Guillermo Fernández

WESTERN SANDPIPERS (CALIDRIS MAURI) DURING THE NONBREEDING SEASON: SPATIAL SEGREGATION ON A HEMISPHERIC SCALE

Abstract The nonbreeding distribution of Western Sandpipers (Calidris mauri) was documented using 19 data sets from 13 sites along the Pacific and Atlantic coasts of the Americas. Western Sandpipers showed latitudinal segregation with regard to sex and age. Females wintered farther south than males. A “U” shaped pattern was found with respect to age, with juveniles occurring at higher proportions at both the northern and southern ends of the range. Distribution of sexes might be affected by differences in bill length and a latitudinal trend in depth distribution of prey. For age class distribution, two different life-history tactics of juveniles might exist that are related to the higher cost of feather wear for juveniles compared to adults. Most juveniles complete three long-distance migrations on one set of flight feathers whereas adults complete two. Juveniles may winter either far north, thereby reducing feather wear induced by ultraviolet light, migration, or both, or far south and spend the summer on the nonbreeding area.

VARIATION IN THE WING MORPHOLOGY OF WESTERN SANDPIPERS (CALIDRIS MAURI) IN RELATION TO SEX, AGE CLASS, AND ANNUAL CYCLE

Abstract We analyzed differences in wing morphology between sexes and among age classes of Western Sandpipers (Calidris mauri) to assess possible relationships with differential migration, acrobatic aerial displays, and potential vulnerability to predation, using size-constrained component analysis. Data on wing morphology of all four sex and age classes were available from birds spending the nonbreeding season in northwestern Mexico, and data on females were available from throughout the annual cycle. During the nonbreeding season, females had longer and more pointed wings than males. Within each sex, adults had longer and more pointed wings than immature birds. Throughout the annual cycle, adult females tended to have longer and more pointed wings than immature females. The longer and more pointed wings of females are consistent with selection on flight efficiency for longer migration distances. The shorter and rounder wings of males are consistent with stronger selection for agility in flight during acrobatic aerial displays, but also with weaker selection for flight efficiency because of shorter migration distances. The rounder wings of immature birds are most consistent with stronger selection for take-off performance as an antipredator adaptation, at a cost of lower flight efficiency during long-distance migratory flight. Considering intraspecific sex and age class differences in wing morphology of species with differential migration complements interspecific comparisons assessing the relative importance of selective agents acting on this character. Variación en la Morfología del ala de Calidris mauri con Relación al Sexo, la Edad y el Ciclo Anual

Differential Migration in Western Sandpipers with Respect to Body Size and Wing Length

Abstract We examined differential migration in the Western Sandpiper (Calidris mauri) with respect to body size and wing chord allometry within sex and age categories. Culmen and wing chord data were collected as indices of structural body size at three sites that vary latitudinally: Ecuador, Panama, and Mexico. Within all sex and age categories, larger individuals (i.e., those with longer culmens and wing chords) and those with a disproportionately longer wing chord relative to the culmen migrated farther south. Our results, coupled with known molting schedules, indicate that i) immature sandpipers that grow disproportionately longer primary feathers on breeding grounds migrate farther during their first southward migration, and ii) adults that fly farther grow disproportionately longer primary feathers on the nonbreeding grounds. Although no single-factor hypothesis accounts for all aspects of age, sex, and size of Western Sandpiper distributions, costs associated with flight during migration play a significant role in determining differential nonbreeding latitudinal distributions.

SEX, AGE, AND BODY SIZE DISTRIBUTIONS OF WESTERN SANDPIPERS DURING THE NONBREEDING SEASON WITH RESPECT TO LOCAL HABITAT

Abstract We documented the local density and sex, age-class, and body size distributions of Western Sandpipers (Calidris mauri) among habitats at Bahía Santa María, northwestern Mexico, during the nonbreeding season. Three habitats were recognized: brackish flats, mangroves, and cattail marshes, which we ranked as richest to poorest in food resources and safest to most dangerous in predation danger. Western Sandpiper population structure differed among habitats. Bird densities were highest in brackish flats, the richest and safest habitat, and males and adults of both sexes were overrepresented. In cattail marshes, which appeared to be the poorest and most dangerous habitat, bird densities were lower, and the sex ratio and age ratios within each sex were more even. In mangroves, bird densities were similar to those in cattail marshes, but sex and age ratios were similar to those in brackish flats. Exposed culmen, an index of structural size, was not related to habitat use in either sex. Body mass of immature males was more variable than that of adults among habitats and immature males gained mass throughout the winter. Birds in brackish flats and mangroves were initially heavier, but tended to lose mass, whereas birds in cattail marshes were initially lighter, but tended to gain mass. Mass distributions thus converged in late winter. While the social and ecological causes and significance of differential sex and age-class distributions among habitats remain largely unquantified, evidence from this and previous studies suggests that nonbreeding population structure is a common phenomenon with important implications for migratory shorebirds.

APPARENT SURVIVAL RATES OF WESTERN SANDPIPER (CALIDRIS MAURI) WINTERING IN NORTHWEST BAJA CALIFORNIA, MEXICO

Abstract To estimate annual apparent local survival, we collected capture–resighting data on 256 individually marked male Western Sandpipers (Calidris mauri) wintering at Estero de Punta Banda, Mexico, between 1994–1997. A hierarchical modeling approach was used to address the effect of age class and year on survivorship rates. The best-fit model included a constant apparent survival probability (ϕ = 0.489; 95% CI = 0.410–0.569), but several models fit nearly as well, and averaging among the top five, to account for model uncertainty, suggested that adults had somewhat higher values than juveniles (ϕ = 0.490 ± 0.051 vs. 0.450 ± 0.067). Detection probability was substantially higher for adults than for juveniles (p = 0.741 vs. p = 0.537). Those apparent survival estimates are low compared with those from other studies of Western Sandpipers at breeding and other nonbreeding locations, and substantially lower than the true survivorship rates expected for small sandpipers in general. We interpret these results as indicating that this site is of below average quality for nonbreeding male Western Sandpipers.

PHENOLOGY AND LENGTH OF STAY OF TRANSIENT AND WINTERING WESTERN SANDPIPERS AT ESTERO PUNTA BANDA, MEXICO

Abstract The Western Sandpiper (Calidris mauri) is one of the most abundant shorebirds in northwestern Mexico; however, little is known about its winter ecology in this area. We studied residency patterns of male Western Sandpipers during the winters of 1995–1996 (1995) and 1996–1997 (1996) at Estero Punta Banda in Baja California, Mexico. We resighted 54 birds in 1995 and 56 birds in 1996. Birds arrived later in 1995 (median 1 December) than in 1996 (median 24 October). The median departure dates (6 March 1996 and 20 February 1997) did not differ between years. We observed two patterns of residency. Wintering birds in 1995 arrived on 17 November (median) and departed on 17 March (median), with length of stay of 120.0 ± 4.2 d; and in 1996 arrived on 12 October (median) and departed on 8 March (median), with length of stay of 146.9 ± 4.3 d. Transient birds in 1995 arrived on 7 December (median) and departed on 17 February (median), with stays of 33.7 ± 4.3 d; and in 1996 arrived on 23 October (median) and departed on 3 December (median), with stays of 33.7 ± 4.1 d. Residence time was independent of sighting effort. Both wintering birds and transients exhibited site fidelity between years. Both categories were independent with respect to age, trapping month, or year. The mid-season departures were not correlated with either age or the banding period. Older birds were more likely to depart earlier and switch their residency pattern from wintering to transient. While the ecological significance of variation in residency patterns remains unknown, evidence from this and previous studies suggest that such variation may be relatively common, with important implications for studies of shorebird populations and conservation strategies.

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.

Foraging Activity and Capture Rate of Large Nearctic Shorebirds Wintering at a Tropical Coastal Lagoon

Abstract. Winter foraging ecology of Marbled Godwit (Limosa fedoa), Long-billed Curlew (Numenius americanus), Whimbrel (Numenius phaeopus) and Willet (Tringa semipalmata) was studied in relation to conspecific density at Estero de Urías, a tropical coastal lagoon in northwestern Mexico. Specifically, the effect of ecological factors in relation to individual prey capture rates for Marbled Godwits and both Numenius spp. was examined. There was a consistent, inverse relationship between conspecific density and foraging activity, but the slope differed among species. The inverse relationship suggests that conspecific density was an important determinant of individual foraging behavior and habitat use across species. For Marbled Godwits, prey capture rate was positively correlated with conspecific density and water depth, but negatively correlated with tidal height, at least during spring tides. In contrast, for both Numenius spp., prey capture rate was significantly higher for territorial birds and tactile foragers, in the case of Curlews, but was independent of tidal conditions. When neap tides resulted in reduced foraging time budget, Marbled Godwits increased prey capture rate, but Numenius spp., did not. Differences in foraging mode, availability of preferred prey in relation to water depth, as well as handling time of prey, could drive these distinct patterns between godwits and both Numenius spp. These results support suggestions that the increased conspecific density during periods of neap tides might explain the reduced prey capture rate of individuals with less competitive ability, especially within large shorebirds showing a sparse foraging distribution.

Seabird Colonies of the Small Islands of Bahía Santa María-La Reforma, Sinaloa, México

Abstract. During the 2010–2011 and 2011–2012 breeding seasons (November to June), there were 21 colonies of 13 species, with at least 40,000 to 50,000 pairs of seabirds on six islands of Bahía Santa María-La Reforma in Mexico. Bahía Santa María maintains the largest breeding congregation of the west coast of Mexico for Laughing Gull (Leucophaeus atricilla; 22,000 pairs), Royal Tern (Thalasseus maximus; 11,000 pairs) and Black Skimmer (Rynchops niger; 1,500 pairs). In the past decade, most of the species that breed on El Rancho Island (coastal dune habitat) have increased their population size, apparently related to a combination of factors such as colonies recently established (and hence still expanding) and increased habitat availability. On the other hand, species breeding on islands with mangroves seem to be decreasing (in particular the Brown Pelican (Pelecanus occidentalis) population), but with no obvious reason to explain these low numbers of some species. Despite the small size of the islands used by breeding seabirds, this coastal system supports important seabird populations and its conservation should be a priority.

Do sex and habitat differences in antipredator behavior of Western Sandpipers Calidris mauri reflect cumulative or compensatory processes

Individuals manage their risk of predation in different ways in different situations. We studied the use of anti-predator behavior by Western Sandpipers (Calidris mauri) at Bahía Santa María, northwestern Mexico, foraging in three habitats that differed in presumed predation danger. Brackish flats are completely open, making them theoretically less dangerous for feeding sandpipers than mangroves and cattail marshes, which have closer visual horizons. Western Sandpipers are sexually dimorphic, with females about 15% longer-billed and 10% heavier than males. We previously showed that male and female sandpipers differed in their habitat choice and relative body mass in ways consistent with differential responses to predation danger (Fernández and Lank in Condor 108:547–557, 2006). Contrary to expectations, however, females were overrepresented in more dangerous habitats. Here, we examine differential usage across habitats and between the sexes of three anti-predator tactics—flock size, density within flocks, and vigilance rate—that may be used cumulatively to reinforce safety, or as trade-offs that compensate for levels of usage of each. We hypothesized, and found, that ordered differences occur among habitats, and that controlling for other factors, females were more cautious than males. For the most part, the use of these three tactics appeared to be cumulative, rather than compensatory. However, with respect to habitat use, birds appeared to compensate for the higher probability of mortality intrinsic to the use of higher-danger habitats by increasing the use of vigilance, foraging in tighter flocks, and maintaining lighter body weights (females only). Thus, both cumulative and compensatory processes operate among anti-predator tactics to determine the net level of safety and trade-off against other factors.