Pick from the Past
BIRDS NESTING in colonies pose some intriguing questions. Some advantages to colonial nesting, such as mutual defense against predators, seem obvious, but so do some disadvantages. Why, for example, build nests in localized, high densities, which appear to maximize competition for nest sites and materials? To try to find some answers, I decided to do a field study on the spectacular white pelican, which nests colonially on a remote island in Great Salt Lake, North Americas inland sea.
The North American white pelican (Pelecanus ervthrorhynchos) is one of six recognized species of pelicans found throughout most of the world. Five of the species are basically white and live in freshwater environments; the marine-living brown pelican is the exception.
Unlike the brown species, the white pelican has not experienced the deleterious impacts of pesticide contamination on eggshell thickness and reproductive behaviors. Levels of DDT and its metabolic derivatives, DDD and DDE, have remained comparatively low in North American white pelicans and their eggs. This disparity in pesticide contamination levels between the two species is due to their nesting site locations. White pelicans nest primarily along the shores of lakes in near-desert and northern prairie regions, which are not agriculturally or industrially developed. Brown pelicans nest along the heavily populated and developed western and gulf coasts of the United Stales, where pesticides (and other wastes) leach into the ocean from agriculturally developed land.
Historically, the continental population of white pelicans declined with the draining of the extensive wetlands of the northern prairies. By the early 1960s the population had stabilized at just over 30,000 birds. A 1972 survey indicated this population nested at fourteen sites on the continent. A fifteenth site was added in 1976 with the colonization of islands in Honey Lake, California, by an estimated 2,000 birds.
A majestic bird, the North American white pelican has a wingspan of about nine feet, only slightly smaller than our largest native bird, the California condor. The large wingspan, along with a relatively light body weight of fourteen pounds, is an adaptation for soaring on thermal air currents; an ability that aids the birds in ranging widely between food resources by reducing the need for energetically costly active flight.
The major morphologic feature of pelicans is their large gular pouch, which is used as a dip net to scoop up fish. This distensible pouch has two other functions. By vibrating one of the pouchs structures, a pelican can cause the tongue to flutter. The fluttering promotes evaporative cooling from the inner surface of the pouch, which in these birds is an energetically more efficient cooling mechanism than panting. The pouch also plays a role in the social life of pelicans. Distension and lateral display of the pouch, accompanied by soft grunts, occurs when mated pairs meet after an absence. Such behavior appears to be an essential part of the mate recognition process.
The first of the more than 5,000 white pelicans that nest annually in the Great Salt Lake region return to Utah in early March from their wintering grounds in western Mexico soon after the ice melts in the extensive marshlands along the lakes east shore. The pelicans visit the traditional nesting site shortly after their spring arrival and lay their first eggs at the beginning of April. Late arrivals come into northern Utah and lay eggs through the end of June.
On 150-acre Gunnison Island in Great Salt Lake, pelicans nest in discrete colonies on low, flat areas, avoiding steep and rocky slopes. Colonies range in size from the definable minimum of two nests to more than six hundred nests, with as many as thirty-five colonies on the island in a given year. Nests within a colony average slightly more than a half yard apart, and the nests of one colony are often only a few yards from nests in another colony.
Within a colony, the reproductive activities of pelicans are highly synchronized. Egg laying, hatching, and fledging of chicks, for example, occur within cycles of five to nine days for all nests. In contrast, reproductive activities in the pelican population as a whole are highly asynchronous. Progressive stages in the reproductive cycle are often separated by four to eight weeks in neighboring colonies. On many days I could easily observe the entire spectrum of reproductive activities-from pair formation through incubation and brood raising-by scanning different colonies on the island.
An aspect of the colonial nesting habit of birds almost totally ignored by researchers is colony formation. With the white pelicans, I found that sexually aroused, unmated birds of both sexes are especially attracted to those displaying courtship behaviors. The result is a flock of courting birds. With each succeeding day, individuals in the courting flock acquire mates, defend territories, construct nest mounds, and ultimately lay eggs, while birds seeking mates continue to join the flock for an additional five to nine days after the flock has settled in a given site. (The timing of these events assures that reproductive synchrony is maintained throughout the nesting period.) Coming together as a flock,
When a sexually active female first enters a courting flock she charges among the massed bodies of unmated males loitering on the periphery. Jabs delivered by the bills of the males impede her progress until she stops and assumes an elaborate bow posture. The closest males respond to her bow with a more subtle bow and overt aggressive behaviors by the surrounding birds diminish. After the exchange of bows, the female usually recommences her attacklike movement toward the center of the flock until she is again forced-or chooses-to stop and bow. Her movement through the flock, together with the high-density social environment she encounters, seems an essential, sexually excitatory stimulus. Males attempt to keep up with a female, and through the course of a few days, one male successfully maintains his position by rebuffing other males and a pair bond is established. Usually neither bird leaves the flock during this period, even to feed.
Bow posturing appears to be a sexual, not an appeasement, behavior. A male only bows in response to a female doing so first, but her bowing stimulates him to do likewise. An over-all effect of this posturing is to reduce the level of aggressiveness between the two. After pair formation and nest site selection, the bow of the female becomes the invitation for copulation.
Once paired, the two birds progress farther into the center of the flock, until they encounter other pairs already defending their nest sites. The newly mated birds move within about a half yard of the territorial birds, select, and then defend their own nest site. Substrate features generally receive little, if any, consideration in the nest site selection process; proximity to other pairs appears to be the major criterion.
Aggressive, behaviors between pairs of birds are frequent and violent during activities associated with courtship, mating, and especially, defending a nesting territory. I noticed, for example, that the rate of aggressive encounters per pair of birds (they respond together to challenges from neighbors) peaks at better than 200 encounters per hour during the initial period of nest-site defense.
During the breeding season, white pelicans have an unusual morphological structure that I speculate directs an agonistic blow to an area where it will do the least harm. Both males and females develop a hornlike, deciduous maxillary protuberance, which drops off during egg incubation-by which time territorial boundaries have been resolved. This structure seems to serve as a target for aggressive jabs, with 95 percent of all agonistic blows being. directed at the "horn" on top of an opponents bill. Just as mountain sheep perform a ritualized combat of head-on clashes and possess highly pneumaticized skulls to absorb such blows, pelicans resolve their conflicts without excessive injury to participants.
Once the selected nest site has been successfully defended, the pair copulate for four to five days. The first egg appears on the fifth day. On the fourth or fifth day the male leaves his mate to visit feeding grounds in marshes along the eastern shore of Great Salt Lake, forty to sixty miles away, and returns three days later when the two-egg clutch is complete.
As more males temporarily leave the courting flock during egg laying, the density of the flock is reduced, the intensity of excitatory stimuli associated with courtship, copulation, and territorial nest defense declines, and a greater proportion of the flock assumes the appearance of an established colony of incubating birds. The flock becomes less attractive to later arriving, unmated birds, which congregate at other sites where courting birds are more in phase with the newcomers physiological state.
Pelicans attend their nests continually throughout incubation (about 30 days) and until the chicks are three to four weeks of age. The male takes the first turn incubating the eggs, enabling the female to go off for her first meal since the pairing process began. During incubation, the male and female exchange places on the nest at 72-hour intervals. They switch to a 24-hour schedule after the eggs hatch. A bird coming to the island to relieve its mate lands near the colony, preens for a moment, then walks directly to the nest. After exchanging bows and greeting behaviors, the birds switch places; the bird from the nest walks outside the colony, stretches and preens briefly, then flies to the distant marshes to feed.
At four weeks of age the nutritional demands of the chicks require the full-time efforts of both parents to feed them. Chicks are often left to wander about unattended as both parents go off to hunt for fish (mainly carp). The chicks are capable of flight at about twelve weeks. At fledging, a chick may weigh as much as 20 percent more than its parents, this excess weight likely representing a metabolic reserve to buffer the lean period when the chick must learn to secure its own food.
During late incubation and the nestling period, both adults undergo a molt of their head feathers. The flowing, white crest feathers of the nuptial plumage are replaced by short, gray or black plumes. The striking aspect of this new plumage is that virtually no two adult pelicans look exactly alike. When pelicans return to the island to feed older chicks, they return to the site of the colony and wait to be approached by their own chicks. The variability in plumage on the heads of adults may provide visual cues that facilitate the initial recognition process for chicks. This is similar to the system used by royal terns; but in these terns, the chicks display the highly variable plumage patterns and the adults visually screen the nursery for their offspring.
One of the more intriguing questions about bird colonies is why they tend to be synchronized. Recently, there has been much speculation that colonial nesting and synchrony within a colony promote the opportunity for birds to share information on the location of food resources. Birds that nest in colonies typically exploit food resources that occur in variable abundance at unpredictable locations. This "information center" hypothesis has received some support from recent work with great blue herons in British Columbia. Observations of these herons revealed that birds on neighboring nests exchanged places with their mates at about the same time, and birds leaving the colonies departed together in small flocks. This information cannot be directly applied to the situation in pelican colonies, however, since nest exchanges are not synchronized between neighbors in a colony. Exchanges in a pelican colony occur at only a third of the nests each day.
I believe, however, that coloniality does enhance the ability of white pelicans to locate and exploit food resources. The white pelican is a social feeder-small feeding flocks work cooperatively by forming a line and herding schools of fish into shallow water where they can be captured, dip-net fashion, with the pouch. When relieved at the nest by its mate, a pelican flies directly from Gunnison Island toward the feeding grounds, joining other birds enroute. Upon arrival at the feeding grounds, the bird will either join other pelicans already foraging or select a new foraging area. Being in a feeding flock likely facilitates selection of a good foraging site, as inferred from the observation that birds rarely leave to feed on their own. By nesting in a colony, pelicans can readily join feeding flocks, and individual members, especially younger ones, can benefit from the combined fishing experiences of all birds in the flock.
For pelicans, synchronization of reproductive events within a colony has at least one observable adaptive value. When parents leave their chicks after the nestling period, the chicks are still awkward and uncoordinated. They wander from the nest site and, on extremely hot days, move to the islands shoreline, where they stand in the water as a means of regulating their temperature. In an unusual instance, I watched such a chick wander within reach of a territorial pelican that was still incubating its eggs. The adult struck the chick on the top of the head and killed it with a single blow. Survival of chicks when they first leave the nests is dependent upon a synchronous breakdown of adult territorial behavior.
Nests in small colonies exhibited a mean productivity similar to that of large colonies. Pelicans, which select their nest sites based upon the social environment, do not appear to benefit from belonging to a larger colony. The social environment (at least as far as its impact upon successful nesting behavior) seems limited to the interactions between neighbors.
Pelicans may be very selective of their neighbors, however. Although the average productivity of a nesting attempt is only .85 chicks, about 11 percent of the attempts do result in both eggs of the clutch leading to fledged chicks. Interestingly, more than 70 percent of the nests producing two chicks are situated near or next to nests from which two chicks also fledged. It may be that older birds more experienced in raising chicks tend to select similar neighbors.
The colony formation process of pelicans forces us to dispel some historical opinions about the species. Although these birds tend to nest on the same islands in subsequent years, the predominant role of the social environment in the selection of a nest site implies that white pelicans are capable of establishing a breeding colony wherever a sexually active flock encounters a barren, predator-free island with open, flat areas, which the birds need as runways for their awkward takeoffs and landings. Such a colonization occurred in a reservoir near Fort Collins, Colorado, in the early 1960s, and at Honey Lake, California.
The future of the white pelican appears secure at present. The continental population has stabilized and, in the absence of further human encroachment on the availability of feeding grounds and islands for nesting, appears capable of coexisting with man.
Efforts to establish white pelican colonies on artificial islands are under way at some of the lakes and reservoirs of eastern Colorado. Encouraging pelicans to nest at additional sites may result in increased numbers of these birds. More significantly, new nesting sites would provide supplemental breeding populations to buffer future human impacts on successful reproduction at any of the current pelican colonies.
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