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Behavior and Nesting Success Relative to Nest Location in Adslie Penguins (Pygoscelis Adeliae)

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Behavior and Nesting Success Relative to Nest Location in Adslie Penguins (Pygoscelis Adeliae) BEHAVIOR AND NESTING SUCCESS RELATIVE TO NEST LOCATION IN ADSLIE PENGUINS (PYGOSCELIS ADELIAE) RICHARD TENAZAl Department of Zoology University of Wisconsin Madison, Wisconsin This study is an experimental evaluation of the predictions for three reasons: (1) it is much hypothesis that differences in nest positions less disturbed in the presence of man than within and outside Ad&e Penguin colonies are most colonial sea birds, (2) it is subject influence behavior, egg loss rates, and nest to predation upon eggs and chicks, and (3) characteristics. “Colony” and “rookery” are theft of nest material from neighbors is con- used in this report as defined for Ad&lies by spicuous and frequent. Also, the breeding Penney (X365:85): “A colony is a geograph- behavior and ecology of the Ad&lie is the ically continuous group of breeding birds best known of the Antarctic penguins. whose territorial boundaries are contiguous,” and a rookery is a “geographical area . that METHODS contains one or more colonies.” The study was conducted at Cape Hallet (72” 18 ’ 50” The actions of predators and conspecifics S, 170” 13 ’ 00” E ), Victoria Land, Antarctica, during within a dense nesting colony of Ad&es or the 1967-1968 austral summer. The Cape Hallet other sea birds are highly predictable. Nests Ad&e Penguin rookery occupies about 40 ha of a are regularly spaced, just far enough apart to low lying spit (“Seabee Spit”), approximately 1000 m long and 200-650 m wide, projecting into the Moubray allow owners of adjacent territories to touch Bay inlet of the Ross Sea (fig. 1). Maximum eleva- bills when reaching toward one another from tion of the spit is about 5mT‘ This rookery was de- their nests. This “social pattern” (Hutchinson scribed and mapped by Reid ( 1964) who recognized 1953) minimizes disturbances within the col- more than 600 colonies at Hallet, ranging in size from less than 10 to more than 1200 nests. Reid (1964:15) ony. A predator hunting eggs or chicks may censused the breeding population at Cape Hallet and have difficulty directing its attention to any found it to be 62,900 pairs in the 1959-1960 breeding single nest because it is always within peck- season and 61,955 pairs in the 1960-1961 season. A ing distance of other nests. For the same census made during the present study identified about reason a conspecific has difficulty approach- 43,000 breeding pairs (Brett Tumbull and Thomas ing a nest to steal nest material (a habit found Choate, pers. comm. ). The experiments described in this report were con- among many colonial sea birds, e.g., see Fisher ducted 5-11 December 1967 during the late incuba- and Lockley 1954) or to otherwise disturb its tion phase of the Ad&lie breeding cycle. Data collec- occupant. Colony perimeters furnish less pre- tion was confined to this brief period to allow a dictable living spaces, and single nest sites temporal cross-sectional comparison among birds nest- entirely separated from colonies are even more ing in different locations. First hatching occurred on 9 December and the peak of hatching was 16-17 unpredictable. Intruders can approach pe- December. The Ad&lies’ incubation period averages rimeter nests from outside the colony without 35-37 days (Penney 1968:93), so the birds evaluated being attacked by potential victims ’ neighbors, in this study had already been exposed to approxi- and isolated nests can be approached from mately 354.5 weeks of nest predation and intra- all directions. specific disturbances. Thus, peripheral and isolated nesters might SUBJECTS suffer heavier losses of eggs and chicks to predators than do residents within the colony. The three experimental groups whose behavior, nests and clutch sizes are compared in this report were 60 They might also be expected to lose more inner-colony ( “central” ) and 60 colony-perimeter nest material (in species where theft occurs) (“peripheral”) nesting Adklies from among 15 and experience greater stress from the higher different breeding colonies, and 22 isolated nesters frequency of predator contacts and interac- (fig. 2). Each “central” nest was surrounded by six tions with conspecifics (see Christian 1963: other nests and had insufficient adjacent space to sup- port another nesting territory. It was therefore central 592, 1964; Barnett 1964). in relation to its six nearest neighbors rather than The Ad&lie Penguin is particularly well central within the colony. Most central nests evalu- suited for an experimental evaluation of these ated were actually near colony edges (to minimize observer disturbance ). Isolated nests were located outside of colonies and at least 2 m from the nearest 1Present address: Department of Zoology, University of California, Davis, California 95616. neighboring nest; such nests comprised less than 1 1811 The Condor 73:81-92, 1971 82 RICHARD TENAZA FIGURE 1. A portion of the Cape Hallet AdClie Penguin rookery on “Seabee Spit.” Colonies appear as pale (guano-covered) areas on the spit. Mt. Herschel (3335 m elevation) is in the left background. per cent of the Hallet rookery. All birds evaluated of the nest next to the displaced egg. Nest radius was were incubating eggs. Penguins with mates present also a measure of the distance from the nest center at their nest were excluded from the study to avoid to the nearest edge of the displaced egg. All of these confounding the results with interactions between measurements were completed in less than 60 sec. birds. I wore the same bulky red parka, black wind trousers, The number of nests in study colonies ranged from and black boots during all experiments. 9 to 470 (5 = 95), and the number of incubating Following these manipulations and measurements birds tested per colony varied from two in the nine- I started my stopwatch, then backed 10 paces away nest colony to 22 in the 470-nest colony. To control from the nest to record the birds’ responses to the for differences among colonies and possible differences displaced egg. The test was ended when the egg was due to time of day or weather, equal numbers of cen- retrieved or pecked hard enough to cause it to roll tral and peripheral birds from the same colony were away from the nest, or after 6 min had elapsed with always tested within a few minutes of each other neither event occurring. Egg retrieval was considered during the same experimental session. complete when the egg was back in the nest cup. Duration of the actual retrieval movement was also PROCEDURE measured. Before collecting the quantitative data in this re- The parameters chosen for measurement were ( 1) port I conducted 75 preliminary trials to familiarize clutch size (the number of eggs in the nest), (2) nest myself with the birds ’ responses, work out appropriate dimensions, (3) nest quality, rated by visual evalua- data recording procedures, and practice recording the tion of compactness and form, (4) relative size of physical and behavioral measures finally adopted. nest stones, (5) responses to the experimenter, and (6) responses to experimentally displaced eggs. The RESULTS AND DISCUSSION rationale for these parameters is given in a later sec- tion. Each experiment was conducted as follows. The RESPONSES TO THE EXPERIMENTER incubating study-subject was approached and nudged Responses of incubating birds to disturbing from its nest (unless it ran away first), and clutch size, nest quality, estimated average nest stone size, stimuli affect the temperature of their eggs and the birds ’ responses to the experimenter were (Baerends 1959:364) and can expose eggs or noted. Then one egg was removed from the nest and chicks to predation if the bird leaves the nest. placed on the ground next to (and touching) the These responses are therefore relevant param- outside edge of the nest. Nest measurements were eters when comparing the relative nesting then taken to the nearest cm. Nest rim height was measured next to the displaced egg, and nest radius potentials of birds nesting in different loca- was measured from the nest center to the outer edge tions. BEHAVIOR AND NESTING SUCCESS IN ADELIE PENGUINS 83 -- ----_--. FIGURE 2. A (upper). The arrow points to a central nester, surrounded by its six nearest neighbors (see the hexagonal arrangement of nests in the hypothetical colony of figure 4A ). B (1ower ) . The bird brooding a large chick on its nest in the center of the picture is an isolated nester. The arrow at right points to a peripheral nest. 84 RICHARD TENAZA TABLE 1. Comparisons of responses to the experi- CLUTCH SIZE menter by central, peripheral, and isolated nesting The smaller mean number of eggs in periph- Adelie Penguins. eral nests (fig. 3) agrees with previous reports Central Peripheral Isolated ( Penney 1968: 107; Taylor 1962: M-200) that I&XXlSC! (n = 60) (n = 60) (n=22) P<O.O58 peripheral nesting Adblies produce fewer Remained on nest young than central birds, as Patterson (1965: and delivered 438) found in Black-headed Gulls (Lam ridi- painful pecks and/or wing bundus). These data have important implica- blows” 42 45 13 None tions regarding the relevance of colony form and size to reproductive success, discussed in Ran away 3 12 4 Cvs.P the following section. Remained on nest but did not attack, RELATION OF COLONY SIZE AND SHAPE or delivered only TO REPRODUCTIVE SUCCESS mild, painless pecks ’ 15 3 5 c vs. P Several observers have reported that small colonies of certain sea birds produced fewer = Two-tailed binomial test used. b Central birds, which neither attacked nor fled, pecked young per breeding adult than larger colonies their experimentally displaced eggs hard enough to cause them to roll away from the nest significantly more often than other (Darling 1938, Herring Gulls, Lams urgenta- central birds (S/15 vs.
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