Roger M. Evans

Brood reduction in the American white pelican (Pelecanus erythrohynchos)

SummaryAmerican white pelicans (Pelecanus erythrorhynchos) breeding in colonies at East Shoal Lake, Manitoba, Canada exhibited a mean hatching asynchrony of 2.5 days in 2-egg clutches. This resulted in a size difference between chicks which facilitated sibling dominance, harassment and lack of food for the subordinate chick. Only one young survived per nest. In marked broods, the secondhatched chick survived in 20% of successful nests. Manipulated clutch sizes (1, 2 and 3 eggs or chicks per nest) revealed that the presence of a second chick contributes significantly to the reproductive success of the parents. Results support the hypothesis that the second egg functions as a form of “insurance” against early loss of the first egg or chick. The parents, by establishing hatching asynchrony, by nonintervention in sibling aggression, and by selectively feeding the dominant chick, maximize their chance of rearing the most viable young.

Cold-induced calling and shivering in young American white pelicans: honest signalling of offspring need for warmth in a functionally integrated thermoregulatory system

Vocal regulation of body temperature in thermally dependent offspring can occur when cold-induced vocalizations elicit appropriate brooding and rewarming by a parent or surrogate. I tested the ability of young American white pelicans to regulate vocally body temperature in the laboratory by exposing them to moderate chilling (20-23°C), either continuously, or punctuated by brief (usually 2 min) periods of warmth in response to cold-induced calling. When continuously chilled, they began to call almost immediately (median latency 35 s), and body temperature began to drop. At 1 and 2 weeks of age, visible shivering also occurred, but was not sufficient to arrest the decline in body temperature. When chilling was then ameliorated by brief periods of warmth in response to cold-induced calling, body temperature shifted upwards, indicating a vocal thermoregulatory ability similar to, but somewhat faster, than that previously reported for late-stage embryos in this species. At 1 day of age, cold-induced calling occurred with a shorter latency and at a higher rate at lower body temperatures, as expected in a compensatory thermoregulatory system. After the onset of shivering thermogenesis at 1 and 2 weeks of age, the compensatory response decreased, but calling still occurred in response to chilling, as required by a simple on-off regulatory system. Begging for food was inhibited at body temperatures low enough to elicit shivering, suggesting that vocal maintenance of a relatively high body temperature may be of importance to normal begging and food acquisition prior to the full development of endothermy in this species. Effective functioning of the vocal thermoregulatory system requires that information about offspring body temperature be accurate. This requirement suggests a strong parallel between vocal thermoregulation and the concept of honest signalling in animal communication. Honest signalling of need by offspring has received recent theoretical support, but is controversial and has rarely been tested experimentally. I used the occurrence of shivering, which is the main source of endothermic heat in birds, as a standard against which to test empirically the honesty of cold-induced calls as signals of need for warmth. When 1 and 2-week old chilled, shivering chicks began to warm up during vocal regulation sessions, shivering eventually became periodic, starting each time a bird was chilled and ending during each subsequent period of rewarming. Calling at this time invariably (84 of 84 cases) began only after the onset of shivering, then ended while shivering was still in progress. To the extent that shivering is an honest, perhaps unfakable indicator of a biologically relevant need for warmth, the vocalizations associated with it appear to be honest signals indicating the presence of that need. Results are also consistent with a recent theoretical model of honest signalling of need by offspring, but it remains to be determined whether honest solicitation of warmth prevails when chicks are in naturally competitive situations at the nest.

Embryonic and neonatal vocal elicitation of parental brooding and feeding responses in american white pelicans

Calls uttered by late-stage embryonic and neonatal American white pelicans, Pelecanus erythrorhynchos, and parental responses to them were observed during natural parent-offspring interactions and during playback experiments. Observations were all made during the hatching period, when the two-egg clutch consisted of various combinations of unpipped or pipped eggs and newly hatched chicks. From pipping onwards, embryos and neonates uttered short harsh squawks, especially when nest contents were exposed to ambient temperatures. Squawks elicited parental brooding responses that were characterized by foot shuffling over the nest, sometimes in association with pushing an egg or chick back towards the centre of the nest. Brooding responses usually ended with the parent resettling over the nest contents. Brooding response frequency increased with the number of squawks uttered per bout of calling, and was the usual response given to playbacks of squawk calls. After hatching, neonates also began to give a distinctive begging call. Long sequences of begging elicited a characteristic parental feeding posture accompanied by feeding. Parental feeding postures, but not actual feedings, were elicited by 1-min playbacks of begging calls. The data indicate that late-stage embryos and neonatal pelicans utter distinctively different calls for thermoregulatory and nutritional needs, and that parents respond appropriately with brooding or feeding responses designed to ameliorate these needs.

Development of an auditory discrimination in domestic chicks (Gallus gallus)

Abstract Ninety-two chicks were exposed to two different sounds, one of which was paired during training with a conspicuous visual stimulus. Visual stimuli maintained subsequent responses to the auditory stimuli, whereas responses to discriminably different sounds that were not paired with visual stimuli habituated. Auditory discriminations were learned at 1 day of age both by chicks that were previously ‘imprinted’ to the visual stimulus and by those that were not. Only the visually imprinted chicks performed significantly better than random when trained at 2 or 3 days, indicating an early critical period for visual, but not auditory, stimuli. It is suggested that visual stimuli enhance responses to species-typical and individually distinctive vocalizations.

Vocal regulation of temperature by avian embryos: a laboratory study with pipped eggs of the American white pelican

Abstract When exposed to temperatures above or below normal mean incubation temperature, embryonic American white pelicans, Pelecanus erythrorhynchos , at the pipped egg stage called quickly and strongly in a graded fashion, then soon stopped calling when temperatures were returned to normal. The ability of pelican embryos to regulate their own body temperature by calling was examined by subjecting pipped eggs to an incubation temperature that was either above (50°C) or below (20°C) normal in a surrogate incubation apparatus that automatically returned incubation temperature to near-normal levels for a brief 4-min period whenever an experimental embryo called in response to heating or cooling. Under these conditions, embryonic temperatures initially drifted about 2°C from normal, then stabilized until the end of a 2-h experimental period. When a return to normal incubation temperature was controlled by calls of an embryo held in another room (yolked control), body temperature drifted erratically, and tended towards ambient, indicating that temperature regulation could not be achieved simply by random calling. Results show that late-stage pelican embryos are capable of vocal regulation of temperature provided a parent or surrogate responds appropriately to the embryos calls. Vocal regulation of embryonic temperature by calling to cooperative parents is potentially operative in a wide range of precocial and some altricial species of birds.

FORAGING STRATEGIES OF AMERICAN WHITE PELICANS

Foraging strategies employed by American white pelicans were studied at a riverine site in Manitoba, Canada, during the breeding season in 1985 and 1986. Six strategies were identified during both diurnal and nocturnal foraging periods. Sit-and-wait was the least common strategy (four instances). Mobile individuals were common but had low rates of bill dipping and prey capture, as did relatively rare and uncoordinated aggregations. A degree of flock coordination occurred in following flocks, characterized by foragers following one after the other, with occasional synchronization of bill dipping among flock members. The largest number of pelicans foraged within more or less circular groups called nuclei. Synchronous bill dipping and apparent herding of prey towards shore were common within nuclei. The most highly coordinated strategy, semicircles, involved small numbers of foragers (2 to 30 birds) that maintained their positions relative to one another, usually in a semicircle but sometimes moving to a closed circular pattern. The greatest degree of synchronized bill dipping occurred in semicircles. Small inter-bird distances and synchronized bill dipping in nuclei and semicircles may enhance their effectiveness in driving or herding clumped fish prey. Foraging strategies could be arranged along a continuum based on degree of coordination, ranging from mobile individuals, then uncoordinated aggregations, through increasing degrees of coordination in following, nuclei, and semicircles. Along this continuum, prey size and capture rats were greatest for the more highly coordinated strategies, while less coordinated strategies appeared to be involved primarily in searching. Switching among strategies fit along the same continuum, with a tendency to switch from less to more coordinated strategies when prey were located and to return to less coordinated search when capture rates declined. Video analysis of captures within large nuclei and observations of positional shifts among foragers in nuclei and following flocks indicated that all individuals within a coordinated group potentially benefited from the presence of others, supporting the view that coordinated foraging strategies in this species are examples of true cooperative foraging. The range of strategies, and interplay among them, appear to provide the American white pelican with a highly effective group foraging system for harvesting mobile, clumped fish prey.

Development of Behavior in Seabirds: An Ecological Perspective

Young seabirds are decidedly terrestrial creatures that spend all or a large part of their prefledging developmental period on or near the surface of the ground, cliffs, or other firm substrates. Consequently, the developing seabird faces problems in adaptation to its natal environment that are common to a wide range of birds, including species whose entire life cycle is restricted to terrestrial habitats. Despite their early terrestrial existence, however, young seabirds are also irrevocably linked to the primarily marine ecology of their parents, whose selection of breeding habitat and foraging behavior imposes strong selective constraints on the developmental adaptations of the young (Lack, 1968). The interaction of these terrestrial and marine influences provides a unique blend of selective forces that underlie the diversity and richness of developmental patterns found in seabirds today. This review is an attempt to describe some of this diversity, and where possible to relate it to the particular ecological conditions to which the individuals of particular species are adapted.

Nocturnal foraging in the American White Pelican

Nocturnal foraging was examined in American White Pelicans (Pelecanus erythrorhynchos) at the Dauphin River, about 50 km from a breeding colony on Lake Winnipeg, Manitoba, Canada. From two to three times as many pelicans foraged at night as in the daytime, with foraging flocks being larger at night. In contrast, more pelicans were present at adjacent loafing sites during the day. Capture rates were highest for flocks of up to 100 foragers in the daytime, but showed no relationship to flock size at night. Rates of bill dipping and mean duration of dips were significantly greater at night, but capture rates were significantly lower. Lower capture rates at night were to some extent offset by the capture of more large fish, probably because they were more accessible at that time. Most of the day-night differences in capture efficiency appeared to be due to lower visual sensitivity of pelicans at night.

Hatching asynchrony and survival of insurance offspring in an obligate brood reducing species, the American white pelican

Abstract American white pelicans (Pelecanus erythrorhynchos) lay two eggs but typically rear only one young owing to siblicidal brood reduction affecting the later-hatched, or B-chick. When the A-offspring fails at an early age, the B-chick may survive as a replacement (insurance) offspring. Using a combination of nests with natural and artificially manipulated hatching asynchrony, I examined the hypothesis that hatching asynchrony in this species is adaptively tuned to permit B-chicks to survive during the time they are most likely to be needed as replacements, with brood reduction following when they become redundant. Hatching asynchrony over the natural range of 0–4 days significantly increased within-brood mass differentials and reduced B-chick lifespan. Full synchrony had a marginally negative effect on A-chick mass. Greater asynchrony did not significantly affect the number of days B-chicks survived after hatching of the A-chick, owing to a corresponding extension of time B-offspring were protected from harassment while still within the egg. This resulted in a high probability (> 0.8) of B-chicks surviving through the initial period (5–7 days) of maximum early A-chick loss. Redundant B-chicks were subject to heavy brood reduction, with both chicks likely to have survived at only one each of 94 natural and 84 manipulated (0, 2, and 4 days asynchrony) nests. Hatching asynchrony in American white pelicans, in combination with a rapid development of senior chick siblicidal competence, appears to result in a time course of brood reduction appropriate for an effective insurance reproductive strategy.

Embryonic fine tuning of pipped egg temperature in the American white pelican

Abstract Upon pipping of the first (a) egg, the second (b) egg of the American white pelican, Pelecanus erythrorhynchos , is sometimes exposed to lower and more variable temperatures. Poor temperature control may arise at this time because the parents lack information about the current temperature state of the b-egg. Once chilled, embryos exhibit a graded response to cooling, which may restore the embryos communicative link with the parent and enhance egg temperature control. This possibility suggested the hypothesis that pipped eggs are maintained at warmer temperatures than unpipped eggs during the final stages of incubation. At the onset of pipping, pipped a-eggs were significantly warmer than unpipped b-eggs in the same nest. This difference was not due to egg size. At more advanced nests, where the older a-egg had already hatched, temperatures of b-eggs increased significantly after pipping. Embryonic heat production was not a confounding variable in either situation. Results support the interpretation that late-stage embryos are able to influence their own incubation temperature by communicating with their parents.