California Gulls Research Articles

FACTORS DETERMINING A CLUTCH SIZE REDUCTION IN CALIFORNIA GULLS (LARUS CALIFORNICUS): A MULTI-HYPOTHESIS APPROACH.

In the thirty-five years since David Lack first highlighted the importance of clutch size, a large number of hypotheses have been proposed relating clutch size variation to various environmental and demographic factors. Despite a great deal of both empirical and theoretical work on clutch size, there has been very little effort to test many of the competing hypotheses in explaining a clutch size difference between two populations of the same species. I have taken the latter approach in an effort to explain a clutch size reduction in the California Gull (Larus californicus) population at Mono Lake, California. I compared the breeding biologies of the gulls at Mono Lake and at Great Salt Lake, Utah, collecting data for three breeding seasons at Mono Lake and one breeding season at Great Salt Lake. These data included measurements of the conditions of 60 adults, growth and mortality measurements for approximately 900 chicks, 4450 nest-hours of parental care observations, and the results of egg-removal experiments on 40 females. I tested seven hypotheses to explain the clutch size reduction: age structure, egg predation, bet-hedging, effort reallocation, most productive brood size, parental mortality, and pre-egg food limitation. Each of these hypotheses is described in detail in the introduction. The pre-egg food limitation hypothesis is best able to explain the clutch size reduction at Mono Lake, although the egg-removal experiments show that the resource limitation is relative and not absolute. Clutch size variation at each site need not be viewed as the result of fine-scaled evolutionary adjustment, although the general clutch size decision machinery is presumably molded by selection. Future research must focus on the details of this clutch size decision machinery and its application to the concept of reproductive effort.

Nest Site Selection and Clutch Predation in the Snowy Plover

Nest Site Selection and Clutch Predation in the Snowy Plover

Increased Reproductive Success with Age in the California Gull: Due to Increased Effort or Improvement of Skill?

Pugesek (1981) recently claimed to demonstrate an increase in reproductive effort with age in the California gull Larus californicus. On the contrary, neither theory nor his own data support Pugesek's claim. Older parents invest no more time in (1) foraging for the young and (2) guarding the nest than do younger parents, but the former are (a) more effective at providing food for the young and (b) more efficient at alternating and coordinating guarding and feeding duties. The increase in territorial defense with age is likely due to older parents having more mature young to defend. Development of parental skills as a result of experience best explains the observed increase in reproductive success with age.

Age-Specific Reproductive Tactics in the California Gull

Age-Specific Reproductive Tactics in the California Gull

Experimental Evidence That Female-Female Pairs in Gulls Result from a Shortage of Breeding Males

The Condor 8 6:472~ 76 © The Cooper Ornithological Society 1984 EXPERIMENTAL EVIDENCE THAT FEMALE-FEMALE PAIRS IN GULLS RESULT FROM A SHORTAGE OF BREEDING MALES MICHAEL R. CONOVER AND GEORGE L. HUNT, JR. ABSTRACT.-We tested the hypothesis that female-female pairings in Ring- billed (Larus delawarensis) and California gulls (L. californicus) result from a shortage of males in the breeding population. This hypothesis was tested by removing males from one Ring-billed and four California gull colonies early in the breeding season. The frequency of 4-6 egg clutches, which we used as an index offemale-female pairings, was significantly higher in these colonies than in nearby control colonies, thus supporting our hypothesis. Our results indicate that female- female· pairings allow females the chance to breed when they are unable to obtain a male partner. Although terns and gulls are usually monog- at some small colonies of Ring-billed and Cal- amous, female-female pairings have recently ifornia gulls. By doing so, we hoped to answer been discovered in Caspian Terns (Sterna cas- the question of why two females would pair pia; Conover 1983), Western Gulls (Larus oc- together rather than with male mates. Previous cidentalis; Hunt and Hunt 1977), Ring-billed authors have suggested that mating systems Gulls (L. delawarensis; Ryder and Somppi are either unrelated to sex ratios (Verner 1964; 1979, Conover et al. 1979a), California Gulls Witten berger 197 6, 1979) or influence sex ra- (L. californicus; Conover et al. 1979a) and in tios (Hamilton 1967, Wilson and Colwell the Great Lakes population of Herring Gulls 1981 ). In the case of these gulls, we were testing (L. argentatus; Fitch 1980). Both females in a the hypothesis that sex ratios determine the same-sex pair defend a single territory and lay mating system used. eggs in the same nest, often resulting in a su- pernormal clutch of 4-6 eggs, double the nor- METHODS mal number of 2-3. Both females incubate and If a shortage of males in a breeding population leads to female-female pairings, then removing share other parental responsibilities. Much interest has centered on why one fe- males from the colony should lead to an in- male would pair with another, because the re- crease in the frequency of such pairings in col- productive success of these females is often onies where female-female pairs already occur. much lower than that of heterosexually-paired In 1981 we removed males from one Ring- females (Hunt and Hunt 1977). One hypoth- billed Gull colony and four California Gull esis (Wingfield et al. 1980a, b, 1982) is that colonies; we compared the frequency of fe- the behavior results from a pairing preference male-female pairings in these colonies to those of some females for the wrong sex, perhaps in nearby (control) colonies where no males owing to behavioral or endocrine masculini- were removed. All colonies were located in zation of these females. Wingfield et al. ( 1980a, Washington and Oregon and are described b, 1982) and Hunt et al. ( 1984), however, found elsewhere (Conover et al. l 979b). We captured gulls from the experimental no significant hormonal or behavioral differ- ences between homosexually- and heterosex- colonies by first baiting them on islands lo- cated within 100-200 m of the colony and in ually-paired females in Western Gulls. A second hypothesis proposes that female nearby garbage dumps, and then firing a rocket gulls pair together when they are unable to net over them. All trapping was done early in obtain male mates, owing to a shortage of the 1981 breeding season (March and April), breeding males (Hunt and Hunt 1977, Con- when gulls were starting territorial defense and over et al. 1979a, Fry and Toone 1981, Pierotti courtship but before they began copulating. We captured gulls at six sites close to the 1981). This hypothesis was supported by the observation that females outnumbered males four experimental colonies but not inside them. three-to-two in a Western Gull colony where Prior experience had taught us that if in-colony female-female pairings occur (Hunt et al. 1980). capture were attempted before the incubation We tested this hypothesis by experimentally period, many of the gulls would desert the col- manipulating the sex ratio of breeding adults ony. Birds from the Potholes Reservoir colony

Oxygen Consumption, Evaporative Water Loss, and Temperature Regulation of California Gull Chicks (Larus californicus) in a Desert Rookery

California gulls (Larus californicus) breeding on barren islets in Mono Lake, California, utilize behavioral responses rather than special physiological adaptations to cope with high environmental temperatures. Young California gulls resemble the chicks of other gull species in oxygen consumption, evaporative water loss rates, thermal conductance, and body temperature. Successful reproduction is dependent on adequate shade for chicks, which are otherwise vulnerable to hyperthermia from high temperatures and intense solar radiation. At present, most shade is provided by parent gulls. Events which decrease nest attentiveness by adults expose the chicks to potentially devastating heat stress.

Frequency, Spatial Distribution and Nest Attendants of Supernormal Clutches in Ring-Billed and California Gulls

This study examined the occurrence and cause of supernormal clutches (SNC; 4-6 eggs) in Ring-billed (Larus delawarensis) and California (L. californicus) gulls, species that normally lay twoto three-egg clutches. In Washington and Oregon, clutches of five to six eggs constituted 0.8% of the clutches in Ring-billed Gulls (n = 20,353) and 0.1% in California Gulls (n = 6,117) during the mid-incubation period. The frequency of supernormal clutches in Ring-billed Gulls varied significantly both among colonies and years on a region-wide basis but this was not true in California Gulls. The frequency of four-egg clutches was correlated with that of fiveto six-egg clutches within each species, but no correlation existed between the two species. Multi-female were responsible for 30% of the examined four-egg clutches (n = 20) in Ring-billed Gulls and 27% of them (n = 11) in California Gulls. All examined clutches of five or six eggs in Ring-billed Gulls (n = 11) and California Gulls (n = 1) were incubated by associations. In Ring-billed Gulls, 79% of the detected were female-female pairs, 16% were polygynous groups, and one was a group of three females but no males. In California Gulls, for these clutches, only female-female pairs were detected. Gulls normally lay clutches of one to three eggs but clutches of four to seven eggs are occasionally found. These unusually large or supernormal (Bonner 1964) clutches have attracted the attention of ornithologists for many years (Call 1891, Jones 1906, Willett 1919). Not until 1977, however, was it discovered that supernormal clutches (SNCs) are produced by female-female pairs in Western Gulls (Larus occidentalis; Hunt and Hunt 1977). Such pairs have since been found also attending SNCs in California Gulls (L. californicus; Conover et al. 1979a), Ring-billed Gulls (L. delawarensis; Conover et al. 1979a, Ryder and Somppi 1979) and Herring Gulls (L. argentatus; Fitch 1980, Shugart 1980). Some SNCs also result from polygyny (Conover et al. 1979a, Lagrenade and Mousseau 1983). Hereafter, I will refer collectively to these where two or more females lay eggs in the same nest and share parental responsibilities (polygyny, female-female pairings, or groups) as multi-female associations (MFAs). It is unclear why female-female pairs or polygynous occur in gulls that are normally monogamous, but one hypothesis is that they result from a shortage of breeding males. Support for this hypothesis comes from the finding that females outnumbered males at a Western Gull colony where there was a high frequency of female-female pairs (Hunt et al. 1980). Conover and Hunt (1984) also showed that SNC frequencies increased in Ring-billed and California gull colonies after the breeding adult sex-ratios were skewed by removing breeding males. These findings, how ver, do not explain why a shortage of b eeding males should exist in some gull populations. One hypothesis is that DDT can feminize male gull embryos so that these individuals do not breed as adults (Fry and Toone 1981). Thi hypothesis may explain the increase in SNC frequencies since the 1940s in Wester Gulls breeding off the California coast (Hunt and Hunt 1977), in Herring Gulls breeding in the Great Lakes (Conover 1984), and in the United States population of Caspian Te ns (Sterna caspia; Conover 1983). In most larids, however, SNC frequencies have not increased significantly in recent years (Conover 1984). Too little is yet known about proximate factors affecting SNC frequencies, or the variability of SNC frequencies, to understand the significance of these large increases in SNC frequencies. In this study, I examined variability in SNC frequencies by testing the hypotheses that within the same population: 1) SNC frequencies change during the incubation period and 2) their frequency varies both among years and colonies. A major difficulty with studying female-female pairs or polygynous in gulls is identifying them. Sexual dimorphism is so slight in these birds that many individuals can be sexed only by head or bill measurements or by laparotomy. For this reason, nests containing SNCs are often used to identify female-

Genetic and morphological similarity of two California gull populations with different life history traits

California Gulls nesting at Mono Lake (ML) have a modal clutch size of two eggs, while at Great Salt Lake (GSL) the mode is three. We used starch gel electrophoresis to determine whether the populations are genically different. Average individual heterozygosity at ML averages 2.67% and at GSL 2.95%, both slightly below the average for other birds. The average number of alleles per locus is 1.2 for both sites. Variation between sites is insignificant, with a genetic distance of 0.001 and an F ST of 0.0044. Therefore, the clutch size differences are without genic correlates. In fact, at the loci studied, the two samples are indistinguishable from two random samples drawn from a panmictic unit. Five morphological characteristics also exhibited little between-site difference, in agreement with the electrophoretic analysis. A marked historical reduction (bottleneck) in the Mono Lake population seems not to have affected levels of genetic variation. We conclude that the clutch size differences are either recently evolved, and determined by loci not surveyed by us, or they are environmentally induced. At the population level, these gulls are similar to passerines in terms of heterozygosity and among population differentiation.

The relationship between parental age and reproductive effort in the California Gull (Larus californicus)

Reproductive effort was compared among California gulls ranging in age from 3 to 18 years old. Results indicated that reproductive effort increased with parental age. Older parents (11–18 years old) invested more in foraging effort and in defense of offspring than did younger parents (<10 years old), and did so over a greater period of time. Increases with age in the efficiency of parenting could not be demonstrated. Increased levels of reproductive effort, therefore, appeared to be the main factor responsible for age-related increases in breeding success. The higher levels of reproductive effort of old parents were associated with higher mortality. Thus, data support the prediction that a cost-benefit system of risk of mortality vs reproductive gain operates in this population. Higher levels of reproductive effort can increase an individual's reproductive success; however, it also increases the likelihood that it will die. Therefore, for the young parent, high risks of mortality involved in current production would be too costly (in terms of loss of future reproductive success) to justify the benefit of high current reproductive output. Parents achieve the highest likelihood of maximizing life-time reproductive output by increasing reproductive effort with age as the cost of mortality diminishes. Data testing the relationship between parental age and reproductive effort demonstrate the existence of age-related variation in reproductive investment. They also demonstrate that the concept of reproductive value proposed by Fisher (1930) has validity as a framework which supports much of our theory on life history evolution.

A Multivariate Study of the Relationship of Parental Age to Reproductive Success in California Gulls

Breeding biology of a California gull population was investigated on Bamforth Lake, Albany County, Wyoming. Data on hatching success, 1st and 2nd wk survival, and fledging success were collected in 1979 and 1980 for 295 breeding pairs. Breeding success increased with parental age as a result of larger clutches and reduced offspring mortality. Linear regressions indicated that nest sites with shrub shelter, central location in the colony, and high nest densities correlated positively with many measures of breeding success. These types of nest sites were most commonly obtained by old parents. Multivariate analyses, however, indicated that while parental age had a major influence on breeding success, all other variables that correlated with breeding success did so through covariation with parental age. Thus, results did not support a hypothesis that more experienced parents have greater breeding success because they obtain better nest sites.

Spacing out at Mono Lake: Breeding Success, Nest Density, and Predation in the Snowy Plover

Abstract Snowy Plovers (Charadrius alexandrinus) nesting on the exposed lake bed surrounding Mono Lake, California lose up to 40% of their clutches. Most are destroyed by predators, predominantly California Gulls (Larus californicus). In 1978 Snowy Plover reproduction was estimated at 0.49-0.70 fledged young per female. Population stability was estimated to require 0.80 fledged young per female. A series of experiments with artificial clutches placed at different densities in the nesting area demonstrates that the predators can have an effect on the plovers' nesting success that is dependent upon their nest density. The maintenance of low nesting density is an important antipredator adaptation. We consider predation on clutches and broods to be the major limiting factor on the Snowy Plover population at Mono Lake.

Possible Sexual Differences in Foraging Patterns in California Gulls and Their Implications for Studies of Feeding Ecology

Possible Sexual Differences in Foraging Patterns in California Gulls and Their Implications for Studies of Feeding Ecology

Increased Reproductive Effort with Age in the California Gull (Larus californicus)

Comparisons of reproductive behaviors of three age classes of California gulls demonstrate that reproductive effort increases with age in this seabird. These findings contradict the assumption that increased reproductive success with age results from increased experience and social status and demonstrate that selection for increased reproductive effort can occur in long-lived species.

Further Studies on Diphyllobothrium cordiceps in Yellowstone Lake

The California gull Larus californicus, has been reported as a host for Diphyllobothrium cordiceps from cutthroat trout, Salmo clarki, (Post, 1971) . A comparison of development and of morphological forms of D. cordiceps in young gulls and hamsters which had previously been proven to be susceptible to infection was considered desirable. Previous experimental exposures of cutthroat trout and grayling, Thymallus arcticus, to plerocercoids naturally infected from cutthroat trout had given mixed results with transfer of plerocercoids from cutthroat to cutthroat but not from cutthroat to grayling (Kingston et al., 1980) though grayling have been cited as a natural host for the tapeworm (Post, 1971). Work in 1980 was directed towards the elucidation of these aspects of the life cycle of D. cordiceps.

Competition and Predation: Herring Gulls versus Laughing Gulls

Gulls of the genus Larus generally nest colonially, often in mixed-species assemblages. For example, Herring Gulls (L. argentatus) in Europe nest with Lesser Blackbacked Gulls (L. fuscus; Brown 1967, MacRoberts and MacRoberts 1972) and Black-headed Gulls (L. ridibundus; Greenhalgh 1974); and in North America Ringbilled Gulls (L. delawarensis) nest with California Gulls (L. californicus; Vermeer 1970) and Herring Gulls (Southern 1970). It is often difficult to determine whether such assemblages are actually mixed, or if monospecific colonies simply exist side by side. When assemblages are mixed it is often impossible to ascertain whether they are so by choice or because of a lack of suitable nest sites. The rate of competition and predation by one or both species has not been investigated. In this paper I examine competitive interactions and predation between Laughing Gulls (L. atricilla) and Herring Gulls on Clam Island, New Jersey, from 1976 through 1978. Laughing Gulls on the east coast of the United States traditionally nest in Spartina salt marshes (Bent 1921), although at the limits of their range they frequently nest on dry land (Nisbet 1971, Dinsmore and Schreiber 1974). Thus, when Herring Gulls expanded southward in the early 1900's into Maine and Massachusetts, they came into direct conflict with Laughing Gulls (Nisbet 1971, Drury and Kadlec 1974). As early as 1943, Herring Gulls nested with Laughing Gulls in a sand dune colony at Muskeget Island, Massachusetts (Noble and Wurm 1943). Despite initial differences in habitat, the Laughing Gulls disappeared (Nisbet, pers. comm.). Increases in Herring Gulls in the northeast also have been associated with decreases in Common Terns (Sterna hirundo; Nisbet 1973). In the 1950's and 1960's Herring Gulls again expanded their breeding range, nesting as far south as North Carolina with Laughing Gulls (Hailman 1963, Parnell and Soots 1975). They nested on the upper parts of domes on man-made islands while Laughing Gulls occupied low swales between the domes. Where Herring Gulls were absent, Laughing Gulls nested farther up the domes (Parnell and Soots 1975). When Herring Gulls began breeding in more southern areas, they chose to nest in salt marshes, the customary nesting areas of the large and stable Laughing Gull colonies (Burger 1977). Although differences in nest site selection were noted initially, these differences decreased as Herring Gull colonies expanded. Overlap in colony and nest site characteristics is obvious in several New Jersey colonies (Burger and Shisler 1978). Since the first report of Herring Gulls on Clam Island (Rogers 1965), the colony has increased from 50 to 1,200 pairs whereas the Laughing Gull colony of 4,000 to 6,000 pairs remained about the same size through 1976. My primary objectives in this paper are to examine how these species interact temporally and behaviorally, and to determine the result of these interactions in terms of nesting space allocation and reproductive suc-

Isolation of Salmonella spp. from Dead Gulls (Larus californicus and Larus delawarensis) from an Idaho Irrigation Reservoir

An epizootic of unknown etiology resulting in the death of about 500 sea gulls (Larus californicus and Larus delawarensis) in 24 hr occurred on an irrigation reservoir in southwestern Idaho in April 1975. Salmonella spp. were isolated from necropsy specimens from 2 of 6 gulls examined. No Salmonella spp. were isolated from fecal specimens or dead gulls collected at a nesting site.

Notes on Behavior of California Gulls and South Polar Skuas off the Washington Coast

Notes on Behavior of California Gulls and South Polar Skuas off the Washington Coast

California Gulls Attack Waterfowl Broods in Alberta

California Gulls Attack Waterfowl Broods in Alberta

Two new species of Maritrema (Trematoda: Microphallidae) from the Pacific Coast of North America.

Maritrema pacifica n. sp. has an incomplete posterior ring of vitellaria, a spiny cirrus, a trilobed ovary, and a well-differentiated metraterm. It differs from M. patulus Coil, 1955 in the larger body, longer esophagus, round rather than slit-like genital pore, and larger eggs. It differs from M. interrupta Oshmarin, 1970, M. eroliae Yamaguti, 1939, and M. kitanensis Shibue, 1953 in the smaller cirrus sac containing sparse prostate cells. M. pacifica has the following definitive hosts: Larus californicus Lawrence, L. occidentalis Audubon, Turdus migratorius L. from Oregon, and Crocethia alba (Pallas) from British Columbia. The metacercariae, from Orchestoidea corniculata from California, became ovigerous in Locke's solution at 39 °C.Maritrema paracadiae n. sp. (synonym M. acadiae of Deblock and Rausch (1972)) differed from the type specimens of M. acadiae (Swales, 1933) in the proportionately larger oral and ventral suckers; the larger, inverted J-shaped cirrus sac containing prominent prostate cells; and the distribution of the uterus within, instead of overlapping, the posterior vitellarian ring. M. paracadiae was found in the intestine of Bucephala islandica (Gmelin) from Washington.

Diplostomosis in North Park, Colorado.

The life cycle of Diplostomum spathaceum (Rudolfi 1819), as it exists in North Park, Colorado, is elucidated. California gulls (Larus californicus), common mergansers (Mergus merganser) and Forster's terns (Sterna forsteri) act as definitive hosts. Two species of water snails, Physa gyrina and Lymneae elodes, act as first intermediate hosts with seven species of fish as second intermediate hosts: brassy minnows (Hybognathus hankinsoni), brook trout (Salvelinus fontinalis), brown trout (Salmo trutta), mackinaw (Salvelinus namaycush), rainbow trout (Salmo gairdneri), western long nose suckers (Catostomus catostomus) and western white suckers (C. commersoni). Infection of fish by larvae of D. spathaceum results in cataracts of the lens and impaired vision.