Journal of Wildlife Diseases, 41(3), 2005, pp. 643–646 ᭧ Wildlife Disease Association 2005

Hailstorms as a Cause of Mass Mortality of Swainson's Hawks in Their Wintering Grounds

Jose HernaÂn Sarasola,1,3 Juan Jose Negro,1 Vero nica Salvador,2 and Juan Jose Maceda21Department of Applied Biology, EstacioÂn BioloÂgica de DonÄana CSIC, Avda. de Ma. Luisa s/n, PabelloÂn del PeruÂ, 41013 Sevilla, EspanÄa; 2 Centro para el Estudio y ConservacioÂn de las Aves Rapaces en Argentina, FCEyNÐUniversidad Nacional de La Pampa, 6300 Santa Rosa, La Pampa, Argentina; 3 Corresponding author (email: sarasola@ ebd.csic.es)

ABSTRACT: We describe a mass mortality in bird populations after hurricanes in event of wintering Swainson’s hawks (SH, Bu- tropical regions are associated with struc- teo swainsoni) in central Argentina during No- vember 2003. One hundred thirteen SH were tural damage to habitat and to loss of food found dead as a consequence of a single hail- supplies rather than to direct mortality storm. In addition, 14 hawks with severe inju- (Wunderle et al., 1992). We provide the ries were recovered alive, but only 10 of these first evidence of hailstorms as a cause of survived 1 wk later. Another 45 dead birds of winter mortality for a long-distance neo- 11 species were collected in the area. Inter- tropical migrant, the Swainson’s hawk (SH, views with local landowners conducted in other areas of the SH wintering grounds provided Buteo swainsoni), and discuss potential further evidence of past hailstorm-related mor- implications for SH demography and con- tality involving SH, suggesting that such events servation. commonly occur in the Argentine Pampas. This The SH breeds in North America and potential cause of mass mortality of SH winter- spends the boreal winter in southern ing in agricultural areas of Argentina may be significant when added to the increased mor- South America (England et al., 1997). It tality associated with poisoning events during winters mainly in the agricultural habitats the last decade. of Central Argentina, commonly called the Key words: Buteo swainsoni, hailstorms, Pampas region or the Argentine Pampas. mortality, Swainson’s hawk, wintering grounds. While breeding, SH are territorial, but in the wintering grounds they become gre- Climate and weather are natural limit- garious, hunting and roosting together in ing factors for bird populations (Newton, large flocks of up to 10,000 birds (Wood- 1998). The former refers to long-term me- bridge et al., 1995). teorological events usually having endur- From November 2003 to February 2004 ing effects during months or even annual (late austral spring and summer seasons), cycles (e.g., droughts, flood, or rainy sea- we surveyed the Argentine pampas for SH. sons). Weather episodes imply occasional, The region is flat and contains a mosaic of short-term experiences (e.g., rainstorms) crops including sunflowers, corn, and soy- that also differ from climate events in the beans, as well as patches of planted pas- spatial scale in which their effects are per- ture and natural habitats such as Stipa spp. ceived (Rotenberry et al., 1995). Both of grasslands. On 17 November, a local land- these natural events could affect bird pop- owner reported a mortality incident in the ulations directly and indirectly in a variety vicinity of Villa Mirasol town (36Њ4’S, of ways, including direct mortality, habitat 36Њ52’W), La Pampa province, due to a loss, breeding failure, and decreased food hailstorm that occurred 1 wk earlier (10 supply (Newton, 1998). In the neotropical November). The incident occurred in an region, tropical storms and hurricanes are elongated grove of approximately 5.3 ha thought of as the catastrophic weather composed mainly of Eucalyptus sp. plus events that can negatively affect most res- other nonnative tree species including Ul- ident and migrant bird populations (Wun- mus pumilla, Pinus sp., and Cuppresus sp. derle et al., 1992; Wiedenfeld and Wie- The landowner had observed SH roosting denfeld 1993). However, marked declines in this grove for several days before the

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TABLE 1. Summary of bird species and number of doves were practically the only remains we birds affected by a hailstorm recorded in northern La Pampa province, November 2003. found of those species, suggesting that scavengers preferred such small birds for Number of feeding and that their total number of Species birds birds was probably underestimated. Rufous hornero (Furnarius rufus) 2 The hawks recovered alive had injuries House sparrow (Passer domesticus) 1 and broken bones. Broken bones were White-tailed kite (Elanus leucurus) 1 mainly in the wings (three birds), skull a Pigeons and doves 32 (one bird), and tarsus (one bird), and in- Chimango caracara (Milvago chimango) 1 Southern caracara (Caracara plancus)b 1 juries were observed in wings (three birds) American kestrel (Falco sparverius) 1 and skull (one bird). Six of the SH had no Whistling heron (Syrigma sibilatrix) 1 evidence of injuries but were unable to fly. Monk parakeet (Myiopsitta monachus) 3 Only 10 of these recovered hawks survived Swainson’s hawk (Buteo swainsoni) 113 for 1 wk. Aplomado falcon (Falco femoralis) 1 Campo flicker (Colaptes campestris) 1 Trees in the grove had evidence of the Total 158 impact of hailstones in their bark as well as defoliation and branch losses. On the a Sample including two spot-winged pigeons (Columba ma- culosa), three Picazuro pigeons (C. picazuro), and two eared basis of marks in the bark and from com- doves (Zenaida auriculata). The remaining birds were not- ments of the landowner, we estimated that identified (see text for further details). the hailstones were approximately 7 cm in b Juvenile. diameter, corresponding to a weight of 165 g (ice density ϭ 0.92 g cm3). A hailstone storm. Surrounding areas were plowed of this size and weight is large enough to fields and perennial pastures. kill large-sized birds like SH with an av- We searched the grove as well as adja- erage body mass of 800 g (Goldstein et al., cent fields for dead and injured birds. One 1999; Sarasola et al., 2004). Neighboring hundred thirteen dead and 14 live SH groves located 2 km away had no evidence were found under roost trees and in ad- of the hailstorm. jacent fields. In addition, 45 dead individ- We estimated the age and gender of uals of 11 different bird species were dead SH by using plumage characteristics found (Table 1). None of them suffered (Wheeler and Clark, 1995) to identify ju- the high mortality of the SH. Most of the venile-immature from adult birds and the dead and live SH were recovered under length of the forewing as a discriminating the trees in the roosting area but 12 hawk variable among genders (Sarasola and Ne- carcasses were collected in the surround- gro, 2004). From 90 carcasses inspected, ing field, all within 120 m of the roost. All the percentage of birds belonging to each dead individuals of other bird species were age and sex group was 16% juvenile male, collected under the trees in the roost site. 38% adult male, 14% juvenile female, and Because we were unable to complete the 33% adult female. survey of the roost site and its adjacent We did not necropsy each dead hawk to area during 1 day of fieldwork, we counted determine the causes of death. Previous and marked all the raptor’s carcasses found reports of massive mortality of wintering until sunset. The next day, 15 hr later, the populations of SH have been related to number of carcasses was the same and chemical poisoning with organophosphate there was no evidence of scavenging activ- compounds during the 1995–96 and 1996– ity, indicating that at the time of our visit 97 austral summers, with an estimate of the estimation of the number of dead 700 and 20,000 birds affected, respectively hawks and other raptor species may not (Woodbridge et al., 1995; Goldstein et al., have been affected by removal by scaven- 1996). Nevertheless, field evidence as well gers. Plucked feathers of pigeons and as landowner remarks and the injuries ob- SHORT COMMUNICATIONS 645 served in surviving birds confirmed that and their relative abundance in the area. this mortality incident was the conse- Only a few individuals of raptor species af- quence of the hailstorm and was unrelated fected by the storm were seen flying in the to insecticide poisoning. area during roost inspection (two southern Birds of prey are rarely mentioned as caracaras, two American kestrels, one suffering direct mortality by catastrophic aplomado falcon, and one white-tailed kite weather events (Newton, 1978) and we are were recorded), whereas at least 20 chi- aware of only one report of raptor mortal- mango caracaras were observed during the ity caused by hail in which SH were grove survey and only one was found dead. among the affected species (Jones, 1952). Chimango caracaras are a raptor species Raptors may be better equipped than oth- well adapted to urban and periurban areas er birds to survive severe weather situa- (Travaini et al., 1995) and this feature may tions because of their relative large size have allowed them to avoid the impact of and the ability to fast for long periods hail by finding refuge in human buildings (Garcı´a-Rodrı´guez et al., 1987). They also near the grove. Swainson’s hawks were not need to devote a smaller proportion of seen near the grove during the field in- their daily energy intake to maintain body spection, presumably because surviving temperature when ambient temperature hawks had moved to another area. Al- declines because of their lower surface though we have no data on the number of area to volume ratio, making them less vul- SH roosting in the grove before the hail- nerable to hypothermia. Their dispersal storm, we suspect that the number of sur- capability and territorial behavior may ex- viving SH was very low and that almost the plain why sporadic but severe weather entire flock was likely killed or injured by events have little effect on their popula- the hail. tions or may simply be too difficult to de- Rainstorms with high-speed winds may tect. cause hawks to fall down from their perch Although communal roosting suppos- while roosting (Goldstein, 1997), but di- edly benefits individuals in terms of a re- rect mortality as a consequence of this duction in thermoregulation demands, a kind of adverse weather condition has not decrease in predation risk (e.g., dilution), been reported for wintering areas. We re- and an increase in foraging efficiency corded further evidence of hawk mortality (Beauchamp, 1999), it may be catastrophic by hailstorms in Buenos Aires province by when there are weather-related causes of mortality such as hailstorms. Patchy distri- interviewing a local landowner. Several bution of suitable habitat (e.g., groves of hundred SH were affected on that occa- alien tree species spread in agriculture sion, although accurate data on the year lands), which results in large congregations and the exact number of individuals was of birds, may also increase the probability unknown. Furthermore, severe hailstorms Ͼ of mortality from such causes. Swainson’s producing ice stones 2.5 cm in diameter hawks and other colonial raptors, such as are frequent in the Argentine Pampas. In the lesser kestrel (Falco naumanni; Negro, some areas of La Pampa province, for ex- 1997), return to the roost site for shelter ample, a mean of 3.2 days during the aus- any time during the day before thunder- tral summer season (December to March) storms (Dehley and Scorolli, 1988); this are expected to have such storms (National behavior would enhance the probability of Meteorological Service of Argentina, inter- mortality in a whole flock if a severe hail- nal report). Consequences of mass mor- storm takes place. tality events, as reported here, may have a We found evidence of differential mor- direct impact on local abundance of SH in tality when considering the number of breeding grounds, which will be more sig- dead birds for each of the raptor species nificant if hawks in wintering areas are seg- 646 JOURNAL OF WILDLIFE DISEASES, VOL. 41, NO. 3, JULY 2005 regated according to their breeding ment of mortality of Swainson’s Hawks on win- ground origin. tering grounds in Argentina. Journal of Raptor Research 30: 106–107. We are indebted to Agustı´n Lanusse for ,P.H.BLOOM,J.H.SARASOLA, AND T. E. giving useful information about the occur- LACHER. 1999. Post-migration weight gain of rence and localization of the mortality, and Swainson’s hawk on the wintering grounds in Ar- Adelmar Funk and Carlos Tucat (Zoo de gentina. Wilson Bulletin 111: 428–432. America, America town, Buenos Aires JONES, G. 1952. Hail damage to wildlife in southwest Oklahoma. Wilson Bulletin 64: 166–167. province) for field assistance and care of NEGRO, J. J. 1997. Lesser kestrel Falco naumanni. surviving hawks. We thank Jorge Urrus- Birds of the Western Palearctic Update 1: 49– puru and his family for logistic assistance 53. during fieldwork. Financial support was NEWTON, I. 1978. Population ecology of raptors. provided by the Wildlife Conservation So- T&AD Poyser, London, UK, 399 pp. . 1998. Population limitation in birds. Aca- ciety (USA) and Universidad Nacional de demic Press, San Diego, California, 597 pp. La Pampa (Argentina). While doing this ROTENBERRY, J. T., R. J. COOPER,J.M.WUNDERLE, research, J.H.S. was supported by a schol- AND K. SMITH. 1995. When and how are popu- arship from Consejo Nacional de Investi- lations limited? The roles of insect outbreaks, gaciones Cientı´ficas y Te´cnicas de Argen- fire, and another natural perturbations. In Ecol- ogy and management of neotropical migratory tina (CONICET) and Consejerı´a de Ed- birds, T. E. Martin and D. M. Finch (eds.). Ox- ucacio´n y Ciencia de la Junta de Andalucı´a ford University Press, Oxford, UK, pp. 55–84. (Spain). We thank three anonymous re- SARASOLA,J.H.,AND J. J. NEGRO. 2004. Gender viewers for helpful comments that greatly identification in the Swainson’s Hawk (Buteo improved this manuscript. swainsoni) using molecular procedures and dis- criminant function analysis. Journal of Raptor Research 38: 357–361. LITERATURE CITED ,J.J.NEGRO, AND A. TRAVAINI. 2004. Nutri- tional condition and serum biochemistry for free- BEAUCHAMP, G. 1999. The evolution of communal living Swainson’s Hawks wintering in central Ar- roosting in birds: Origin and secondary losses. gentina. Comparative Biochemistry and Physiol- Behavioral Ecology 10: 675–687. ogy A 137: 697–701. DELHEY, R., AND A. SCOROLLI. 1988. El aguilucho TRAVAINI, A., A. RODRIGUEZ,O.CEBALLOS,J.A. langostero en el sur y sureste de Buenos Aires. DONAZAR, AND F. H IRALDO. 1995. 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