO,C. 0. 1955. Vegetation of the Huachuca Range, through U.S. Geological Survey Biologi- Mountains, Arizona. American Midland Natural- cal Resources Division Cooperative Research ist 54:466-480. Units, Reston, Virginia. WEBER, D. J. 1992. The Spanish frontier in North America. Yale University Press, New Haven, Con- Submitted19 November1999. Accepted10 July 2001. necticut. AssociateEditor was William H. Baltosser