Lewis W. Oring

Sexual selection, arrival times, philopatry and site fidelity in the polyandrous spotted sandpiper

Summary1.Spotted sandpipers have a resource defense polyandrous mating system in which females compete for control of breeding territories and mates, and males provide most of the parental care.2.Data from a nine year study of a local breeding population showed that females preceded males to the breeding ground in spring. Within each sex, experienced local breeders arrived before new breeders. The mean arrival time of new females coincided with egg-laying of first clutches by experienced females, and at this time new females experienced their greatest success at entering the breeding population. Arrival of new males coincided with the onset of incubation by experienced males.3.Breeding males and females returned from year to year at similar rates. Birds returned at lower rates following breeding failure. This effect was strongest among birds after their first local breeding year, i.e. inexperienced birds, and weakest among experienced females.4.Significantly more locally hatched females than males returned and bred on the study site in their first year. Similarly, there were more locally hatched female than male chicks which returned but did not breed.5.These results are consistent with the hypothesis that the intensity of sexual selection drives the arrival timing of individuals on breeding grounds, and affects patterns of natal philopatry. The importance of territorial control in obtaining mates causes females to precede males, which is a complete reversal of the normal avian pattern. In both sexes, inexperienced birds arrive when intrasexual competition is lessened by the commitment of older birds to reproductive or parental activities. Greater female natal philopatry may reflect an increased importance of site familiarity for females as they attempt to become breeders in this resource defense polyandrous system.

A comparison of the breeding ecology of birds nesting in boxes and tree cavities

ABSTI~ICT.-We compared laying date, nesting success, clutch size, and productivity of four bird species that nest in boxes and tree cavities to examine whether data from nest boxes are comparable with data from tree cavities. Western Bluebirds (Sinlin nrexicnnn) gained the mast advantage from nesting in boxes. They initiated egg laying earlier, had higher nesting success, lower predation rates, and fledged marginally more young in boxes than in cavities but did not have laracr clutches or hatch more eggs. Plain Titmice (Pnnis iriorrmtrrs) nesting earlier in boxes. ~oise Wrens (~roglo&tes ~eiion) nesting in boxes laid larger clutches, hatched more eggs, and fledged mare young and had marginally higher nesting success and lower predation rates. Ash-throated Flycatchers (Myinrclirrs cincmscens) experienced no apparent benefits from ncsting in bows versus cavities. No significant relationships were found between clutch size and bottom area or volume of cavities for any of these species. These results suggest that researchers should use caution when extrapolating results from nestbox studies of reproductive success, predation rates, and productivity of cavity-nesting birds. Given the different responses of these four species to nesting in boxes, the effects of the addition of nest boxes on community structure also should be considered. Rrcrived 19 September 1996, nccepted 21 April 1997. NEST BOXES MAY BE USED AS A CONSERVATION

Testosterone-induced inhibition of incubation in the spotted sandpiper (Actitis mecularia)

The spotted sandpiper (Actitis macularia) is characterized by intense female intrasexual competition and predominantly male parental care. Females often are polyandrous. Incubating males were implanted with testosterone (T)-filled Silastic tubes. Plasma T levels were significantly elevated by T implants while prolactin (Prl) remained unchanged. Birds implanted with T deserted clutches (30%) or exhibited reduced incubation constancy (50%) while controls incubated normally. The T implants appeared to heighten sexual receptivity and hence reduce incubation constancy. Variation in male incubation behavior may have been further influenced by the degree to which males were exposed to courting females.

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.

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.

Mate and Nutrient Limitation of Egg‐laying in a Polyandrous Shorebird

Spotted Sandpipers are unusual among animals in that a females annual reproductive effort is regulated primarily by her ability to obtain mates. Males provide most of the parental care in this species. We recorded data on the abundance of food, and on the timing, number, and size of eggs laid by marked females over a 9-yr period on an island in central Minnesota, USA. The onset of egg-laying varied from year to year in association with the abundance of flying insects early in the season. After laying began, there was no relationship between food abundance and the number or size of eggs laid, the time taken to produce a clutch, or the intervals between clutches. The rate of egg- laying per female was affected, however, by rates of nest predation and by the operational sex ratio, which became increasingly female-biased over the season. Females ceased laying eggs in early July, even in years when food and mates were still available. We conclude that nutrient availability acts as a proximate factor in determining the onset of breeding, and that it may have been an ultimate factor causing cessation of egg-laying. Within the season, however, the reproductive rate of individual females is primarily determined by competitive interactions with other females for males. Thus both intra- and intersexual selection have important effects on fitness in this species.

Are Nest Exclosures an Effective Tool in Plover Conservation

Abstract High levels of egg predation by numerous vertebrates has been implicated as a limiting factor in many avian populations. In response, ornithologists often attempt to control potential egg predators through lethal methods or large scale fencing of breeding habitat. More recently, individual nest exclosures have been employed to deter egg predators. During 1999 and 2000, we experimentally tested nest exclosure effectiveness at enhancing Killdeer (Charadrius vociferus) nest success in the Great Basin, USA. We demonstrated that nests with exclosures had significantly lower daily mortality rates compared to nests without exclosures. Although nest exclosures increased Killdeer nest success in this study, it is necessary to identify predators impacting nest success prior to the exclosure design. Nest exclosures are unlikely to be equally successful among sites, and variation in exclosure design may be required across both the range of a species and/or breeding seasons at a single site. The exclosure design we used did not deter small mammals and birds, or reptiles; and we observed small mustelids killing incubating birds. However, these exclosures were effective at increasing hatching success by deterring larger predators (ravens, gulls, canids). Any benefit derived from the nest exclosure used may potentially be offset by increased predation on incubating adults. Nest exclosures are not a means of preserving biological diversity over geologic time, but rather a potential temporary site-specific tool that increases nest success while long-term conservation plans are developed.

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.

Life-history and viability analysis of the endangered Hawaiian stilt

The Hawaiian stilt Himantopus mexicanus knudseni is an endangered, endemic subspecies of black-necked stilt. We present life-history data required to perform population viability analysis (PVA), and the results of a series of PVAs under two scenarios, treating (a) the subspecies as a single population, and (b) six subpopulations as a metapopulation. We performed sensitivity analyses on model parameters and used results to address various management options. Both basic models predicted that stilts would increase to fill available habitat with no chance of a significant decline. Catastrophe, maximum age, and density-dependent reproduction had little effect on population projections. Rapid declines in the probability of stilt populations persisting occurred when clutch failure rate or first-year mortality rate increased above 70%, or when adult mortality rate increased above 30%. Model predictions of mean population size at 200 years tracked changes in carrying capacity. If current conditions change such that rates of clutch failure or stilt mortality increase, population declines and eventual extinction becomes more likely. Managers, therefore, should maintain predator control, limit water level fluctuations, and maintain current habitat area. Downlisting is not warranted because wetland management and predator control are necessary for Hawaiian stilts to persist.