Mark A. Colwell

Nonrandom shorebird distribution and fine-scale variation in prey abundance

Spatial variation in the abundance of nonbreeding shorebirds can be predicted to varying degrees by the density of their prey species; strongest relationships obtain from studies encompassing large spatial scales (e.g., entire estuaries). We examined variation in shorebird distribution and abundance within microhabitats of the Mad River estuary, California, with the following null hypotheses: (1) shorebird spatial distribution was random, and (2) no relationship existed between bird abundance and invertebrate densities. Shorebirds exhibited nonrandom spatial distributions; species were highly clumped within the study area. Most foraging calidridine sandpipers (Calidris minutilla, C. mauri, and C. bairdii) aggregated in sandy areas within 1 m of the tide edge, where they foraged by probing for a burrow-dwelling amphipod, Corophium spp. By contrast, Semipalmated Plovers (Charadrius semipalmatus) and especially Ruddy Turnstones (Arenaria interpres) foraged by pecking in drier, coarse-grained substrates gmater than 1 m from the tide edge. Corophium densities in sand exceeded those in cobble; Corophium densities explained significant variation (r2 = 0.36, 0.31 and 0.22) in the abundance of Least Sandpipers, Western Sandpipers and all shorebirds, respectively. These findings emphasize the importance of understanding variation in processes across spatial scales.

Population identification of western hemisphere shorebirds throughout the annual cycle

Identification of relationships among geographically distinct populations of migratory species can provide an understanding of breeding and natal philopatry, migration pathways, and population mixing during winter. We used random amplified polymorphic DNA (RAPD) analyses to search for markers specific to difficult‐to‐differentiate shorebird species (e.g. long‐billed dowitcher Limnodromus scolopaceus and short‐billed dowitcher L. griseus) as well as geographically distinct breeding populations of Hudsonian godwits Limosa haemastica, red‐necked phalaropes Phalaropus lobatus, semipalmated plovers Charadrius semipalmatus, dunlin Calidris alpina, pectoral sandpipers C. melanotos, semipalmated sandpipers C. pusilla and western sandpipers C. mauri. Markers clearly differentiated all shorebird species. Estimates of population differentiation varied greatly among species (FST= 0.095–0.685) and correlated with interspecific variation in philopatry and geographical separation of breeding populations. We assigned individuals to putative breeding locales with greater certainty in well‐differentiated species than in poorly differentiated species. Our findings indicate specific phylogeographical structure varies among species, which has strong implications for conservation of habitats within migratory corridors. We suggest that RAPDs are useful in identifying geographical populations of migratory species and that molecular markers should be considered for tracking migratory birds throughout the annual cycle.

Extra-pair mating in the spotted sandpiper: a female mate acquisition tactic

Abstract Extra-pair mating behaviour was studied in the spotted sandpiper, Actitis macularia , a sex-role reversed, polyandrous shorebird, in which females are larger than and socially dominant to males. On a population basis, intra-pair mating events (0.39 events/h) exceeded extra-pair events (0.05 events/h) by about a factor of 10; 15% ( N =26) of extra-pair events were copulations. Extra-pair events were not synchronized with the fertile period of females. Extra-pair mating events were more likely to end in copulations when males were either unpaired or paired to females that had not started laying a clutch compared with males that were incubating or paired to a laying female. Overall, males moved further than females during extra-pair events, but distance moved was associated with male reproductive status. Within each sex, extra-pair events were not randomly distributed among individuals in most years, indicating that few individuals were responsible for most extra-pair events. For both sexes, extra-pair behaviour was correlated with number of breeding attempts annually and was unrelated to individual morphology and breeding experience. Extra-pair events increased the likelihood that females would subsequently pair with the males concerned. It is proposed that extra-pair mating events in spotted sandpipers in part serve the novel function of establishment and maintenance of social relationships that may lead to subsequent pair formation and mate acquisition, a pattern correlated with the species sex-role reversed mating system.

Sexual selection and spring arrival times of red-necked and Wilson's phalaropes

SummarySeasonal changes in sex ratios during a 4-year study of red-necked phalaropes and a 3-year study of Wilsons phalaropes showed that females preceded males to breeding and/or courtship areas. The degree to which females preceded males may have been constrained by harsh weather, as the arrival of red-necked phalaropes was synchronous in 1983, when spring was unusually late. Neither sex defended territories; females competed vigorously for direct access to males. These findings show that selection for sexually asynchronous arrival need not act through territoriality. We interpret early female arrival as an adaptation for obtaining mates and coclude that sexual selection may be an important determinant of arrival times in mate defense social systems.

Sex ratios and intrasexual competition for mates in a sex-role reversed shorebird, Wilson's phalarope (Phalaropus tricolor)

SummaryOperational sex ratio (OSR) was proposed by Emlen and Oring (1977) as an empirical measure of the intensity of sexual selection. Few studies, however, have examined the link between OSR and levels of intrasexual competition, which may influence selection. We studied the seasonal relationship between OSR and female-female competition for mates in Wilsons phalarope (Phalaropus tricolor), a sex-role reversed, non-territorial shorebird. Positive correlations between four measures of OSR (Scan Ratio, Day Ratio, Focal Ratio, and Chase Ratio) indicated seasonal changes in the availability of mates for females. Changes in mate availability resulted from the interaction between paternal care and female emancipation, asynchronous spring arrival schedules of the sexes (Reynolds et al. 1986), and the effect of clutch failure on renesting opportunities. Measures of intrasexual competition (courtship chases, percent males defended, rate and intensity of mate defense, and female-male proximity) varied significantly within and among years. Univariate and multivariate correlations indicated general agreement between measures of OSR and estimates of intrasexual competition. Our results suggest that OSR may provide a useful estimate of the opportunity for sexual selection, especially in species with matedefense mating systems.

Waterbird communities in rice fields subjected to different post-harvest treatments

-In Californias Sacramento Valley, the potential value of rice fields as habitat for waterbirds may vary with harvest method, post-harvest treatment of rice straw (chopped, burned, plowed), and extent of flooding. Recent changes in rice harvesting methods (i.e., use of stripper-headers) and a legislative mandate to decrease burning of rice straw after harvest may alter habitat availability and use. Thus, we investigated species richness and community composition of nonbreeding waterbirds during October-March 1993-94 and 1994-95 in rice fields of the northern Sacramento Valley. Most (85-91% of land area) rice was conventionally harvested (i.e., cutter bar), and the remainder was stripped. Rice straw was left untreated in more than half of fields (52% in 1994 and 54% in 1995), especially in stripped fields (56-70%). In fields where farmers treated straw, the most common management methods were plowing (15-21%), burning (19-24%), and chopping (3-5%). Fields became increasingly wet from October through March as seasonal precipitation accumulated and farmers flooded fields to facilitate straw decomposition and provide habitat for ducks. Species richness of waterbirds was greater (P 0.23). Species richness in stripped fields probably was low because foraging opportunities were limited by tall dense straw, decreased grain density, and infrequent flooding. We recommend that land managers wishing to provide habitat for a diverse waterbird community harvest rice using conventional methods and flood fields shallowly. Received 3 April 1998, accepted 21 July 1998.

Hormonal changes associated with natural and manipulated incubation in the sex-role reversed Wilson's phalarope

Wilsons phalarope (Phalaropus tricolor) is characterized by intense female intrasexual competition and exclusive male parental care. Females occasionally are polyandrous and no territories are defended. Prolactin (Prl) and testosterone (T) were analyzed from plasma samples obtained from individuals of different reproductive stages. Males tended to have higher plasma Prl levels than females throughout the breeding season. Prolactin levels of males declined posthatch, reaching baseline levels when chicks were approximately 9 days old--the time when chicks approach thermal independence and brooding is minimal. Testosterone levels dropped at the onset of incubation and remained low through the brooding period. Eggs were manipulated such that some males incubated clutches for shortened periods, others for extended periods. Regardless of the length of incubation, Prl levels were determined primarily by environmental events, i.e., incubation, hatching, and brooding.

Plasma steroid hormone levels in free-living Wilson's phalaropes, Phalaropus tricolor

Blood samples collected from free-living Wilsons phalaropes during the reproductive season were analyzed for testosterone, dihydrotestosterone, estradiol-17 beta, and progesterone. Levels of testosterone were seven times greater in nonincubating males than in females. During incubation males underwent a reduction in testosterone and dihydrotestosterone to levels similar to those of females. Estradiol-17 beta values were higher in females than in incubating males and comparable to values reported for other avian species. Progesterone was significantly greater in females than in incubating or nonincubating males. Maximum levels of progesterone were detected in laying females. Progesterone levels increased in males during incubation while testosterone and dihydrotestosterone levels decreased. These results indicate that the greater intensity of competition for mates among females and exclusive male parental care characteristic of this species is not based upon a reversal of the typical avian levels of androgens and estrogens in males and females.

Age-Related Survival and Behavior of Snowy Plover Chicks

Abstract Abstract. Precocial young often experience high mortality prior to achieving flight, especially in the first days after hatching. We quantified relationships between chick age, survival, behavior, and response to natural and anthropogenic danger for the threatened Western Snowy Plover (Charadrius alexandrinus nivosus) in coastal northern California, USA. Plover chicks were most likely to die in the first three days of their 28-day prefledging period, especially on sandy ocean beaches compared to coarser substrates of river habitats. Chick survival in both habitats increased across the ∼120-day chick-rearing period. Improved survival of older chicks coincided with an age-related reduction in brooding and increased distance from the tending parent, which was not related to season. Lower survival of younger chicks was correlated with a tendency to lie motionless when approached by humans; nearly all older chicks responded to human approach by running to evade danger. Chicks of all ages were more likely to lie motionless when potential avian predators flew nearby. Age-dependent survival of Snowy Plover chicks paralleled a trend of increasing development, thermal independence from adults, and capability of evading predators.

Staging of Pacific flyway brant in relation to eelgrass abundance and site isolation, with special consideration of Humboldt Bay, California

Spring distributions of black brant (Branta bernicla nigricans) are closely related to that of their primary food plant, eelgrass (Zostera marina).Using historical (1931–2001) brant data from Humboldt Bay, California, we show that proportional use of two main feeding areas in the bay strongly reflects food abundance.We used multiple regression to investigate whether the spatial relationship between brant and eelgrass holds at the flyway level.We related peak brant numbers at 11 staging areas in the Pacific flyway to the sites’ Z. marina abundance and isolation from other important staging areas.We explained 90% of the variation in peak brant numbers across sites with these variables; isolated bays with high eelgrass abundance supported the most brant.Our results emphasize the importance of protecting large eelgrass habitats along the Pacific Coast, particularly in bays that are geographically isolated from other large staging areas. # 2003 Elsevier Ltd.All rights reserved.