The Diet of the Aplomado Falcon (Falco Femoralis) in Eastern Mexico
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The Condor 87:336-342 0 The CooperOrnithological Society 1985 THE DIET OF THE APLOMADO FALCON (FALCO FEMORALIS) IN EASTERN MEXICO DEAN P. HECTOR ABSTRACT. -1 describehere breeding seasondiets of Aplomado Falcons (F&co femoralis) at 18 sites in Veracruz, Campeche, and Chiapas, Mexico, based on 256 animals in prey remains and 234 prey that I detected while watching the falcons’ feeding behavior. Birds comprised 94% of individuals in prey remains, but only 35% of prey that I saw being taken. Although the remainder and majority of the prey that I saw being taken were insects, 97% of prey biomass in this sample was birds. Common prey were moths, beetles, doves, cuckoos, and grackles. Prey animals ranged in weight from lessthan 1 g to over 500 g. Avian prey that I saw being taken averaged 67 g. In at least one case, prey size may have influenced prey selection within speciessince the falcons preferentially took female Great- tailed Grackles (Quiscalus mexicanus), which are smaller than males. The swift- nessof Aplomado Falcons in flight, coupled with their agility on foot and tendency to hunt cooperatively, may account for their broad prey preferences.They do not, however, capture swifts and swallows. The high proportion of birds in the diet may explain the falcon’s heavy contamination with residues of DDT. The Aplomado Falcon (F&co femoralis) in- tris;Kellogg in Bailey 1928); Cassin’s Sparrow habits neotropical savannasand desert grass- (Aimophilacussinii; Strecker 1930); plovers, lands from the southwesternUnited States to sandpipers,nighthawks, Ruddy Ground-Dove Tierra de1 Fuego (Blake 1977). Before 1930, (Columbinatalpacotl], Scaled Dove (Scarda- this falcon nested regularly in Arizona, Texas, fella squammata),Pale-vented Pigeon (Co- and New Mexico, but today, it is rare north of lumbacayennensis), Rock Dove (C. livia), and Mexico (Ligon 196 1, Phillips et al. 1964, Ob- a fledgling hummingbird (Friedmann and erholser 1974). Smith 1950, 1955); Lark Bunting (Calamos- In eastern Mexico, Aplomado Falcons are piza melanocorys),and Lark Sparrow (Chon- heavily contaminated with residues of DDT destesgrammacus; Ligon 1961); doves and and lay eggs with shells nearly 30% thinner quail (Wetmore 1965); tinamou (Johnson than pre-DDT (pre-1947) eggshellsfrom the 1965); Plain-breasted Ground-Dove (Colum- same region (Kiff et al. 1980). Unlike raptors bina minuta; Howell 1972); and Dickcissel that eat insectsor small mammals, bird-eating (Spiza americana;ffrench 1973). Other ver- and fish-eating raptors have been seriously af- tebrate prey are: bats(Ligon 196 1, ffrench 1973, fected by pesticide contamination (Newton and Mader 198 1); kangaroo rats (specimen 1979). Situated at the upper levels of extensive notes of R. D. Camp); pocket mice (Perog- food webs, thesespecies ingest greater amounts nathussp.), white-footed mice (Peromyscussp.; of environmental contaminants than those at Strecker 1930); lizards (Bendire 1887, Wet- lower trophic levels. In the Peregrine Falcon more 1965); frogs (Haverschmidt 1968); and (F. peregrinus)and Osprey (Pandionhaliae- small fish (Cherrie 19 16). Invertebrate prey tus), levels of pesticide contamination are include locusts, beetles, dragonflies, crickets, tightly correlated with the degree of eggshell butterflies, wasps, and bees (Bendire 1887, thinning and eggbreakage (Ratcliffe 1980). The Cherrie 1916, Bailey 1928, Brooks 1933, and contamination of Aplomado Falcons with Mitchell 195 7). DDE (the primary metabolite of DDT) sug- This list of prey speciesindicates the variety gests that the species feeds mainly on birds, of animals captured by Aplomado Falcons, but and that it might be subjectto pesticide-related saysnothing about the relative importance of reproductive problems. various prey in the diet. Many older descrip- Many brief descriptions of the food of the tions of these birds stated that they subsiston Aplomado Falcon have been reported. Known insects,small mammals, and reptiles (Bendire avian prey include: quail, teal (Anas sp.), pi- 1887, Sclater and Hudson 1889, Bailey 1928, geons(Columba sp.), and sandpipers(Grayson Sprunt 1955, and Ligon 196 1). These asser- in Lawrence 1874); ground-doves (Mader tions conflict with evidence that the speciesis 198 1); a wren (Cumpylorhynchussp.), snipe an upper trophic-level predator (Kiff et al. (Gallinagosp.), antshrike (Thamnophilussp.; 1980) and have led at least one recent au- Cherrie 1916); Horned Lark (Eremophilaalpes- thority to suggestthat the Aplomado Falcon 13361 DIET OF THE APLOMADO FALCON 337 should not be sensitive to pesticide contami- species,however, I assignedit the weight of a nation (Bond 1972). Other authorities, how- known prey speciesof similar size. In the field, ever, reported that these falcons feed mainly I weighed specimens with a triple-beam bal- on small birds (Lawrence 1874, Crawshay ance or pesola scales. Most weight estimates 1907, Cherrie 1916, Wetmore 1965, and John- came either from specimensI collected or from son 1965). Owing to suchcontradictory views, specimens at the Texas A&M Cooperative it has been unclear whether this specieshunts Wildlife Collections. Weights for the following birds, as the peregrine does, or insectsand ro- specieswere obtained from the literature: Plain dents, as the American Kestrel (F. sparverius) Chachalaca (Ortalis vetula; Leopold 1972), does. In order to resolve this question, I stud- Pale-vented Pigeon (Howell 1972) Mourning ied the feeding behavior of Aplomado Falcons (Zenaida macroura)and White-winged doves breeding in Veracruz, Chiapas,and Campeche, (2. asiatica;Cottam and Trefethen 1968) and Mexico. My data form the basisof the present Lesser Nighthawk (Chordeilesacutipennis ; paper. Improved knowledge of the falcon’s di- Johnson 1968). Becauseof the difficulty of as- etary niche might help to account for its dis- signing weights to categories of insect prey, I appearance from the United States, and ex- assumed all insects to weigh 1 g. This may plain the high levels of pesticides found in slightly overestimate the contribution of in- Aplomado Falcons in eastern Mexico. sectsto prey biomass, since most examples of typical insect prey weighed a gram or less. STUDY AREAS AND METHODS Feathers collected as prey remains were I found Aplomado Falcons by driving through identified to species and were classified ac- likely-looking habitat, then exploring on foot cording to type (e.g., right remex, left remex, areaswhere I had spotted birds from the road. or rectrix). I next divided the number of feath- All areas regularly frequented by falcons were ers in each group by the number of feathers of true territories in that they were defended that type found in that species.At times, this against conspecifics(and other raptors). procedure gave different values for each feath- During 10 March-15 June 1977,25 March- er type. I used the largest of these values to 15 April 1978, and 12 May-15 June 1979, I estimate the number of individuals in the sam- located 18 territories in northern and central ple. Body featherswere used for identifications Veracruz, northern Chiapas, western Cam- when no flight feathers could be found. peche, and eastern Tabasco between 17-23”N Becausethe falcons did not seem apprecia- and 91-99”W. This is a region of rolling hills bly disturbed by my presence,I observedhunt- and low volcanic mountain rangescovered by ing activities from exposed, ground-level van- patches of farmland, open pasture, and trop- tageposts located 50-200 m from nestsor perch ical forest. Meandering streams and marshes trees. The opennessof typical nesting habitat are common in most areas. allowed me to view most feeding activities. Aplomado Falcons were found in various Whenever possible, I identified prey by ex- savanna associations.Dominant plants at fal- amining remains left after feedings that I con territories were oaks (e.g., Quercus watched. In many cases,however, I was forced oleiodes),palms (Sabal mexicana,Acrocomia to make distant, visual identifications. mexicana,or Scheelialiebmanii), Crescentia Small items were under-represented in my cujete,huisache (Acaciafarnesiana), and palo samples of prey remains, a known bias (Er- de rosa (Tabebuiarosea). In addition, I found rington 1932, Snyder and Wiley 1976). Con- two nests in fallow cornfields. sequently, I assumed that the sample of prey During each visit to a territory, I collected that I saw being taken provides the best in- all remains of prey animals. Some remains formation on the relative importance of var- came from nest platforms or beneath nest trees; ious classesof prey in the falcon’s diet. My however, I found the majority beneath perches estimates of dietary importance are given in where falcons had eaten or defeathered prey. terms of percentagetotal prey individuals (O/o I seldom found food pellets, and did not use TPI) and percentage total prey biomass (O/o them as a sourceof dietary information in this TPB). report. I saw no other raptor speciesusing fal- con feeding perches. This, plus the fact that RESULTS Aplomado Falcons aggressivelyexcluded oth- er raptors from their territories, made it un- CONTENTS OF REMAINS likely that I collected remains of animals not I collected prey remains at 18 sites; however, captured by the study animal. 65% of detected prey animals (n = 168) came When possible, I estimated weights of prey from seven sites. A total of 192 prey animals animals using weights of freshly-killed speci- (75%) was collected at nine sites during nest- mens. When I could not identify an animal to ling and fledgling periods; remains of 46 prey 338 DEAN P. HECTOR TABLE 1. Speciesof prey detected in remains and observed captured or eaten by adult Aplomado Falcons in eastern Mexico. % TPI = percent total prey individuals. O/oTPB = percent total prey biomass. Weight Prey remains Observedprey Prey animals (9) n % TPI % TPB n 96 TPI % TPB Birds Plain Chachalaca,Ortalis vetula 577 1 0.4 2.8 - - - Northern Bobwhite. Colinus virginianus 156 6 2.3 4.5 1 0.4 2.8 Killdeer, Charadrius vociferus - 88 1 0.4 0.4 - - - Pale-vented Pigeon, Columba cayennensis 260 1 0.4 1.2 - - - Mourning Dove, Zenaida macroura 126 21 10.5 16.4 6 2.6 13.6 White-winged Dove, Z.