Density and Habitat Use of Raptors Along the Rio Bavispe and Rio Yaqui, Sonora, Mexico

]. RaptorRes. 24(3):47-51 ¸ 1990 The Raptor ResearchFoundation, Inc.

DENSITY AND HABITAT USE OF RAPTORS ALONG THE RIO BAVISPE AND RIO YAQUI, SONORA, MEXICO

RICARDO RODRIGUEZ-ESTRELLA Centrode InvestigacionesBiologicas, Apdo. Postal 128, La Paz, 23000, Baja CaliforniaSur, Mexico

BRYAN T. BROWN P.O. Box 3741, Tuscon, AZ 85722

ABSTRACT.--Raptorpopulations of the Rio Yaqui and one of its principal tributaries, the Rio Bavispe, of Sonora, Mexico, were surveyedby boat in the early spring of 1987 and 1988. The purposesof this surveywere to documentpopulation status,to examine patterns of habitat use, and to compareraptor abundancebetween rivers. Turkey Vulture (Cathartesaura) and Black Vulture (Coragypsatratus) were the two most abundantraptors in the study area, although mostvultures observedduring the study period appearedto be migrants.Nesting raptors included Bald Eagle (Haliaeetusleucocephalus), Red-tailed Hawk (Buteojarnaicensis), Common Black-Hawk (Buteogallusanthracinus), and PeregrineFalcon (Falcopere- grinus).Common Black-Hawk was the mostabundant nesting raptor, exhibiting a densityof 0.35 pairs/ km in riparian woodland along the Rio Bavispe,the highestreported density for this speciesin Mexico. Common Black-Hawks were strongly associatedwith riparian areas dominatedby densestands of large Honey Mesquite (Prosopisglandulosa) and Pithecellobium(Pithecellobiurn mexicanurn), a habitat that has apparently increasedin extent due to the operationof an upstreamdam and reservoir.Bald Eagles and Peregrine Falcons nestedon riverside cliffs.

Densidady usodel habitat por avesrapaces a lo largo del Rio Bavispey del Rio Yaqui, Sonora,M•xico EXTRACTO.--Durante el inicio de la primavera de 1987 y 1988 se censaronlas poblacionesde aves rapacesdel Rio Yaqui y del Rio Bavispe,en Sonora,M•xico. Los rios fueron recorridosen canoascon el prop6sitode registrarla abundanciade avesrapaces y determinarlos patronesdel usodel habitat que las diferentesespecies tienen en los dos rios. El Aura Comfin (Cathartesaura) y el Zopilote Negro (Coragypsatratus) fueron las rapacesmils abundantesen los censos,aunque la mayorla de los individios eran problamentemigratorios. Las especiesque se reproduclanen la zona incluyen el Aguila Calva (Haliaeetusleucocephalus), el Aguililla colirroja(Buteo jarnaicensis), el Aguililla Cangrejera(Buteogallus anthracinus)y el Halc6n Peregrino(Falco peregrinus). La especieque anidabamils abundantementefue el Aguililla Cangrejera,con una densidadde 0.35 pares/km en el bosquede galeria del Rio Bavispe. Esta es la mits alta densidadreportada para la especieen Mfixico. El Aguililla Cangrejera estuvo particularmenteasociada a fireasdensas dominadas por Mezquite (Prosopisglandulosa) y por Palo Fierro (Pithecellobiurnmexicanurn), un habitat que se ha incrementadodebido aparentemente a la operaci6nde una presaubicada en la parte alta del rio. El Aguila Calva y el Halc6n Peregrinofueron vistos anidando sobrelos riscosque bordeanel rio.

Few studiesexist regarding raptor populationsof associatedwith riparian zones,particularly Com- Mexico (Wetmore 1945, Friedmann et al. 1950, mon Black-Hawk (Buteogallusanthracinus). Thiollay 1977, 1978, 1979, 1980). Most of these studies were conducted in eastern Mexico and few STUDY AREA relate to raptor status and habitat use in the more The Rio Yaqui, the largestriver of Sonora(Fig. 1), is arid northwesternportion of Mexico. Here we pre- formedby the confluenceof its major tributaries,the Rio sent information on the density and habitat use of Aros and Rio Bavispe. The study area consistedof the Rio Bavispe from the town of Granados downstream to raptorsalong the Rio Yaqui and one of its principal its confluencewith the Rio Aros (65 km) and the main- tributaries, the Rio Bavispe,in Sonora,Mexico. Spe- stream Rio Yaqui from the Aros/Bavispe confluence cial attention has been focusedon nesting raptors downstreamto the El RaspaderoMine at the head of the

47 48 RICARDO RODRIGUEZ-ESTRELLAAND BRYAN T. BROWN VOL. 24, NO. 3

(Sapindussaponaria), and a few scattered cottonwoods (Populusfremontii) and palms (Erythearoezlii). Riparian vegetation also invaded narrow floodplains in the river canyons(White 1948). Riparian vegetationof the Rio Bavispe,and to a lesser extent the Rio Yaqui, has been modified by the upstream constructionand operationof the AngosturaReservoir on the Rio Bavispesince the mid 1930s.The dam eliminated the large annual floodswhich had formerly scouredaway all vegetationbelow the pre-dam high water mark, allow- ing a densenew zone of woody vegetationto developat the post-dam river's edge. Although the extent of this increase in riparian habitat is undocumented,our quali- tative observationssuggest that Mesquite- and Pithecel- 1obium-dominatedriparian woodland along the Rio Ba- vispeincreased several hundred percent, from an estimated pre-dam 50 ha to a post-dam200 ha. A continuous,dense bandof closed-canopy,even-aged riparian woodlanddom- inated by Mesquite and Pithecellobiumaveraging 12 m in height had formed along the Rio Bavispeby the 1980s. This is analogous to the increase in extent of riparian vegetation documentedalong the Colorado River in Ari- zona after the completionof Glen Canyon Dam (Turner and Karpiscak 1980). Riparian vegetationwas lesswell- developedalong the Rio Yaqui due to the recurrenceof annual floods from the undammed Rio Aros. The riparian zone of the Rio Yaqui was greatly disturbed by human activitiesincluding agriculture, grazing, mining, roads, and the presenceof several small ranches and a village. The Rio Bavispewas relatively undisturbed exceptfor grazing and the presenceof two small ranches.

METHODS We surveyedraptor populationsin the studyarea from 24 March to 2 April 1987, and from 7-14 April 1988. The surveyswere made from canoesas we floateddown- stream,censusing a distanceof 15-20 km/day. Up to eight skilled observersparticipated in the surveys. Eachsurvey was designed as a continuoustransect (Fuller and Mosher 1981) wherethe transectlength was the length of the river traversedin km. All raptors observedwithin Figure 1. The Rio Bavispeand Rio Yaqui study area 0.5 km of the river were recorded between 1000 and 1800 was surveyedin 1987-1988. Bold line showsthe portions H, the range of times when we were actively moving of each river censused. downstream.For eachraptor we recordedspecies, location, indicationsof breeding,if any, and habitat.We classified the studyarea into fivemajor habitats: Pithecellobium and E1 Novillo Reservoir (80 km). Elevationsalong the river Mesquite, Gooding Willow and Pithecellobium,Mes- range from 520 m at Granadosto 330 m at the E1 Novillo quite,upland mountain or cliff,or disturbedareas. Habitat Reservoir; mountain peaks adjacent to the rivers rise to occurrencefor raptorswhose nest location was knownwas over 1800 m. The Rio Bavispe flows through relatively classifiedaccording to the nestlocation; habitat occurrence narrow canyons, while the Rio Yaqui flows through a for raptorswhose nest location was not knownwas clas- broad canyon. Cliffs up to 100 m in height occur infre- sifiedaccording to the initial habitat in which they were quently along both rivers. Upland vegetationconsists of observed. subtropical,winter-deciduous thornscrub averaging 10 m Several techniques,including the continuousvisual in height dominatedby Lysilomadivarzcata, Acacia cochlia- transect,nest searches,and observationsof territorial be- cantha,Ceiba acuminata, Cercidium praecox, Mimosa dyso- havior, were used to determine raptor abundance. Nest carpa, and Fouquieriamacdougalii (White 1948). sitesof cliff-nesting specieswere locatedby visual inspec- Riparian vegetationconsisted of Pithecellobium(Pithe- tion of cliff habitat; tree nests were often visible from the cellobiummexicana), Honey Mesquite (Prosopisglandulo- river as we floated past. sa), Gooding Willow (Salix gooddingii),Fig (Ficuspetio- Observationof the territorial behaviorof singleor paired laris),Canyon Ragweed (Ambrosia ambrosioides), Soapberry Common Black-Hawks was useful in determining the AUTUMN1990 RIO YAQUIRAPTORS 49

Table 1. Raptorsobserved along the Rio Bavispeand the Rio Yaqui,in Sonora,Mexico, 1987-1988.

BAVISPE YAQUI TOTAL (65 river-km) (80 river-km) (145 river-km) 1987 1988 1987 1988 1987 1988

Black Vulture (Coragypsatratus) 77 141 54 64 131 205 Turkey Vulture (Cathartesaura) 122 67 179 57 301 124 Bald Eagle (Haliaeetusleucocephalus) -- -- 4 4 4 4 Sharp-shinnedHawk (Accipiterstriatus) 1 1 2 4 3 5 Cooper'sHawk (Accipitercooperi) -- 2 1 1 1 3 CommonBlack-Hawk (Buteogallusanthracinus) 37 42 15 16 52 58 Gray Hawk (Buteo nitidus) -- Red-tailedHawk (Buteojamaicensis) 11 3 13 8 24 11 AmericanKestrel (Falcosparverius) 1 -- 2 -- 3 -- PeregrineFalcon (Falcoperegrinus) 2 -- 2 -- 4 -- Prairie Falcon (Falco mexicanus) 2 ------2 -- No. of species 8 6 10 7 11 7 Total individuals 253 256 273 154 526 410 Individuals/kin 3.8 3.9 3.4 1.9 3.6 2.8

numberof pairspresent, particularly along the Rio Bavis- gyps atratus) were the two most abundant species pc. Single or paired adults would regularly precedeor encountered (Table 1). The next most abundant accompanyus, screamingloudly, as we floated down- specieswas the Common Black-Hawk, a common streamthrough their territory. The adult(s) would break away and fly back upstreamafter a certain point, which nesting species. we interpreted to be the territorial boundary. Here, the We observeddifferent patterns of habitat occu- raptorswere often replacedby other singleor paired adults pationamong selected raptor species(Table 2). Black which would likewise precedeor accompanyus through Vultures were almost entirely associatedwith dis- their territory.Constant observation of theactivity patterns of Common Black-Hawks allowed us to estimate their turbed areas,while Turkey Vultures were primarily populationsize along the river. Nests attendedby single associatedwith riparian areas dominatedby Mes- or paired adults were interpretedto indicatethe centerof quite and Pithecellobium. Common Black-Hawks activity of a pair. Single adults exhibiting territorial be- were alsolargely found in riparian areasdominated haviorwere interpreted as a pair, unlesssingle adults were by Mesquite and Pithecellobium.Bald Eagles(Hal- observed< 1 km from anothersingle adult exhibitingter- ritorial behavior. In the latter case,the two adults may iaeetusleucocephalus) and Peregrine Falcons (Falco have beenpaired but were temporarilyin different areas peregrinus)occurred at siteswith suitablecliffs, while of their territory. Pairs of Common Black-Hawks were Red-tailed Hawk (Buteojamaicensis) observations not includedin the censusas long as they continuedto fly were at bothmountain cliffs and riparian areasdom- downstreamahead of the advancingboats, but only when they were seento fly back upstreamor away from the inated by Mesquite and Pithecellobium.The abun- river on a course that would not result in their being danceof differenthabitats within the studyarea was counted twice. not examined,precluding a quantitativecomparison of habitat preferences. We observedonly four speciesnesting in the study RESULTS area, althoughmore speciesmay nest during sum- A total of 526 individualsof 11 raptor species mer. Two occupiedBald Eagle nestswere attended were observedalong the two rivers in 1987; 410 by adults in both 1987 (Brown 1988) and 1988. An individualsof sevenspecies in 1988 (Table 1). Mean occupiedPeregrine Falcon eyrie was attendedby values of 3.6 and 2.8 individuals/km occurredin the two adults in 1987, but was unoccupiedin 1988. studyarea in 1987 and 1988, respectively.Turkey Courtshipwas observedat two probableRed-tailed Vulture (Cathartesaura) and Black Vulture (Cora- Hawk nests in 1987 and 1988 where incubation had 50 RICARDO RODRIGUEZ-ESTRELLAAND BRYAN T. BROWN VOL. 24, No. 3

Table 2. Proportionsof raptorsobserved in five riparian habitatsalong the Rio Bavispe,and Rio Yaqui, Sonora, Mexico, 1987.

GOODDING PITHE- WILLOW/ NUMBERSOF CELLOBIUM/ PITHE- MOUNTAIN/ DISTURBED SPECIES INDIVIDUALS MESQUITE CELLOBIUM CLIFFSa AREASb

Black Vulture 131 0.04 0 0 0.96 Turkey Vulture 306 0.57 0.02 0.13 0.28 Bald Eagle 4 0 0 1.0 0 Common Black-Hawk 52 0.70 0.25 0.03 0.02 Red-tailed Hawk 24 0.38 0.04 0.58 0 Peregrine Falcon 4 0 0 1.0 0 Total 521 224 20 64 213 Mountain/cliffswere largelyassociated with uplandvegetation. By humaninfluence (i.e. fields,pastures, cattle-ranches).

not yet begun. The fourth nestingspecies was the The greatestconcentrations (up to 50 individuals/ Common Black-Hawk, which exhibited courtship site) of Black Vultures were observed near small behavior and strong territorial behavior during the villagesor ranches.The primary differencebetween study period. Five and sevenprobable Common the 1987 and 1988 abundanceof vulturesin the study Black-Hawk nests were located in 1987 and 1988, area was apparentlydue to greaternumbers of Black respectively,although incubation had evidentlynot Vultures near the ranchesand villagesin 1988, and yet begunfor mostpairs. Common Black-Hawk was greaternumbers of Turkey Vultures overall in 1987. the most abundant riparian-nesting raptor in the These differencesmay have been due to the timing study area, where it occurredat densitiesof up to of vulture migration during the two study years, 0.35 pairs/km alongthe Rio Bavispe(Table 3). sincemost of the vulturesobserved were probably migrants. Lower raptor densityon the Rio Yaqui may be DISCUSSION explainedby the presenceof humansper se,or subtle habitat differences between the two rivers. Direct We observeda proportionatelygreater concentra- human interference of sensitiveraptors is a well- tion of Turkey Vultures and Black Vultures than documented detriment to their occurrence (Newton other raptors on the Rio Yaqui. These speciesare 1979, Thiollay 1984, 1985). The Rio Yaqui expe- clearly associatedwith human activities(Table 2). riences more human disturbance than the Rio Ba- vispe. However, the Rio Bavispe has also experi- enced substantial human-related modification (i.e., Table 3. Pairs of Common Black-Hawks and their den- increasein the extent of riparian woodland due to sity/river-km on the Rio Bavispe,and Rio Ya- the Angostura Reservoir).These modificationsap- qui, Sonora, Mexico, as determinedthrough downriver continuous transect surveys per- parently increasedthe complexityand extent of the formed in late March and early April, 1987 riparian environment. and 1988. Single adults exhibiting territorial The Common Black-Hawk was a specialistin the behavior were countedas a pair. use of habitatsnot favoredby other species(dense riparian woodland dominated by Mesquite and RIO BAVISPE RIO YAQUI TOTAL Pithecellobium). This habitat was most developed along the Rio Bavispe, where the highest nesting DEN- DEN- DEN- densityof CommonBlack-Hawks occurred(Table YEAR NO. SITY a NO. SITY No. SITY 3). Their high densitythere may have been associ- 1987 23 0.35 8 0.10 33 0.22 ated with the increasein riparian habitat brought 1988 21 0.32 8 0.10 30 0.20 about by the constructionand operationof the An- a Densityexpressed as a numberof pairs/km. gostura Reservoir. AUTUMN 1990 RIo YAqU! RAPTORS 51

Little comparativeinformation on CommonBlack- LITERATURE CITED Hawk densityexists from otherportions of its range BROWN,B.T. 1988. Additional Bald Eagle nesting rec- (8 pairs/3000 ha, Palma Sola, Veracruz; Thiollay ords from Sonora, Mexico. J. Raptor Res. 22:30-32. 1978), but its abundanceon this 65-km stretch of , P.L. WARREN AND L.S. ANDERSON. 1987. First the Rio Bavisperepresents the highestreported for Bald Eagle nestingrecord from Sonora,Mexico. Wilson this speciesin Mexico. Densitiesreported in Table Bull. 99:279-280. 3 shouldbe conservativelyinterpreted as minimum FRIEDMANN, H., C. GRISCOM AND R.T. MOORE. 1950. populationsizes. Only one surveywas made each Distributional checklist of birds of Mexico, Part 1 Pacific CoastAvifauna No. 29. CooperOrnithol. Club, year, when severalsurveys per nestingseason would Berkeley, California. have providedmore accurateinformation. FULLER, M.R. AND J.A. MOSHER. 1981. Methods of Common Black-Hawks are restrictedto riparian detecting and counting raptors: a review. Studiesin habitat in the arid Southwest(Millsap 1981), and Avian Biol. 6:235-246. our limited observationson the configurationsof their HITCHCOCK,M.A. 1977. A surveyof the Peregrine Fal- territoriesin the study area suggestedlinear terri- con population in northwestern Mexico, 1976-77. toriesnearly equally spacedalong the Rio Bavispe. Contrib. No. 40, Chihuahuan Desert Research Insti- Their affinities for riparian habitat, territorial re- tute, Alpine, Texas. sponsesto intruders early in the breedingseason, MCEWAN, L.C. AND D.H. HIRTH. 1979. Southern Bald and nature of the habitat they occupysuggest that Eagle productivity and nest site selection.J. Wildl Manage. 43:585-594. surveysby boat, when feasible,may be an efficient MILLSAP, B.A. 1981. Distributional status of falconi- method of censusingCommon Black-Hawk popu- formesin west-centralArizona, with noteson ecology, lations. reproductivesuccess, and management.Tech. Note 355, The occurrenceof the two rarer nesting species, U.S.D.I. Bureau of Land Management, Phoenix, Ar- Bald Eagle and Peregrine Falcon, was of special izona. interest.The Rio Yaqui drainageis the only known NEWTON,I. 1979. Populationecology of raptors. Buteo localityon the mainlandof Mexico (excludingBaja Books, Vermillion, SD. California) where Bald Eagles are known to nest SWENSON,J.E., K.L. ALT ANDR.L. ENG. 1986. Ecology (Brown et al. 1987, Brown 1988). Two young ap- of Bald Eagles in the greater Yellowstoneecosystem. parently fledgedfrom one of the occupiednests in Wildlife Monogr. 95. THIOLLAY,J.M. 1977. La migration d'automne sur la 1987, and bothnests contained two youngnear fledg- cote orientale du Mexique. Alauda 45:344-346. ing in 1988, suggestingthat the quality of the Rio 1978. Comparaisonsentre les peuplementsde Yaqui territorieswas adequate. Swenson et al. (1986) falconiformesdes plaines cotieresdu Mexique et de found that a stable food sourceappeared to be the Cote D'Ivoire. Le Gerfaut 68:139-162. mostimportant factor influencing selection of nesting 1979. L'importance d'un axe de migration: la territoriesby Bald Eagles,but the presenceof suit- cote est du Mexique. Alauda 47:235-245. able sites for nest-placementis also critical (Mc- 1980. Spring hawk migration in easternMex- Ewan and Hirth 1979). We observedmany fish in ico. Raptor Res. 14:13-20. the Rio Yaqui near the two Bald Eagle nests.Both 1984. Raptor communitystructure of a primary rain forest in French Guiana and effect of human hunt- nestswere locatedon high cliffs abovethe river. The status and abundanceof Peregrine Falcons ing pressure.Raptor Res. 18:117-122. 1985. Compositionof falconiform communities in Sonorais poorly known (Hitchcock 1977). The along successionalgradients from primary rainforest singleoccupied eyrie we observedalong the Rio Ba- to secondaryhabitats. Intl. Council for Bird Preser- vispein 1987 suggestedthat this specieswas rare in vation, Tech. Publ. No. 5:181-190. the studyarea, although other pairs could have gone TURNER, R.M. AND M.M. KARPISCAK. 1980. Recent undetectedin our survey. vegetationchanges along the ColoradoRiver between

ACKNOWLEDGMENTS Glen Canyon Dam and Lake Mead, Arizona. U.S GeologicalSurvey Prof. Pap. 1132, U.S. Government Specialthanks are due to Alfredo Ortega, M.S. Laura Printing Office, Washington, D.C. Arriaga, and Peter L. Warren for their commentson pre- WETMORE, A. 1943. The birds of southern Veracruz, vious drafts of this manuscript. S. Anderson, J. Arenas, Mexico. Proc. U.S. Nat. Mus. 93:215-340. W. Leibfried, R. Mesta, S. Reyes, and P. Warren gave valuable assistancein the field. The illustration is by R. WHITE, S. 1948. The vegetationand flora of the region Lomeli. Financial supportwas providedby the U.S. Bu- of the Rio Bavispe in northeastern Sonora, Mexico. reau of Reclamation,the Salt River Project,and Secretaria Lloydia 11:229-302. de Programaciony Presupuestoand Centro de Investi- gacionesBiologicas of Mexico. Received19 December 1989; accepted5 July 1990