Identification: a medium-sized (10 inch) sandpiper-like bird with mottled buff and black plumage; adult males have a black face and underparts bordered with white.

Social Organization

Oystercatchers and Golden Plovers commonly associate in flocks. The mating system typically involves monogamous pair-bonding, although many alternative arrangements also occur (see below). Nonbreeding Oystercatchers tend to aggregate in groups known as CLUBS.

Description

Behavioral Expression: Oystercatchers sometimes participate in same-sex courtship and copulation. This behavior typically occurs within bisexual trios, that is, an association of three birds—two of one sex and one of the other—in which all three members have a bonded sexual relationship. For example, two males and a female sometimes form a trio, and in addition to heterosexual activity between the opposite-sex partners, the two males may court and mount each other. Several different courtship and pair-bonding displays are used in both same-sex and opposite-sex contexts. For example, while walking around each other, two males might perform BALANCING, in which they make seesaw movements with their bodies, or the THICK-SET ATTITUDE, a stylized posture in which the head is drawn down between the shoulders with the tail and back horizontal, all the while bending the legs and making tripping steps. Sometimes two males also perform ritualized nest-building activities as part of their mutual courtship, such as THROWING STRAWS, in which they toss straw and other materials backward, or PRESSING A HOLE, in which they repeatedly sit down, pressing their breasts and wings against the ground as if fashioning a nest. As a prelude to copulation, one male approaches the other in the STEALTHY ATTITUDE, similar to the thick-set attitude except that the head is held to one side and the tail is pressed down and spread. One male may mount and try to copulate with the other, although sometimes his sexual advances are thwarted by an attack from the other male. Interestingly, all three members of such a trio may be nonmonogamous, engaging in heterosexual courtship or copulations with birds other than their primary partners.

Homosexual activities also occur between two female Oystercatchers that form part of a bisexual trio with a male. Most associations of this type start off the way heterosexual trios do, with considerable aggression between the females, but eventually they develop a strong bond with each other. They preen one another while remaining close together and also cooperate (along with their male partner) in mutual defense of their territory. Employing the same behavior patterns seen in heterosexual mating, the two females also regularly copulate with one another: one female approaches the other in a hunched posture, making soft pip-pip noises while her partner tosses her tail upward. Then, while mounting, the female flaps her wings to maintain balance and may push her tail under the other female’s in order to achieve genital (cloacal) contact, at which point she utters soft wee-wee sounds. The two birds may take turns mounting one another, and about 47 percent of lesbian copulations include full genital contact (compared to 67 percent of matings by heterosexual pairs and 74 percent of male-female copulations in heterosexual trios). The females also mate regularly with their male partner, eventually building a joint nest together in which they each lay eggs. This results in a SUPERNORMAL CLUTCH of up to 7 eggs (compared to a maximum of 4—5 in nests of heterosexual pairs, or in each of the two separate nests of heterosexual trios). All three partners take turns incubating the eggs and they cooperate in raising their chicks. However, because each bird is usually unable to adequately cover all 7 eggs simultaneously, bisexual trios generally hatch and raise fewer offspring than do heterosexual pairs. Bisexual trios can remain together for up to 4—12 years, comparable to Oystercatcher heterosexual pairs, and are actually more stable and longer-lasting than heterosexual trios (which typically do not extend beyond 4 years).

Male Golden Plovers occasionally court and pair with each other in the early spring. Courtship activities often begin with ground displays, in which one male chases the other with his head lowered, wings half-spread, and back feathers ruffled, all the while raising and lowering his fanned tail. This may develop into a spectacular twisting aerial pursuit flight, in which the two males synchronously dip and climb, careening and skimming over the ground in a dramatic, high-speed chase that may take them far from their home territories.

Frequency: Homosexual behavior occurs occasionally in Oystercatcher and Golden Plover populations. Less than 2 percent of Oystercatchers, for example, live in trios of two females with one male, although 43 percent of such associations involve homosexual bonding and sexual activities. Overall, about 1 in every 185 copulations is between two females; lesbian matings take place roughly once every 6—7 hours within each bisexual trio, compared to roughly once every 3—6 hours for heterosexual matings (in a pair or trio). Likewise, approximately 1 out of every 400 Oystercatcher bonds involves a trio of two males with a female, and only some of these include same-sex activity. As in bisexual trios with two females, however, homosexual behavior may be fairly frequent within the association: in one such trio, for instance, almost two-thirds of all courtship activities were homosexual, and 15— 19 percent of all mounting activities were same-sex.

Orientation: Oystercatchers that participate in same-sex activities are usually bisexual, being part of a bonded trio with a member of the opposite sex and sometimes also engaging in promiscuous heterosexual activities. Within the trio, however, one bird may be more homosexually oriented than the other, i.e., it may have a closer bond with a bird of the same sex, while the other may have a stronger heterosexual bond. In one bisexual trio involving two males and a female, for example, 85 percent of one male’s courtship activities and more than a third of his mounting activities were homosexual; for the other male, about 70 percent of his courtships and a quarter of his mounting activities were same-sex. Some female Oystercatchers in bisexual trios also end up leaving their trio and pairing with a male, although this occurs less frequently than for females in heterosexual trios.

Nonreproductive and Alternative Heterosexualities

Polygamous heterosexual trios (without same-sex activities) sometimes form in Oystercatchers (as mentioned above), and the same phenomenon also occurs in Golden Plovers. In addition, several other variations on the long-term, monogamous, male-female parenting unit have developed in these species. Although pair-bonds in Oystercatchers and Golden Plovers sometimes last for life, heterosexual partners may divorce and re-pair with new mates. In some Oystercatcher populations 6—10 percent of couples divorce, and the average length of a pair-bond is only two to three years. Some birds (particularly females) divorce repeatedly and may have as many as six or seven different partners during their lives, and only about half of all birds remain with the same partner for life. A female Golden Plover sometimes deserts her mate during the breeding season (often to start a second family with a new male); her former mate must then raise their young on his own. In addition to single parenting, “double-family parenting” sometimes occurs: two Plover families occasionally share the same territory (with one couple breeding earlier than the other) and may help defend each other’s brood. Oystercatcher pairs sometimes foster-parent chicks of other related species such as lapwings (Vanellus vanellus) and avocets (Recurvirostra avosetta), occasionally even “adopting” and hatching foreign eggs.

Infidelity is a prominent feature of Oystercatcher pair-bonds. Up to 7 percent of all copulations are nonmonogamous, often between a paired female and a single male (and usually initiated by the female). Females often have an extended “affair” with a particular male over several years and may eventually leave their mate to pair with him; some females are even unfaithful to their new partner by continuing to copulate with their “ex” after they have remated. However, nonmonogamous mates are not generally more likely to divorce than strictly monogamous pairs, and in fact some evidence suggests that Oystercatchers who engage in outside sexual activity are actually more likely to stay together. One study found that 0—5 percent of unfaithful birds divorced, while 11 percent of monogamous ones did. Many nonmonogamous matings are nonreproductive, occurring too early in the breeding season for fertilization to be possible, or between nonbreeders; in fact, only 2—5 percent of all chicks are the result of infidelities. There are several distinct categories of nonbreeders in this species, including nonbreeding pairs with territories (about 5 percent of all pairs) and FLOATERS without any territories. Overall, about 30 percent of the adult population is nonbreeding. Nevertheless, such birds still engage in sexual behavior, both with each other and with paired birds. Nonbreeding pairs and individuals also occur in Golden Plovers, and on average about half of the population is nonreproductive at any time.

Many within-pair copulations are also nonprocreative, with about 40 percent occurring too early or too late in the breeding season (for Oystercatchers), or during incubation. In addition, it has been estimated that each Oystercatcher pair copulates about 700 times during the breeding season—far in excess of the amount required for reproduction. Oystercatchers also sometimes practice nonreproductive REVERSE copulations, in which the female mounts the male. And as mentioned above, a quarter to a third of mounts between heterosexual mates do not involve genital contact; many such copulations are incomplete because the female throws the male off her back or otherwise refuses to participate. Much more rarely, a male will rape or forcibly copulate with a nonconsenting female. Adult-youngster interactions are also sometimes marked by violence and neglect: Oystercatcher chicks have been viciously attacked and even killed when they stray into another bird’s territory. In addition, LEAPFROG parents often starve their chicks by failing to bring them enough food. Leapfrog birds are those whose nesting territories are located farther inland, separate from the feeding territories, hence to obtain food they must “leapfrog” over birds that nest directly adjacent to the shore. Studies have shown that the territories of such Oystercatchers do not, however, place undue time or energy constraints on them compared to nonleapfrogs. Thus, the fact that their chicks sometimes starve is due more to inadequate parental care than to their suboptimal territories.

GULLS AND TERNS RING-BILLED GULL

IDENTIFICATION: A medium-sized (21 inch) gull with a gray back and wings; spotted black-and-white wing tips; yellow bill, legs, and eyes; and a black band on the bill.

COMMON GULL

IDENTIFICATION: Similar to Ring-billed Gull, except slightly smaller (up to 18 inches) and with a more slender, plain yellow bill. DISTRIBUTION: Nearly circumpolar in Northern Hemisphere; winters south to North Africa, East Asia, California. HABITAT: Coasts, mudflats, beaches, lakes. STUDY AREA: Fair Isle on the Shetland Islands, Scotland; subspecies L.c. canus.

Social Organization

Common Gulls are fairly sociable, often associating in flocks of up to 100 individuals; sometimes tens of thousands of birds congregate outside of the breeding season. Ring-billed Gulls are also gregarious. Birds of both species generally form monogamous pair-bonds (although several variations exist—see below). Nesting colonies in Common Gulls contain a few dozen to several hundred pairs, while Ring-billed colonies can be much larger, in the tens of thousands of pairs.

Description

Behavioral Expression: Both Ring-billed and Common Gull females sometimes develop homosexual bonds, build nests together, lay and incubate eggs, and successfully raise chicks. Same-sex pairs are often long-lasting (as are heterosexual pair-bonds in these species), and females generally return to the same nest site with their female partner each year. Of five homosexual pairs of Ring-billed Gulls tracked over time, for example, all remained together over more than one mating season, while homosexual bonds lasting for at least eight years have been documented in female Common Gulls. Some pairs do divorce between mating seasons, however, as do some heterosexual pairs. In addition, female Ring-billed Gulls on rare occasions switch mates during a mating season, first pairing with one female, then with another. Same-sex bonds in this species also show a number of other interesting features that are rarely found in female homosexual bonds in other animals. For example, one or both partners in a homosexual pair are often younger females: couples in which there is an age difference between the two females—one an adult, the other an adolescent or younger adult—are particularly common in some populations. In addition, some female Ring-billed Gulls form homosexual trios consisting of three birds that are all simultaneously bonded to one another. In Common Gulls, homosexual pairs may after several years develop into bisexual trios. This occurs when a male joins them and is accepted into their association, bonding with one or both females (who nevertheless still retain their bond with each other). In Ring-billed Gulls, the opposite scenario may occur: in at least one case, two females in a bisexual trio remained paired to each other after the male left their association. Ring-billed Gulls in homosexual pairs probably do not engage in a great deal of courtship activity (unlike heterosexual or homosexual pairs in other species).

The first breeding season that female Common Gulls begin a pair-bond, they may build a “double nest” consisting of two separate but touching nest cups; in subsequent years, they will build only a single nest in which they both lay eggs (like most Ring-billed female pairs). Since both partners usually lay eggs, nests of homosexual pairs often contain “oversize” or SUPERNORMAL CLUTCHES of 5—8 eggs (Ring-billed Gull) and 6 eggs (Common Gull)—up to twice the number found in heterosexual nests. Some Ring-billed Gulls in female couples may also lay their eggs in the nests of other (heterosexual) pairs. One or both partners in pairs of female Ring-billed Gulls may mate nonmonogamously with a male so that some of their eggs will be fertilized. Female Common Gulls in bisexual trios can also lay fertile eggs by mating with their male partner. Both females share incubation duties and also cooperate in parenting the chicks that they hatch. Homosexual parents in Ring-billed Gulls invest as much time as their heterosexual counterparts do in feeding their chicks, spending time on the nesting territory, and defending their territory. They may actually work harder than male-female pairs in brooding and defending their chicks, with the result that offspring of female pairs often have a faster growth rate than chicks of heterosexual parents.

Nevertheless, chicks belonging to female pairs are often less robust on hatching, and female parents generally raise less than half the number of chicks that male-female pairs do. However, these traits are also characteristic of supernormal clutches belonging to heterosexual trios and are therefore undoubtedly related to the larger clutch size rather than the sex or abilities of the parents. In addition, in some populations female pairs are relegated to smaller, substandard territories on the periphery of the breeding colony or in between other territories (as are less experienced heterosexual pairs). In some cases, homosexual pairs actually appear to form clusters of up to ten nests in close proximity, or else small groups of two or three (sometimes in a straight-line or triangular formation). Many of these nests are located in areas where it is more difficult to parent successfully—places with little or no vegetation, or else away from the beach—and therefore it is remarkable that female pairs are able to successfully raise chicks as often as they do in such less than optimal conditions.

Frequency: There is wide variation in the incidence of female pairing in Ring-billed Gulls. In some populations—especially in growing colonies—as many as 6—12 percent of the pairs are homosexual. In other colonies, they are less common—1—3 percent of all pairs—and in some locations as few as 1 in 700 or 1 in 3,400 nests may belong to a female pair. Overall, pair-bonds between females probably occur only occasionally in Common Gulls, although at one study site, 1 pair out of a total of 12 was homosexual.

Orientation: There is an equally wide variation in the proportion of heterosexual, bisexual, and homosexual orientations among Ring-billed Gulls. In some populations, less than a third of all eggs laid by female pairs are fertile, indicating that the majority of such females are exclusively homosexual (at least for the duration of their pair-bond). In other colonies, egg fertility of female pairs is much higher—two-thirds to nearly 90 percent—indicating a greater prevalence of bisexual activity. Even among such females, however, there is further variation. In some same-sex couples, both females mate with males and lay fertile eggs; in others, only one partner does, or each partner might lay both fertile and infertile eggs at different times, indicating temporal variation in bisexual activity. Similarly, in a Common Gull bisexual trio, one female remained exclusively homosexual even though her female partner mated with the male. In addition, a large number of “heterosexual” Ring-billed females may have a “latent” bisexual potential, since many are able to develop bonds with other females if single males are not available.

Nonreproductive and Alternative Heterosexualities

Heterosexual pairs in Ring-billed and Common Gulls exhibit a variety of bonding and parenting arrangements (like homosexual pairs). Not all males and females couple for life: the heterosexual divorce rate is about 28 percent in both species. Polygamous heterosexual trios—two females bonded to the same male, but not to each other—are also found in both species, as are occasionally even quartets (three females with one male). Common Gull pairs sometimes foster-parent chicks, while another form of “adoption” occurs in these species when females occasionally lay eggs in nests belonging to other pairs or roll eggs from other nests into their own. Moreover, because of parental ineptitude or inefficiency (such as poor feeding), at least 8 percent of Ring-billed chicks abandon or “run away” from their own families; most of these are adopted and cared for by other families.

About 4 percent of Ring-billed pairs continue to engage in courtship and copulation after the hatching of their eggs—when sexual activity is not directly reproductive—and about 5 percent of adults court and mount chicks. Most of this activity involves females behaving incestuously with their own offspring, including full copulatory REVERSE mounts of young birds. Mounted chicks may be as young as two weeks old, and they usually collapse under the weight of the adult mounting them and cry out in distress. Some individuals appear to be “habitual molesters” in that they repeatedly interact sexually with chicks, including their own. In addition to sexual molestation, Ring-billed chicks are often subjected to vicious attacks from neighboring adults when their parents are away, or if they stray outside of their home territory. About 1 in 300 chicks is killed by such assaults, and infanticide can account for between 5 percent and 80 percent of all chick deaths (depending on the population).

Other Species

Female pairs that lay supernormal clutches also occur in California Gulls (Larus californicus ), where they constitute about 1 percent of all pairs.

WESTERN GULL