Dietary Shifts Based Upon Prey Availability in Peregrine Falcons and Australian Hobbies Breeding Near Canberra, Australia
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J. Raptor Res. 42(2):125–137 E 2008 The Raptor Research Foundation, Inc. DIETARY SHIFTS BASED UPON PREY AVAILABILITY IN PEREGRINE FALCONS AND AUSTRALIAN HOBBIES BREEDING NEAR CANBERRA, AUSTRALIA JERRY OLSEN1 AND ESTEBAN FUENTES Institute for Applied Ecology, University of Canberra, ACT, Australia 2601 DAVID M. BIRD Avian Science and Conservation Centre of McGill University, 21111 Lakeshore Road, Ste. Anne de Bellevue, Quebec, Canada H9X 3V9 A. B. ROSE2 The Australian Museum, 6 College Street, Sydney, New South Wales 2010 DAVID JUDGE Australian Public Service Commission, 16 Furzer Street, Phillip ACT, Australia 2606 ABSTRACT.—We collected prey remains and pellets at 16 Peregrine Falcon (Falco peregrinus) nest territories (975 prey items from 152 collections) and one Australian Hobby (F. longipennis) territory (181 prey items from 39 collections) during four breeding seasons in two time periods: 1991–1992 and 2002–2003, a total of 60 peregrine nest-years and three hobby nest-years. By number, European Starlings (Sturnus vulgaris) were the main prey taken by both falcons in 1991–1992 and 2002–2003, but starlings made up a smaller percentage of the diet by number in the latter period, apparently because their numbers had declined in the wild. Although the geometric mean of prey weights and geometric mean species weights were similar in the two time periods, both falcons compensated for the decline in European Starlings in the latter period by taking a greater variety of bird species, particularly small numbers of mostly native birds, rather than taking more of one or two other major prey species. Peregrines took 37 bird species in the latter period not found among their prey remains in the earlier period, and more individuals of some large species such as Gang-gang Cockatoos (Callocephalon fimbriatum), Galahs (Cacatua roseicapilla), and Rock Pigeons (Columba livia). Prolonged drought and competition from increasing numbers of Common Mynas (Acridotheres tristis) may have caused European Starlings to decline, but neither falcon species compensated by catching Common Mynas, even though they are of optimal prey size. Though both falcons, especially peregrines, took the more common bird species found in surveys in the Australian Capital Territory, both tended to avoid large or dangerous species, or agile species that foraged close to cover, such as Common Mynas. KEY WORDS: Peregrine Falcon; Falco peregrinus; Australian Hobby; Falco longipennis; European Starling; Stur- nus vulgaris; Australia; Canberra; diet. CAMBIOS EN LA DIETA CON BASE EN LA DISPONIBILIDAD DE PRESAS EN POBLACIONES REPRO- DUCTIVAS DE FALCO PEREGRINUS Y F. LONGIPENNIS EN LAS CERCANI´AS DE CANBERRA, AUSTRA- LIA RESUMEN.—Recolectamos restos de presas y egagro´pilas en 16 territorios de anidacio´n de Falco peregrinus (975´tems ı presa provenientes de 152 recolecciones) y en un territorio de F. longipennis (181´tems ı prove- nientes de 39 recolecciones) durante cuatro e´pocas reproductivas en dos perı´odos: 1991–1992 y 2002–2003, correspondiendo a un total de 60 an˜os-nido para F. peregrinus y a tres an˜os-nido para F. longipennis.En nu´mero, Sturnus vulgaris fue la principal presa cazada por ambas especies de halcones tanto en el per´odoı 1 Email address: [email protected] 2 Present address: 61 Boundary Street, Forster, New South Wales, Australia 2428 125 126 OLSEN ET AL. VOL. 42, NO.2 de 1991–1992 como en el de 2002–2003, aunque el porcentaje contribuido a la dieta en te´rminos de nu´meros por S. vulgaris fue menor durante el segundo perı´odo debido a que aparentemente esta especie ha disminuido en los ambientes silvestres. A pesar de que la media geome´trica de los pesos de las presas y la media geome´trica de los pesos de las especies fueron similares en los dos periodos de tiempo, ambas especies de halcones compensaron la disminucio´n de S. vulgaris durante el segundo periodo cazando una mayor variedad de especies de aves. Lo hicieron cazando particularmente nu´meros pequen˜os de aves principalmente nativas, en vez de cazar mayor cantidad de una o dos especies de presas principales. Los individuos de F. peregrinus cazaron 37 especies de aves durante el segundo periodo, las cuales no se encontraron entre los restos de presas recolectados durante el primer periodo, y ma´s individuos de algunas especies de gran taman˜o como Callocephalon fimbriatum, Cacatua roseicapilla y Columba livia. La sequı´a prolongada y la competencia debido al nu´mero en aumento de Acridotheres tristis, pueden haber causado la disminucio´n de S. vulgaris, pero ninguna de las especies de halcones compenso´ cazando individuos de A. tristis a pesar de que e´stos tienen un taman˜o de presa o´ptimo. A pesar de que las dos especies de halcones, especialmente F. peregrinus, cazaron la especie de ave ma´s comu´n que se registra en los muestreos del territorio de la capital australiana, ambas especies tendieron a evitar especies grandes o peligrosas, y a especies a´giles que forrajean cerca de a´reas con mayor cobertura como A. tristis. [Traduccio´n del equipo editorial] The Peregrine Falcon (Falco peregrinus) and Aus- ble (Olsen 1992). The same nest cliff may be occu- tralian Hobby (F. longipennis) breed throughout pied for many years and these cliffs are ideal sites Australia and are typical fast-flying, bird-catching for repeated annual collections of prey remains. falcons (Marchant and Higgins 1993). There have Peregrines near Canberra tend to lay eggs in Au- been no major studies of the diet of the Australian gust–September and fledge young in November– Hobby outside the Northern Territory, and no stud- December. Hobbies lay eggs about November and ies comparing the diet of Australian Hobbies and fledge young in January–February (Olsen 1992). Peregrine Falcons breeding in the same region, or Because there have been no long-term studies comparing their diets over time (Marchant and Hig- (.10 yr) of trends in diet for peregrines or hobbies gins 1993). The diet of the Peregrine Falcon has in Australia, our purpose in this study was to com- been well studied in rural and coastal parts of south- pare the diets of breeding Peregrine Falcons during eastern Australia, particularly in Victoria and in two time periods, 1991–1992 and 2002–2003. We open, lower-elevation parts of the Australian Capital also examined diet differences at one Australian Territory (ACT) and New South Wales (Pruett-Jones Hobby breeding territory in 1991, 2002, and 2003 et al. 1981, Olsen 1992, Marchant and Higgins 1993, in order to look for similar patterns in the prey Olsen et al. 1993, Olsen and Stevenson 1996, Rose captured, as both falcons eat primarily birds. 2001, Olsen and Tucker 2003, Olsen et al. 2004). They eat primarily flocking birds, particularly pi- METHODS geons, parrots, and European Starlings (Sturnus vul- Study Area. We monitored 16 Peregrine Falcon garis), but also larger species up to the size of herons, territories, mostly within 60 km of Canberra. At low- ibises, and waterfowl (Debus 1998). The diet of the er elevations this area comprised urban areas, rivers, Australian Hobby has been less well studied (Cze- grazing land, open forest, tall woodland with dom- chura and Debus 1986, Debus et al. 1991, Marchant inants of Scribbly Gum (Eucalyptus rossii), Brittle and Higgins 1993, Aumann 2001). Outside the Gum (E. mannifera), Red Stringybark (E. macro- Northern Territory (Aumann 2001), there has been rhyncha), Blakely’s Red Gum (E. blakelyi) with Red only one diet study of a single nest of Australian Box (E. polyanthemos) and Yellow Box (E. melliodora), Hobbies (Debus et al. 1991, N 5 100 prey items). and Casuarina in river bottoms (NCDC 1988). Four Peregrines and hobbies are sympatric in Australia territories were in Namadgi National Park in the (Simpson and Day 1999). They compete for some southern half of the ACT, a 106 000-ha reserve cov- prey species, like European Starlings and small par- ering much of the Brindabella Range, with peaks up rots, but hobbies specialize in smaller birds such as to 1900 m. Namadgi National Park consists primar- larks, pipits, grassfinches, doves, and swallows, as well ily of tall wet sclerophyll forest and dense shrub as insectivorous bats and flying insects (Debus 1998). understory, with alpine woodland at the highest el- Near Canberra, there is a shortage of nest cliffs evations, and open, drier forest with open grassy for peregrines and pair densities are remarkably sta- valleys at the lower elevations (Taylor and COG JUNE 2008 DIETS OF PEREGRINES AND HOBBIES 127 1992, Olsen and Rehwinkel 1995). Eight of the low- one prey bird, because nestlings share prey items er-elevation cliff nests were above rivers and dams, and more than one species was often found in each and four were in deserted or semi-active quarries, all pellet. at approximately 500 m above sea level (asl); the We also included observations of kills and prey four nests on cliffs in Namadgi National Park were consumed in situ, if these items were not reflected at around 1000 m asl. in the prey remains and pellets collected. For exam- Collection of Prey Remains and Pellets. Pere- ple, if we saw a European Starling carried to the grine Falcons. Sixteen Peregrine Falcon nest cliffs brood, but did not find European Starling feathers were monitored during the 1991, 1992, 2002, and in the collection for that day or the following day, 2003 nesting periods (total 5 60 nest-years; 4 nest- we counted that European Starling as a prey item. years not sampled). We attempted to collect prey However, if we did find European Starling in the remains and pellets at the nest cliffs two or three pellets or prey remains for that day or the following times during the nestling period.