Demography and Populations

Age At First Breeding; Intervals Between Breeding

Ferruginous Pygmy-Owls usually court during first fall and nest as yearlings (W. Richardson pers. comm., GAP). Adults of both sexes breed annually (GAP).

Clutch

In U.S. and Mexico, average 3.3 eggs/clutch (range 2–5, n = 43; Johnsgard 1988a Johnsgard, P. A. (1988). North American Owls: Biology and Natural History. Smithsonian Institution Press, Washington, DC, USA. Close ); in Texas, average 4.9 (range 3–7, n = 58; see Breeding; eggs, above).

Annual And Lifetime Reproductive Success

In Texas from 1994 to 1999, average annual hatching efficiency (i.e., number of hatchlings per egg) from natural cavities (n = 33) 77% and from nest boxes (n = 53) 72%; fledging efficiency (i.e., number of fledglings per hatchling) 70 and 78%, respectively; nest productivity (i.e., number of fledglings per egg) 54 and 56%, respectively. Average number of fledglings per nest attempt from natural cavities 2.61; from nest boxes, 2.81. Because nest predation may increase with nest age (Sonerud Sonerud 1985b Sonerud, G. A. (1985b). Nest hole shift in Tengmalm's Owl Aegolius funereus as defense against nest predation involving long term memory in the predator. Journal of Animal Ecology 54:179-192. Close , Sonerud 1989 Sonerud, G. A. (1989). Reduced predation by pine martins on nests of Tengmalm's Owl in relocated boxes. Animal Behaviour 37:332-343. Close ), efficiency differences may be associated with nest box age. Renesting attempts are less successful than first nests (GAP). Lifetime reproductive information is lacking and needs further study.

Number Of Broods Normally Reared Per Season

One.

Proportion Of Total Females That Rear At Least One Brood To Nest-Leaving Or Independence

Of 29 females using nest boxes in Texas 1993–1998, 22 (76%) fledged ≥1 young; of 28 females using natural cavities in Texas 19951998, 18 (64%) fledged ≥1 young.

Little information available; in Texas, 3 pairs banded in April 1994 nested in same areas for 4 yr. Of 57 pairs monitored in Texas 1993-1998, mortality of 2 adults recorded. Because not all adults were banded and not all banded adults are accounted for, information is limited. Immigration and emigration studies are needed.

Diseases

No information.

Body Parasites

Several body parasites have also been identified in fledglings, but their overall affect on Ferruginous health is unknown. In Texas, no blood parasites (Hematozoa—e.g., Haemoproteus spp., Leucocytozoon spp.) from 63 individuals examined (Proudfoot and Radomski 1997 Proudfoot, G. A. and A. A. Radomski. (1997). Absence of hematozoa from Ferruginous Pygmy-Owls (Glaucidium brasilianum) in southern Texas. J. Helminthol. Soc. Wash. 64:154-156. Close ). Hippoboscid flies are common in nestlings ≤3 wk old in Texas and less common in adults; mites and lice are common in wing axilla of nestlings and less common in adults. In addition to these, other parasites documented include Philornis mimicola and Ornithodoros concanensis in Texas populations (Proudfoot et al. 2006), and Protocalliphora sialia and Hesperocimex sonorensis in Arizona (Proudfoot et al. 2005). Known health consequences include blood loss and anemia (Proudfoot et al. 2006).

In Texas from 1994 to 1998, 94% (n = 17) of nestling mortality resulted from depredation, primarily by raccoons; 6% due to starvation. Fledgling mortality resulted from avian (e.g., Great Horned Owl, Cooper's Hawk, and Harris's Hawk [Parabuteo unicinctus]) and mammalian (e.g., raccoon, possibly bobcat [Lynx rufus] and ocelot [Felis pardalis]) predation. Similarly, adult mortality was avian and mammalian. One adult female and 1 fledgling using a nest box near human residence were killed by a domestic cat 3 d after nestlings fledged (Proudfoot 1996 Proudfoot, G. A. (1996). Natural history of the Cactus Ferruginous Pygmy-Owl. Master's Thesis, Texas A&M Univ., Kingsville. Close ). The adult male became sole provider, and 3 remaining fledglings survived to dispersal (GAP).

Home Range

In Texas, areal use by 9 radio-tagged adult males, monitored from approximately 1 wk before incubation to approximately 1 wk after, ranged from 1.3 to 23.1 ha (15–42 coordinates); 1 unmated adult male used 110 ha during same time period (37 locations). From fledging-dispersal of young, 5 radio-tagged families (i.e., 2 adults [1 male, 1 female] and 3 fledglings/family) used 9.3, 13.7, 16.9, 36.7, and 59.5 ha (46, 58, 126, 117, and 125 coordinates recorded). One hatch-year young and 2 adult males monitored from Oct to Nov used 19.6, 72.8, and 116.4 ha, respectively (10–27 locations recorded). In Arizona, 2 pairs used 2–16 ha during nesting (W. Richardson pers. comm.).

Numbers

Few studies; density estimates limited; this species is considered common throughout most of its range; see Habitat: breeding range, above. Alteration of as much as 90% of historical habitat in U.S. may have drastically reduced numbers (Oberholser 1974 Oberholser, H. C. (1974). The Bird Life of Texas. University of Texas Press, Austin, Texas, USA. Close , Monson and Phillips 1964 Monson, G., and A. R. Phillips (1964). A Check-list of the Birds of Arizona. University of Arizona Press, Tucson, Arizona, USA. Close ). Populations in ne. Mexico may be experiencing similar effects (Tewes 1995 Tewes, M. E. (1995). Status of the Ferruginous Pygmy-Owl in southern Texas and northeast Mexico. Kingsville: Texas Parks and Wildl. Dept. and Texas A&M Univ.-Kingsville. Close ). During the Texas Breeding Bird Atlas (BBA) project (1987–1992), only 2 nest sites confirmed below Falcon Dam in Starr Co., TX, and 6 probable sites between Rio Grande River and 27°N (Proudfoot 2001 Proudfoot, G. A. (2001). "Ferruginous Pygmy-Owl." In the Texas breeding bird atlas., edited by K. L. P. Benson and K. A. Arnold. College Station: Texas A&M Univ. Press. Close ). Estimated 1,308 individuals in Brooks, Kenedy, and Willacy Cos., TX (Wauer et al. 1993 Wauer, R. H., P. C. Palmer and A. Windham. (1993). The Ferruginous Pygmy-Owl in southern Texas. American Birds 47:1071-1075. Close ). In Kenedy Co., TX, estimated 745–1,823 individuals in 29,000 ha of live oakmesquite habitat (Mays 1996 Mays, J. L. (1996). Population size and distribution of Cactus Ferruginous Pygmy-Owls in Brooks and Kenedy Counties, Texas. Master's Thesis, Texas A&M Univ.-Kingsville, Kingsville. Close ), and 99 nests monitored 1994–1999 (GAP). In Arizona, 16 nests recorded in Tucson Basin from 1994 to 1999; 59 individuals banded (W. Richardson pers. comm., GAP). Proudfoot et al. (2006) estimated the population size of all owls in Arizona at 13-117 individuals (Proudfoot et al. 2006). In Mexico (Enríquez-Rocha et al. 1993 Enríquez-Rocha, P., J. L. Rangel-Salazar, and D. W. Holt (1993). Presence and distribution of Mexican owls: A review. Journal of Raptor Research 27(3):154–160. Close ) and Central and South America, Ferruginous Pygmy-Owl is considered a common resident; fairly common to locally common in Panama and Colombia, and fairly common in Costa Rica; in some countries the only common pygmy-owl. Occurs in a considerable number of protected areas in most countries throughout its extensive range.

Trends

Formerly considered common resident along lowland rivers of s. Arizona as far west on Gila River as Agua Caliente, west of Gila Bend (Gilman 1909c Gilman, M. F. (1909c). Some owls along the Gila River in Arizona. Condor 11:145-150. Close ); as far north as New River, north of Phoenix (Fisher 1893c Fisher, A. K. (1893). The hawks and owls of the United States in their relation to agriculture. U.S. Department of Agriculture, Division of Ornithology and Mammalogy, Washington, DC, USA. Close ); possibly restricted to valley of upper Gila, and its tributaries, where it was not uncommon (Swarth 1914a Swarth, H. S. (1914). A distributional list of the birds of Arizona. Pacific Coast Avifauna 10. Close ). Into the 1940s, considered common breeding bird (Friedmann et al. 1950 Friedmann, H., L. Griscom, and R. Moore (1950). Distributional checklist of the birds of Mexico, Part I. Pacific Coast Avifauna 29. Close ), and by mid-1900s, regarded as local and generally sparse resident in s. Arizona (Phillips et al. 1964a Phillips, A. R., J. T. Marshall Jr., and G. B. Monson (1964). The Birds of Arizona. University of Arizona Press, Tucson, AZ, USA. Close ). For Phoenix region, a sparse resident in the county until 1950s (Witzeman et al. 1997 Witzeman, J. L., S. R. Demaree, and E. L. Radke. (1997). Birds of Phoenix and Maricopa County, Arizona. Maricopa Audubon Society, Phoenix, Arizona, USA. Close ); previously casual resident (Demaree et al. 1972 Demaree, S. R., E. L. Radke and J. L. Witzeman. (1972). Birds of Maricopa County, Arizona. Pheonix: Maricopa Audubon Soc. Close ); no known records from 1960s, 1980s, or 1990s; 1 record from 1970s (Millsap and Johnson 1988 Millsap, B. A. and R. R. Johnson. (1988). "Status report: Ferruginous Pygmy-Owl." In Southwest raptor management symposium and workshop proceedings., edited by R. L. Glinski, B. G. Pendleton, M. B. Moss, Jr M. N. La Franc, B. A. Millsap and S. W. Hoffman, 137-139. Washington, D.C: Natl. Wildl. Fed. Close ). However, large areas of its former range are never visited by experienced observers, and the species may be more common than perceived (Monson 1998 Monson, G. (1998). "Ferruginous Pygmy-Owl." In The raptors of Arizona., edited by R. L. Glinski, 159-161. Tucson: Univ. of Arizona Press. Close ).

Populations have also been shrinking in most of Texas since the 1920’s (Flesch and Steidl 2006; Flesch and Steidl 2007; Johnson et al. 2003; Proudfoot et al. 2006b), particularly along the peripheries of habitation, and populations may be experiencing similar decline more recently in northern Mexico (Sonoma; Flesch and Steidl 2006; Johnson et al. 2003; Flesch and Steidl 2010), although research is needed. Some data exist on current population densities in Arizona and Texas, but historical accounts of Ferruginous populations are dominated by anecdotal evidence, so the rates of population shrinkages are difficult to calculate.

Southern subspecies G. b. tucumanum found to be "rather abundant" near Tartagal, between Salta in N Argentina and Bolivian border (König and Weick 2008 König, C., F. Weick, and J.-H. Becking (2008). Owls of the World. Second Edition. Christopher Helm, London, UK. Close ). Numbers perhaps decreasing, but no quantitative estimates of population size or trend. Habitat destruction, especially burning, a threat, as is trapping for traditional superstitious customs (König and Weick 2008 König, C., F. Weick, and J.-H. Becking (2008). Owls of the World. Second Edition. Christopher Helm, London, UK. Close ).

Few data; needs study. Nest depredation, low prey abundance, lack of nest cavities, habitat manipulation, and extreme weather changes may directly affect population numbers; variations in population numbers and resulting fluctuations in levels of intraspecific competition may have indirect effects. Although nest box depredation in Texas increased with box age, predator-resistant nest boxes have proven successful and may augment declining populations in U.S.; see Measures of breeding activity, above.