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Predation by feral cats on birds at Isla Socorro, Mexico

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Ricardo Rodriguez-Estrella
Ricardo Rodriguez-Estrella
Ricardo Rodriguez-Estrella
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Gustavo Arnaud at Centro de Investigaciones Biológicas del Noroeste S.C.
Gustavo Arnaud
Sergio Alvarez Cardenas
Sergio Alvarez Cardenas
Sergio Alvarez Cardenas
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A. Rodríguez at Centro de Investigaciones Biológicas del Noroeste S.C.
A. Rodríguez
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Abstract

Socorro Island, the largest and biologically most diverse of the four islands of the Revillagigedo Archipelago (Brattstrom 1990), confronts serious problems related to human activities. Eight endemic taxa of land birds now breed on Socorro: Yellow-crowned Night Heron, Nyctanassa violacea gravirostris; Socorro Red-tailed Hawk, Buteo jamaicensis socorronensis; Socorro Ground Dove, Columbina passerina socorroensis; Green Parakeet, Aratinga holochlora brevipes; Socorro Wren, Thryomanes sissonii; Socorro Mockingbird, Miraodes graysoni; Tropical Parula Warbler, Parula pitiayumi graysoni; and Socorro Towhee, Pipilo erythrophtalmus socorroensis (McLellan 1926, Jehl and Parkes 1982, Brattstrom 1990). Jehl and Parkes (1983) have proposed that domestic cats (Fells catus), introduced since 1957, are responsible for the extirpation of the endemic Socorro Dove, Zenaida graysoni, and reductions of other species, especially the Socorro Mockingbird. However, no quantitative data on predation by feral cats on Socorro Island have been previously reported in the literature. We present here an analysis of the diet of feral cats on Socorro Island. Socorro Island, located approximately 450 km south of the tip of the peninsula of Baja California, covers about 14,000 hectares (16 x 11 km). This island is a volcanic cone whose highest point is Mount Evermann (1050 m). Climate is arid tropical with an average annual temperature of 24.8 ø C and average annual precipitation of 327.7 mm. Fresh water is found in the puddles formed after tropical storms and continuously in several caves. From 16 to 28 February and from 7 to 12 May 1990 we conducted our field research on Socorro Island, intensively searching for scats, tracks, and other indirect evidence of feral cats, specially in the south portion of the island. We searched in Croton scrub (40 m elevation), disturbed grassland, and forest (650 m elevation). Presumably, the scats we found were from different individuals, as they were widely dispersed. We found and analyzed 31 scats, comparing the remains of scales, bones, feathers, and hairs with those of the collection of Centro de Investigaciones Bio16gicas. Table 1 shows the occurrence of items in 31 scats found during February and May 1990. Feral cats were feeding mainly on House Mice (Mus musculus), endemic lizards (Urosaurus a u riculatus), some birds, and also crabs, insects, scorpions, and various vegetable materials (Table 1). Birds were present in 22.6% of the cat scats (February and May samples combined). The Socorro Ground Dove and Tropical Parula Warbler were the two bird species found most often in the scats, five and three times, respectively. A recent colonizer, the Mourning Dove (Zenaida macroura), was present in two of the 31 analyzed scats. Although we did not find remains of another recent colonizer, the Northern Mockingbird (Mimus polyglottus), in May we found feathers of one in the field, presumably eaten by a feral cat. Jehl and Parkes (1982) reported predation by cats on Townsend's Shearwater (Puffinus auricularis), but we did not find evidence of this perhaps because our searches were not made near the shearwater colonies. Our results suggest that predation on birds by feral cats is not very important. However, even limited predation may seriously affect a species as endangered as the Socorro Mockingbird. This species' population may be approximately 60-70 pairs (Castellanos and Rodriguez-Estrella 1990, L. Baptista pers. comm.), and we found evidence that it is preyed upon by Socorro Red-tailed Hawks (a Socorro Mockingbird
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NOTES
PREDATION BY FERAL CATS ON BIRDS AT ISLA
SOCORRO, MEXICO
RICARDO RODRIGUEZ-ESTRELLA, GUSTAVO ARNAUD, SERGIO ALVAREZ
CARDENAS, and ANTONIO RODRIGUEZ, Centro de lnvestigaciones Bio16gicas,
Div. Biol. Terr., Apartado Postal 128, La Paz, Baja California Sur 23000, M•xico
Socorro Island, the largest and biologically most diverse of the four islands of the
Revillagigedo Archipelago (Brattstrom 1990), confronts serious problems related to
human activities. Eight endemic taxa of land birds now breed on Socorro: Yellow-
crowned Night Heron, Nyctanassa violacea gravirostris; Socorro Red-tailed Hawk,
Buteo jamaicensis socorronensis; Socorro Ground Dove, Columbina passerina
socorroensis; Green Parakeet, Aratinga holochlora brevipes; Socorro Wren,
Thryomanes sissonii; Socorro Mockingbird, Miraodes graysoni; Tropical Parula
Warbler, Parula pitiayumi graysoni; and Socorro Towhee, Pipilo erythrophtalmus
socorroensis (McLellan 1926, Jehl and Parkes 1982, Brattstrom 1990). Jehl and
Parkes (1983) have proposed that domestic cats (Fells catus), introduced since
1957, are responsible for the extirpation of the endemic Socorro Dove, Zenaida
graysoni, and reductions of other species, especially the Socorro Mockingbird.
However, no quantitative data on predation by feral cats on Socorro Island have been
previously reported in the literature. We present here an analysis of the diet of feral
cats on Socorro Island.
Socorro Island, located approximately 450 km south of the tip of the peninsula of
Baja California, covers about 14,000 hectares (16 x 11 km). This island is a volcanic
cone whose highest point is Mount Evermann (1050 m). Climate is arid tropical with
an average annual temperature of 24.8 ø C and average annual precipitation of
327.7 mm. Fresh water is found in the puddles formed after tropical storms and
continuously in several caves.
From 16 to 28 February and from 7 to 12 May 1990 we conducted our field
research on Socorro Island, intensively searching for scats, tracks, and other indirect
evidence of feral cats, specially in the south portion of the island. We searched in
Croton scrub (40 m elevation), disturbed grassland, and forest (650 m elevation).
Presumably, the scats we found were from different individuals, as they were widely
dispersed.
We found and analyzed 31 scats, comparing the remains of scales, bones, feathers,
and hairs with those of the collection of Centro de Investigaciones Bio16gicas. Table 1
shows the occurrence of items in 31 scats found during February and May 1990.
Feral cats were feeding mainly on House Mice (Mus musculus), endemic lizards
(Urosaurus a u riculatus), some birds, and also crabs, insects, scorpions, and various
vegetable materials (Table 1). Birds were present in 22.6% of the cat scats (February
and May samples combined). The Socorro Ground Dove and Tropical Parula Warbler
were the two bird species found most often in the scats, five and three times,
respectively. A recent colonizer, the Mourning Dove (Zenaida macroura), was
present in two of the 31 analyzed scats. Although we did not find remains of another
recent colonizer, the Northern Mockingbird (Mimus polyglottus), in May we found
feathers of one in the field, presumably eaten by a feral cat. Jehl and Parkes (1982)
reported predation by cats on Townsend's Shearwater (Puffinus auricularis), but we
did not find evidence of this perhaps because our searches were not made near the
shearwater colonies.
Our results suggest that predation on birds by feral cats is not very important.
However, even limited predation may seriously affect a species as endangered as the
Socorro Mockingbird. This species' population may be approximately 60-70 pairs
(Castellanos and Rodriguez-Estrella 1990, L. Baptista pers. comm.), and we found
evidence that it is preyed upon by Socorro Red-tailed Hawks (a Socorro Mockingbird
Western Birds 22:141-143, 1991 141
NOTES
Talkie 1 Prey Remains Identified in 31 Scats of Feral Cats
on Socorro Island, 1990
February May
(12 samples) (19 samples)
Prey N ø % N %
Mammals 11 91.7 17 89.5
Mus musculus 9 75.0 17 89.5
Felis catus 2 16.7 0 0.0
Birds 3 25.0 4 21.1
Columbina 2 16.7 3 15.8
Thryomanes 1 8.3 1 5.3
Parula 2 16.7 1 5.3
Miraodes 1 8.3 0 0.0
Zenaida macroura 0 0.0 2 10.5
Reptiles 4 33.3 10 52.6
Urosaurus auriculatus 4 33.3 10 52.6
Invertebrates 9 75.0 14 73.7
Orthoptera 8 66.7 12 63.2
Lepidoptera 2 16.7 0 0.0
Coleoptera 2 16.7 4 21.1
Hymenoptera 1 8.3 1 5.3
Scorpiones 0 0.0 4 21.1
Decapoda 0 0.0 2 10.5
Miscellaneous 9 75.0 17 89.5
Seeds 1 8.3 4 21.1
Vegetable fibers 8 66.7 14 73.7
Unidentified animal matter 0 0.0 2 10.5
aNumber of scats in which each species or item was identified.
wing and feathers showing predation by a hawk) and cats. The endemic Red-tailed
Hawk and Socorro Mockingbird have presumably long been in predatory-prey
equilibrium, while cats are of recent introduction. "Alien predation" (Moors and
Atkinson 1984) by cats may be a serious problem because it could lead to declines in
some bird species, as has happened with other island landbirds and seabirds (Jehl
1972, Taylor 1979, Diamond 1982, Moors and Atkinson 1984, Fitzgerald and
Veitch 1985, Veitch 1985, Ebenhard 1988).
The population of feral cats seems to be densest in the southeast portion of the
island apparently because this area is the zone most perturbed by human activities.
Here water and domestic garbage are available for cats, and the food supply in
human refuse may favor an increase of the cats' population. The actual numbers of
cats is hard to determine, although their distribution on the island is increasing
(Arnaud et al. unpubl. data). Because feral cats take their prey mainly on the ground,
birds foraging on the ground, such as the endemic ground dove and wren, are
especially vulnerable. The Socorro Wren population is very dense, but such is not the
case for Socorro Ground Dove, which is still fairly common in the open lowlands of
the island, especially in the southeast and eastern portions near the coast (W. Wehtje
pers. comm.). This species is no longer as tame as reported by Villa (1960)--"son
mansas y se les captura simplemente con las manos"--but it seems that their
foraging and reproductive activities are normal. However, whenever we approached
142
NOTES
the ground doves, they immediately flew, suggesting that predation by cats is
affecting the endemic dove's tameness and possibly its population size.
At the Centro de Investigaciones Bio16gicas we are studying the process of
apparent extinction of some bird species, and we want to call the attention of the
scientific community and organizations concerned with the conservation of
biodiversity about the need to eradicate the feral cats from Socorro Island and thereby
protect the endemic avifauna. Otherwise, cats may put in risk some of the land birds
of the island, particularly the Socorro Mockingbird.
Special thanks are due to Alfredo Ortega for encouraging our studies on Socorro
Island. Philip Unitt, William T. Everett, and an anonymous reviewer made helpful
suggestions on a previous draft. Luis Baptista kindly provided us the information on
the 19 Socorro Mockingbirds he recorded in the north-central part of the island in
November 1990. For its support on the island we thank the Mexican Navy.
Economic support was provided by World Wildlife Fund, the Centro de
Investigaciones Bio16gicas, Consejo Nacional de Ciencia y Tecnologia, Secretaria de
Programaci6n y Preseupuesto, Secretaria de Desarrollo Urbano y Ecologia, and
Fundaci6n Ricardo J. Zevada.
LITERATURE CITED
Brattstrom, B. H. 1990. Biogeography of the Islas Revillagigedo, Mexico. J. Bio-
geogr. 17:177-183.
Brattstrom, B. H., and Howell, T. R. 1956. The birds of the Revilla Gigedo Islands,
Mexico. Condor 58:107-120.
Castellanos, A., and Rodriguez-Estrella, R. 1990. Endangered Socorro Mockingbird
(Nlimodes graysoni) on Socorro Island, Mexico. Proceedings of the 108th
Stated Meeting of the American Ornithologists' Union and 60th Annual Meet-
ing of the Cooper Ornithological Society, Univ. of Calif., Los Angeles.
Diamond, J. M. 1982. Man the exterminator. Nature 298:787-789.
Ebenhard, T. 1988. Introduced birds and mammals and their ecological effects.
Swedish Wildlife Res. 13:1-107.
Fitzgerald, B. M., and Veitch, C. R. 1985. The cats of Herekopare Island, New
Zealand; their history, ecology and effects on birdlife. New Zealand J. Zool.
12:319-330.
Jehl, J. R. 1972. On the cold trail of an extinct petrel. Pacific Discovery 25:24-29.
Jehl, J. R., and Parkes, K. C. 1982. The status of the avifauna of the Revillagigedo
Islands, Mexico. Wilson Bull. 94:1-19.
Jehl, J. R., and Parkes, K. C. 1983. "Replacements" of landbird species on Socorro
Island, Mexico. Auk 100:551-559.
McLellan, M. E. 1926. Expedition to the Revillagigedo Islands, Mexico, in 1925. VI.
The birds and mammals. Proc. Calif. Acad. Sci., 4th ser., 15:297-322.
Moors, P. J., and Atkinson, I. A. E. 1984. Predation on seabirds by introduced
animals, and factors affecting its severity. ICBP Tech. Publ. 2:668-690.
Taylor, R. H. 1979. How the Macquarie Island parakeet became extinct. New
Zealand J. Ecol. 2:42-45.
Veitch, C. R. 1985. Methods of eradicating feral cats from offshore islands in New
Zealand. ICBP Tech. Publ. 3:125-141.
Villa, B. 1960. Vertebrados terrestres. La Isla Socorro. Monogr. Inst. Geofisica.
Univ. Nacional Aut6noma de M•xico 2:203-216.
Accepted 15 April 1991
143
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Full-text available
  • Jul 2023
The family of Columbidae, including pigeons and doves, remains understudied despite their patrimonial value and high ecological and conservation relevance. Currently, 353 extant columbid species are listed in the IUCN red list, with about 20% of them being threatened with extinction. However, there has been little effort so far to synthetize the available information on factors influencing extinction risk and the allocation of research effort among columbid species. In this context, using random forest models, the present study aims at quantitatively assessing to what extent environmental, life history and socio-political factors may drive the extinction risk of pigeons and doves and explain differences in scientific attention among species. We found that high risk of extinction in columbids is associated with small historical range, exposure to invasive alien mammals and living in isolated islands and/or at low altitudes, while the probability of population decline is associated with species body size, surrounding human density and narrow habitat breadth. We also evidenced a large disparity between species or population extinction risk and scientific interest. Indeed, most of the studies on columbids have been conducted by scientists from North America and Western Europe on their local species, whereas species from biodiversity hotspots, which are more at risk of extinction, have comparatively received little attention. This unequal acquisition of knowledge creates gaps that deserve to be filled in order to have a good appreciation of extinction risk in columbids and associated threats, through fair transnational cooperation, academic training and regional coordination in conservation-oriented research on columbids.
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A New Species of Johngarthia from Clipperton and Socorro Islands in the Eastern Pacific Ocean (Crustacea: Decapoda: Gecarcinidae)
Article
  • Apr 2019
  • PAC SCI
The taxonomy of the East Pacific land crab Johngarthia planata (Stimpson, 1860) is reviewed and a neotype designated to stabilize the taxonomy of this species. The population of J. planata is found to be restricted to inshore islands and the continental mainland coast of the tropical and subtropical Pacific coast of America. A new species, Johngarthia oceanica sp. nov., is described for populations that occur on the oceanic Clipperton and Socorro islands. Johngarthia oceanica sp. nov., can be separated from J. planata by the broad, subrectangular mesial lobe of the infraorbital margin (narrow, subtriangular in J. planata) and a dorsal carapace that is dark yellow to brightly orange with or without black areas of different extension (homogeneously red to maroon in J. planata). The distribution of J. planata is modeled using MaxEnt and the biogeography of East Pacific species of Johngarthia is discussed. The conservation status of the Clipperton population of J. oceanica sp. nov. deserves special attention because this population may be threatened by introduced rats.
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“Replacements” of Landbird Species on Socorro Island, Mexico
Article
  • Jul 1983
The endemic dove Zenaida graysoni of Socorro Island, an oceanic island in the Pacific south of Baja California, became extirpated between 1958-1978, and the endemic mockingbird Mimodes graysoni diminished during the same years and was approaching extinction by 1981. The mainland mourning dove Z. macroura became established on Socorro between (probably) 1971-1978, and the mainland northern mockingbird Mimus polyglottos between 1978-1981. Extermination of the endemics was probably caused by predation by feral cats Felis catus introduced in 1957 or later. There is no evidence of any actual competition between the pairs of species, and their preferred habitats overlap only slightly. The nearly concurrent establishment of the two invading species, known to have occurred frequently as casual visitors to Socorro and other islands, is attributed largely to the provision of fresh water made available as a result of human settlement.-from Authors
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The status of the avifauna of the Revillagigedo Islands, Mexico.
Article
  • Jan 1982
  • Wilson Bull
Isla Socorro is in a state of change as a result of increased human activities since 1958 and the introduction of domestic animals. Nine endemic avian taxa have been described for Socorro. In the past 20 yr, several of these have shown sharp declines, largely attributable to predation by domestic cats Felis catus. Pipilo erythrophthalmus socorroensis seems far less common than in the recent past. Mimodes graysoni, once described as the most common landbird on Socorro, is extremely rare and local and its condition seems precarious. Zenaida graysoni is apparently extinct. Thryomanes sissonii, Aratinga holochlora brevipes, Columbina passerina socorroensis and Parula pitiayumi graysoni remain common; indeed the warbler may have increased. Two species have invaded Socorro. Zenaida macroura became well established between 1971-78 and Mimus polyglottos between 1978-81. Resident landbirds are absent from San Benedicto but seabird numbers have increased since the eruption of Volcan Barcena in 1952. Notes on 25 species recorded for the first time are included. -from Authors
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Biogeography of the Islas Revillagigedo, Mexico
Article
  • Mar 1990
The Islas Revillagigedo consist of four volcanic, oceanic islands located 400 miles west of Colima, Mexico. The islands in order of increasing size are: Roca Partida, San Benedicto, Clarion, Socorro. The terrestrial fauna of the islands is depauperate and disharmonic and consists of birds, three species of reptiles, and flying and rafting arthropods. Many of the species of sea birds and shore birds are cosmopolitan or cosmotropic, while the plants, reptiles and land birds show their greatest affinity to the mainland Sonora-Sinaloa are of Mexico. Prevailing currents in the Gulf of California favour chance dispersal to the islands. The percentage of the biota which is endemic is high. The degree of endemism of birds seems to be favoured by the size of the island and the diversity of habitats, while endemism and success of establishment by reptiles seems favoured by the age of the island. This seems consistent with methods of dispersal (flying versus rafting). There is no evidence of adaptive radiation of the terrestrial vertebrates within the islands. Recent introductions of exotic species have upset the apparent stability of the fauna and flora of these islands.
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The cats of Herekopare Island, New Zealand; their history, ecology and affects on birdlife
Article
  • Jul 1985
House cats (Felis catus) were introduced to Herekopare Island, a mammal-free island of about 28 ha near Stewart Island, in about 1925. In winter 1970, the total population of 33 cats (20 males, 13 females) — a density of 1.2 cats/ha — was killed, mainly by trapping. Examination of stomach contents and scats showed that the cats fed mainly on petrels, supplemented by land birds and insects. The bird life of Herekopare Island was studied by H. Guthrie-Smith in 1911, L. E. Richdale in the early 1940s, and by New Zealand Wildlife Service staff in 1968 and 1970. Their accounts indicate that a vast breeding population of diving petrels and thousands of broad-billed prions were probably exterminated by the cats, though fairy prions and sooty shearwaters persisted. Among land birds, the yellow-crowned parakeet, robin, fernbird, brown creeper, Stewart Island snipe, and banded rail were exterminated. Two other species, the red-crowned parakeet and tomtit, probably disappeared but subsequently recolonised the island. Although cats had the greatest influence on the bird life over this period, wekas, which were present for some years, together with changes in the vegetation, may have affected some bird populations.
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Endangered Socorro Mockingbird (Nlimodes graysoni) on Socorro Island
  • Jan 1990
  • A Castellanos
  • R Rodriguez-Estrella
Castellanos, A., and Rodriguez-Estrella, R. 1990. Endangered Socorro Mockingbird (Nlimodes graysoni) on Socorro Island, Mexico. Proceedings of the 108th Stated Meeting of the American Ornithologists' Union and 60th Annual Meeting of the Cooper Ornithological Society, Univ. of Calif., Los Angeles.