Eric Mellink

Distribution and conservation status of prairie dogs Cynomys mexicanus and Cynomys ludovicianus in Mexico

Abstract The two living species of black-tailed prairie dogs Cynomys mexicanus and C. ludovicianus are found in Mexico. Cynomys mexicanus, a Mexican endemic, is restricted to a 600-km2 region in northwestern Mexico. It is found in six large arid grassland valleys associated with gypsum soils and surrounded by arid scrub. Due to the small geographic range and destruction of its habitat this species is considered endangered. Cynomys ludovicianus is found in northwestern Mexico. Its present distribution comprises a very large complex covering approximately 55 000 ha, eight major dogtowns, and more than one million prairie dogs. Indeed, this population represents the largest continuous prairie dog complex left in North America. However, its present conservation status is considered as threatened, mainly because of the rapid deterioration of its habitat.

Occasional inter-sex differences in diet and foraging behavior of the Blue-footed Booby: maximizing chick rearing in a variable environment?

Labor division in parental care and reduction of inter-sex competition by division of the foraging niche have been suggested as part of the evolutionary basis for reversed sexual size dimorphism in boobies (Sulidae). To test the potential viability of both hypotheses for the maintenance of dimorphism, we studied parental care and foraging behavior of the Blue-footed Booby (Sula nebouxii) in two colonies in the Gulf of California, Mexico. Proportion of time spent at the nest, feeding frequency, and time devoted to nest defense were used as indicators of parental care. Foraging ecology was evaluated through the number and duration of foraging trips, proportion of time dedicated to flight and to active feeding, rate and depth of diving, and diet. We found some inter-sex differences in foraging only under high demand conditions, such as food shortage or large broods. Inconsistent inter-sex differences seem to be part of a strategy to maximize chick rearing under a variable environmental regime.

Population demise and extinction threat of the Angel de la Guarda deer mouse (Peromyscus guardia)

We analyze the status of the Angel de la Guarda deer mouse (Peromyscus guardia), a species endemic to Mexico, based on our own fieldwork, bibliographic records and information from colleagues. This species, with different subspecies on three islands and a population of an undetermined subspecies on a fourth, was apparently common until the mid-1960s. Currently the species is critically endangered, if not already extinct. One of the subspecies and the undetermined population are presumed extinct. Of the two other subspecies, one is at least in critical condition, and the other at least reduced. The demise of the species can be attributed to the introduction of domestic cats. The case of P. guardia is a good example of the vulnerability of other taxa of endemic rodents on islands in the Gulf of California. # 2002 Elsevier Science Ltd. All rights reserved.

Organchlorine content and shell thickness in brown booby (Sula leucogaster) eggs in the Gulf of California and the southern Pacific coast of Mexico

We determined egg concentrations of organochlorines and thickness of eggshells from brown boobies at eight colonies ranging from the northern Gulf of California to southern Mexico. The only common residue was that of DDE, which was found in almost all eggs. DDE content apparently reflected pre-1990 DDT use in nearby agricultural areas and, at one site, intensive mosquito control for high-end tourism development. There were no inter-colony differences in eggshell thickness, and variation in this variable likely reflected individual bird characteristics and/or individual feeding source. This variable was not a good proxy to DDE exposure of brown boobies, under current DDE levels in the brown booby trophic chain. In the northern Gulf of California, eggshell thickness has recovered to pre-DDT conditions. Our data indicate that the Gulf of California and southwestern coast of Mexico have a healthy near-shore marine environment, as far as organochlorines are concerned.

Colonies of Four Species of Terns and the Black Skimmer in Western Mexico

Abstract During the 2003 breeding season 367 historic, potential, and current nesting sites of larids (Fam. Laridae) along the coast of western Mexico were surveyed. In 2004 and 2005, three areas: Guerrero Negro, in Baja California Sur; Marismas Nacionales, in Nayarit-Sinaloa; and Laguna Cuyutlán, in Colima were surveyed more intensively. In 2003-2005, there were nine nesting sites and probably between 200 and 300 breeding pairs of Caspian Tern (Sterna caspia); eleven confirmed nesting sites and at least 13,000 breeding pairs of Royal Tern (S. maxima); two nesting sites, one of which included almost the entire population, and 90,000 breeding pairs of Elegant Tern (S. elegans); four nesting sites and about 20 breeding pairs of Forster’s Tern (S. forsteri); and 13 nesting sites and about 1,000 breeding pairs of Black Skimmer (Rynchops niger). Breeding of Caspian and Elegant Terns on the Pacific coast of Mexico is confined to the northern areas, whereas that of Royal Terns and Black Skimmers occurs at selected sites along most of the coast. Forster’s Terns nest in two widely separated areas, one in the northern section of the coast, and one in the southern section.

Fish Abundance and the 1995 Nesting Season of the Least Tern at Bahía de San Jorge, Northern Gulf of California, México

Abstract During 1995, the relationship between the nesting chronology of the Least Tern (Sterna antillarum) and the near-shore abundance of fish was examined at La Purinera, Sonora, the site of one of the largest known Least Tern colonies in northwest Mexico. Fish abundance was measured adjacent to the shoreline during early mornings using a beach seine net, and the sampling periods corresponded to different phases of the Least Tern’s annual cycle. During this year, the nesting season (April to July) coincided with peak prey fish abundance, and peak hatching of tern eggs (early July) coincided with the greatest abundance of fish of adequate size for chicks. Departure of the terns from the area coincided with a marked reduction in suitable food in September. Collections of fish dropped and left uneaten by Least Terns in the colony contained five species, which were also present in the fish samples obtained with the beach seine net. The observations on foraging adult Least Terns and the collections of fish dropped in the colony suggested that the terns were preying mainly upon silversides (Leuresthes sardina, Colpichthys regis) and anchovies (Anchoa spp.). Terns foraged mainly on the bay side of the barrier beach where fish were more abundant. The results support the idea that seabird breeding and food abundance or availability coincide at the time of maximum need, when parents feed rapidly growing chicks.

Nesting Waterbirds of Isla Montague, Northern Gulf of California, Mexico: Loss of Eggs Due to Predation and Flooding, 1993-1994

-We studied seven species of waterbirds that nested on Isla Montague, Mexico, at the mouth of the Colorado River, during the 1993 and 1994 breeding seasons: Great Blue Heron (Ardea herodias), Snowy Egret (Egretta thula), Black-crowned Night Heron (Nycticorax nycticorax), Laughing Gull (Larus atricilla), Least Tern (Sterna antillarum), Gull-billed Tern (Sterna nilotica), and Black Skimmer (Rynchops niger). Breeding occurred along several tidal channels of the Estero del Chayo and on shell mounds near the lighthouse. There were low levels of predation, by at least one Coyote (Canis latrans), and unknown, but likely avian, predators. Tidal inundation was the major factor affecting the nesting of the birds, as it destroyed almost all nests, eggs and chicks on at least five occasions during one breeding season. It seems likely that the pattern of inundations we recorded does not happen all years. Received 21 August 2000, accepted 20 March 2001.

MAXIMUM DIVING DEPTH IN FLEDGING BLUE-FOOTED BOOBIES: SKILL DEVELOPMENT AND TRANSITION TO INDEPENDENCE

Abstract We evaluated maximum diving depth and time spent at the nest of fledging Blue-footed Boobies (Sula nebouxii) at Isla El Rancho, Sinaloa, in the Gulf of California, Mexico. Within three consecutive 10-day post-fledging intervals, maximum diving depth was highly variable, but was not affected by sex, weight, or body condition. During the first days of post-fledging flight, maximum diving depth increased rapidly. By the second week after first flight, the plunge-dives of juveniles were almost as deep as those of adults. Parental care and attachment to the nest lasted several additional weeks (up to 40 days after first flight). Although their diving capacity rapidly reached a level similar to that of the adults, it appeared that juvenile boobies took much longer in acquiring other foraging skills.

Ecology of Mexican Prairie Dogs, Cynomys mexicanus, in El Manantial, Northeastern Mexico

The Mexican prairie dog ( Cynomys mexicanus ) is an endangered species that is endemic to northeastern Mexico. To obtain basic data on diet, parasites, reproduction, relationship with other small mammals, and predators, during the spring and summer, we studied a prairie dog colony at El Manantial, San Luis Potosi, northeastern Mexico. Mexican prairie dogs preferentially fed on grasses in the spring, but as grasses matured, prairie dogs changed to a diet principally composed of forbs. Prairie dogs exhibited low levels of parasitism. Breeding peaked from mid-January to early February, but extended, at low levels, into early April. Nocturnal rodents and lagomorphs were more abundant and had a higher species richness on an abandoned prairie dog colony than on the active one, except for grasshopper mice ( Onychomys arenarius ) which were less abundant on the abandoned colony site. Coyotes ( Canis latrans ) appeared to be the most important predators of prairie dogs.

Diurnal Abundance, Foraging Behavior and Habitat Use by Non-Breeding Marbled Godwits and Willets at Guerrero Negro, Baja California Sur, México

Abstract. Seasonal variation in abundance, time activity budgets and foraging behavior of non-breeding Marbled Godwits (Limosa fedoa) and Willets (Tringa semipalmata) were compared at four sites at the Laguna Ojo de Liebre—Guerrero Negro saltworks complex, Baja California, Mexico. Habitat use varied between species, seasons and sites. Marbled Godwits (182 ± 44.1 individuals per site) were more abundant than Willets (70 ± 16.1 individuals per site), reflecting their overall pattern in northwestern Mexico. Overall abundance diminished throughout the season (270 ± 69 individuals per site in Oct–Nov, 85 ± 21 in Dec–Jan and 60.2 ± 24 in Feb–Mar), although it remained high at mudflat. Consistent with bill length, Willets foraged mostly by pecking, while Marbled Godwits did so mostly by probing (proportion of pecks: 0.95 ± 0.17 and 0.4 ± 0.27, respectively). Marbled Godwits changed their time activity budgets through the wintering season: time devoted to vigilance changed from 8–23% in Oct–Nov, to 0.4–8% in Feb–Mar whereas time devoted to feeding changed from 12–40% in Oct–Nov to 59–74% in Feb–Mar. This pattern seems to reflect a change in priorities; surviving early in the season and accumulating energy to migrate, later. In both species, differences in use of habitat appeared to be related to site characteristics such as substrate hardness and risk of predation. The mudflat was the site most used and the saltmarsh, the least used. Some individuals in both species used the more risky saltmarsh, but increased the time devoted to vigilance. Thus, habitat quality for non-breeding shorebirds depended on both benefits and costs for foraging birds, and habitat choice by specific individuals was complex and probably involved condition- or state-dependent tradeoffs that balanced metabolic requirements, safety priorities, and, perhaps, social status or dominance.