Removing cats from islands in NW Mexico

Wood, B., Tershy, B. R., Hermosillo, M. A., Donlan, C. J., Sanchez, J. A., Keitt, B. S., Croll, D.A. Howald, G.R. & Biavaschi, N. (2002). Removing cats from islands in north-west Mexico. In Veitch, C. R., & Clout, M. N. (2002). Turning the Tide: The Eradication of Invasive Species: Proceedings of the International Conference on Eradication of Island Invasives,[University of Auckland, 19 to 23 February 2001] (No. 27). IUCN, 374-380.


Feral cats have been associated with extinctions of endemic island species throughout the world. Removing cats from islands is an effective way to protect biodiversity, but compared to other invasive alien mammals, cats are difficult to eradicate. Here we describe the techniques we used to eradicate cats from 15 islands in north-west Mexico between <1 and 43 km²  These eradication techniques were developed and refined on small islands (<1 km²) and then adopted successfully on larger islands (1– 43 km² . Experienced hunters and trappers, and high quality hunting dogs were critical for successful cat eradication. The most effective technique was trapping and the most critical components of trapping were trap design and placement.

Introduced mammals as the first threat to Xantus' Murrelet

KEITT, B.S. 2005. Status of Xantus’s Murrelet and its nesting habitat in Baja California, Mexico. Marine Ornithology 33: 105–114.

A preliminary survey was conducted in 1999 to establish the status of the Xantus’s Murrelet Synthliboramphus hypoleucus in Baja California, Mexico. Seven island groups with prior evidence of breeding (Coronado, Todos Santos, San Martín, San Jerónimo, San Benito, Asunción and San Roque) and two potential breeding islands without prior evidence of nesting (Natividad and Adelaida) were examined. In 2004, additional work was conducted at Afuera Islet off Guadalupe Island. Presence of murrelets was detected through nest searches and by rough estimation of birds in nocturnal at-sea congregations using boat-based and land-based vocalization counts. Vocalizations were heard at six island groups (Coronado, Todos Santos, San Martín, San Jerónimo, San Benito and Guadalupe) and nests were found at four island groups (Coronado, San Jerónimo, San Benito and Guadalupe). Land-based and boat-based vocalization surveys both detected presence or apparent absence of murrelets at potential nesting islands, although boat-based vocalization rates were higher on average. Vocalization surveys cannot readily be converted to breeding population estimates, but overall population size of murrelets in Baja California appears to about 2300 pairs (range: 1000–4000 pairs), similar to previous estimates. Historically, nonindigenous mammals were introduced to most islands in Baja California; recent progress in removing introduced mammals should benefit Xantus’s Murrelets.

Eradication of non-native mammals and the status of insular mammals on the California and Baja California Islands

Knowlton, J. L., Josh Donlan, C., Roemer, G. W., Samaniego-Herrera, A., Keitt, B. S., Wood, B., Aguirre-Muñoz, A., Faulkner, K.R. & Tershy, B. R. (2007). Eradication of non-native mammals and the status of insular mammals on the California Channel Islands, USA, and Pacific Baja California Peninsula Islands, Mexico. The Southwestern Naturalist, 52(4), 528-540.

The California Channel Islands, USA, and Pacific Baja California Peninsula Islands, Mexico (hereafter referred to as the California islands), are known for their high levels of biodiversity and globally important colonies of seabirds. We document the history, impacts, and management of non-native mammals and summarize the current status of native, non-volant mammals on the California islands. Of the 26 species of native mammals on the California islands, including 6 species and 41 subspecies that are endemic, ≥10 populations have suffered extirpation or global extinction. All recent extirpations and extinctions resulted directly from non-native mammalian predators or indirectly via habitat degradation by non-native herbivores. In light of the devastating effects non-native mammals have had on the native insular biotas of the California islands, a variety of organizations have collaborated to eradicate 44 populations of non-native mammals from 19 California islands. Documentation of impacts of non-native mammals and timely implementation of successful eradication efforts are essential to the conservation of these and other insular ecosystems.

Effect of feral dogs and cats on native mammals of Isla de Cedros

Cortés-Calva, P., Gallo-Reynoso, J. P., Delgadillo-Rodríguez, J., Lorenzo, C., & Álvarez-Castañeda, S. T. (2013). The Effect of Feral Dogs and Other Alien Species on Native Mammals of Isla de Cedros, Mexico. Natural Areas Journal,33(4), 466-473.

We report the status of alien species on Isla de Cedros, Mexico, and analyze the information from different years that together with a collaborative effort between academic biologists, Mexican governmental agencies, and local individuals has resulted in major information about the alien species on this island. We also report species richness of distinct endemic mammal species and the presence of feral dogs (Canis lupus familiaris Linnaeus), which is the principal problematic situation in this “Marine Priority Region.” The possible origin of feral dogs could be the migratory movement of stray dogs to find food in the inner part of the island, first moving to garbage dumps, reproducing in the area, and later hunting goats (Capra aegagrus hircus Linnaeus) in packs. The combination of the high density of stray dogs, urban and industrial food garbage dumps, and the large number of marine resources scattered along the seashore made ideal conditions for the establishment of feral dog packs that are affecting native species. The island does not have any natural mammal predator, but the presence of feral species, dogs and cats (Felis silvestris catus Schreber), could disturb the occurrence of endemic fauna as it has happened on other islands in the world.

Population and social biology of free-ranging dogs

Daniels, T. J., & Bekoff, M. (1989). Population and social biology of free-ranging dogs, Canis familiaris. Journal of Mammalogy, 754-762.

Population size and density, age structure, survivorship patterns, sex ratios, and social organization of urban, rural, and feral dog (Canis familiaris) populations were examined in Cd. Juarez, Mexico (urban site) and on the Navajo reservation (rural and wild sites) between June 1983 and December 1984. Urban and rural dogs were less social than expected whereas feral dogs characteristically lived in packs. Seasonal variation in the structure of feral dog packs was influenced by reproduction, both directly (pups born into the pack) and indirectly (pregnant females may temporarily emigrate form the pack to give birth).

Planning cat eradication on Guadalupe Island

Luna-Mendoza,L., J. M. Barredo-Barberena, J. C. Hernández-Montoya, A. Aguirre-Muñoz, F. A. Méndez-Sánchez, A. Ortiz-Alcaraz, & M. Félix-Lizárraga. 2011. Planning for the eradication of feral cats on Guadalupe Island, México: home range, diet, and bait acceptance. Pages 192-197 In: Veitch, C. R.; Clout, M. N. and Towns, D. R. (eds.). 2011. Island invasives: eradication and management. IUCN, Gland, Switzerland.

Feral cats (Felis catus) introduced to new environments have caused the extinction of many vertebrate species, including six species of birds on Guadalupe Island, México. To save species from extinction and restore natural processes, cats have been eradicated from islands using a variety of techniques. Eradication campaigns have to be planned carefully; ideally supported by information about the population to be eradicated. Our study focuses on home range estimation (fixed kernel); bait consumption by feral cats and non-target species; and diet of feral cats on Guadalupe Island. Home range was 76 to 1098 ha (KE 95) and core areas 21 to 196 ha (KE 50). Feral cats and non-target species including Guadalupe junco (Junco hyemalis insularis), Guadalupe rock wren (Salpinctes obsoletus guadalupensis), western gull (Larus occidentalis), and house mouse (Mus musculus) consumed baits. Items most commonly found in diet samples were mice (66.5%) and birds (16.8%). Male cats were 2.9 ± 0.6 kg, and females 2.4 ± 0.9 kg. The results of this study will inform eradication decisions for Guadalupe Island, especially regarding the use of poison baits.

More on stable isotopes to know feral cat diet on islands

Keitt, B. S., Wilcox, C., Tershy, B. R., Croll, D. A., & Donlan, C. J. (2002). The effect of feral cats on the population viability of black‐vented shearwaters (Puffinus opisthomelas) on Natividad Island, Mexico. Animal Conservation, 5(3), 217-223.

Insular breeding seabirds are likely to be particularly vulnerable to introduced mammalian predators because they often lack behavioural, morphological and life-history defenses against predation. We studied the life-history of the black-vented shearwater (Puffinus opisthomelas) on Natividad Island, Baja California Sur, Mexico, to examine its vulnerability to introduced feral cats. Using an allometric equation, we estimated that feral cats consumed 328 g of food day-1 to satisfy their nutritional requirements. We used stable isotope analysis of cat scat to estimate that 90% of the cats' diet was composed of shearwaters. Using data from our focal species and from the closely related manx shearwater (Puffinus puffinus), we created a demographic model to evaluate the effects of cat population size on the annual growth rate (λ) of the shearwater. The annual growth rate for black-vented shearwaters was estimated to be 1.006 in the absence of cat predation. With predation, we estimated that annual growth rate declined approximately 5% for every 20 cats in a population of 150,000 birds. Persistence times of bird colonies decreased both with an increase in the size of the feral cat population and with a decrease in the size of the initial bird population.

Feral cat impact on seabirds populations on Natividad Island

Keitt, B. S., C. Wilcox, B. R. Tershy, D. A. Croll & C. J. Donlan. 2002. The effect of feral cats on the population viability of black-vented shearwaters (Puffinus opisthomelas) on Natividad Island, Mexico. Animal Conservation, 5: 217–223


Insular breeding seabirds are likely to be particularly vulnerable to introduced mammalian predators because they often lack behavioural, morphological and life-history defenses against predation. We studied the life-history of the black-vented shearwater (Puffinus opisthomelas) on Natividad Island, Baja California Sur, Mexico, to examine its vulnerability to introduced feral cats. Using an allometric equation, we estimated that feral cats consumed 328 g of food / day to satisfy their nutritional requirements. We used stable isotope analysis of cat scat to estimate that 90% of the cats’ diet was composed of shearwaters. Using data from our focal species and from the closely related manx shearwater (Puffinus puffinus), we created a demographic model to evaluate the effects of cat population size on the annual growth rate (λ) of the shearwater. The annual growth rate for black-vented shearwaters was estimated to be 1.006 in the absence of cat predation. With predation, we estimated that annual growth rate declined approximately 5% for every 20 cats in a population of 150,000 birds. Persistence times of bird colonies decreased both with an increase in the size of the feral cat population and with a decrease in the size of the initial bird population.

Eradication of feral cats on a forested tropical island

Rodríguez C., Torres R. & Drummond H. (2006) Eradicating introduced mammals from a forested tropical island. Biological Conservation, 130, 98-105.

Methodologies developed in New Zealand for eradication of exotic mammals from temperate oceanic islands were adapted for use on a topographically complex, 194-ha, densely forested, tropical island off the Pacific coast of Mexico. Isla Isabel is a National Park and a nesting refuge for nine species of marine birds, but the island’s extraordinarily dense population of cats (113/k2) was annually killing 23–33% of nesting sooty terns.
We provide quantitative descriptions of the implementation and outcome of the methodologies over a period of three years, culminating in successful eradication of cats, but not rats. We also provide quantification of the effects of cat eradication on sooty tern mortality over a period of 11 years.

Socorro island's endemics and cats

Arnaud, G., A. Rodriguez, A. Ortega-Rubio, S. Alvarez-Cardenas. 1993. Predation by Cats on the Unique Endemic Lizard of Socorro Island (Urosaurus auriculatus), Revillagigedo, Mexico. Ohio Journal of Science, 93 (4): 101-103

During 1990 the distributions of the endemic lizard (Urosaurus auriculatus) and the feral cat (Felis catus) of Socorro Island were observed and scats of the last species were collected. A total of 46 cat scats were analyzed, showing that lizards were an important prey item, varying in frequency from 33.33% in February to 66.66% in November. Because of the impact of sheep (Ovis aries) on the natural vegetation and because of the impact of cat predation, we recommend that a plan be developed for the removal of exotic species and the restoration of natural vegetation to Socorro Island.

Jehl, J.R.& K.C. Parks. 1983. 'Replacements' of landbird species on Socorro Island, Mexico. Auk 100: 551-559.
The endemic dove Zenaida graysoni of Socorro Island, an oceanic island in the Pacific south of Baja California, became extirpated between 1958 and 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 and 1978 and the mainland northern mockingbird Mimus polyglottos between 1978 and 1981. Superficially an instance of replacement of island endemics by mainland relatives through competitive exclusion, the case is actually more complicated. It is argued that extermination of the endemics was probably caused by predation by feral cats 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 2 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. Simple comparison of island species lists compiled at intervals is an inadequate base for theories of island species turnover.

More info about Socorro's threatened birds:
BirdLife International (2013) Species factsheet: Zenaida graysoni. Downloaded from http://www.birdlife.org on 12/04/2013.
BirdLife International (2013) Species factsheet: Mimus graysoni. Downloaded from http://www.birdlife.org on 12/04/2013. 

BirdLife International (2013) Species factsheet: Aratinga brevipes. Downloaded from http://www.birdlife.orgon 12/04/2013.
BirdLife International (2013) Species factsheet: Troglodytes sissonii. Downloaded from http://www.birdlife.orgon 12/04/2013.
BirdLife International (2013) Species factsheet: Puffinus auricularis. Downloaded from http://www.birdlife.orgon 12/04/2013.

Introduced mammals on California Channel and Baja California islands

McChesney, G. J., & Tershy, B. R. (1998). History and status of introduced mammals and impacts to breeding seabirds on the California Channel and northwestern Baja California Islands. Colonial Waterbirds, 21 (3) 335-347.

The California Channel Islands, U.S.A., and Northwestern Baja California Islands, Mexico, host important breeding populations of several seabird species, including the endemic Black-vented Shearwater (Puffinus opisthomelas) and Xantus' Murrelet (Synthliboramphus hypoleucus). Mammals introduced to nearly all of the islands beginning in the late 1800s to early 1900s include: cats (Felis catus), dogs (Canis familiaris), Black Rats (Rattus rattus), rabbits and hares (Leporidae), goats (Capra hirca), sheep (Ovis aries), and other grazers. Cats, dogs and rats are seabird predators, grazers such as goats and sheep cause habitat degredation, and rabbits destroy habitat and compete with hole-nesting seabirds. Cats, which were introduced to at least 19 islands and currently occur on ten islands, have had the greatest impacts on seabirds, including the extinction of the endemic Guadalupe Storm-Petrel (Oceanodroma macrodactyla). Cats are known to have eliminated or severely reduced colonies of Black-vented Shearwaters, Cassin's Auklets (Ptychoramphus aleuticus) and Xantus' Murrelets. Black Rats have occurred on a minimum of seven islands and have reduced numbers of small, hole-nesting alcids on at least one island. At many islands, defoliation and erosion caused by rabbits and large grazing mammals has been severe. Their effects on seabirds are not well documented but potentially are serious. Impacts from introduced mammals have been most severe on islands with no native mammalian predators. On the Northwestern Baja California Islands, temporary and permanent human settlements have led to a greater diversity and source of introductions. Programs to remove introduced mammals and to reduce the possibility of future introductions are needed to restore seabird populations and to preserve the biodiversity of the region. Surveys are needed particularly on the Northwestern Baja California Islands to update the status and distribution of seabirds and to further assess impacts from introduced mammals.