Response of adult Leach's storm-petrels to increased food demand at the nest

-In a colony of Leach's Storm-Petrels (Oceanodroma leucorhoa) on Kent Island, New Brunswick, i chicks were rotated on a daily basis through n nests to produce i/n chickequivalents of food requirement, increasing by small increments from 1 (i = 6, n = 6) to 2 (i = 2, n = 1), between ages 10 and 50 days. Food delivered each night (SUM) was estimated by 24-h increments in chick mass. In nonexperimental nests SUM varied significantly among pairs, and chick mass varied in direct relation to SUM. Adults tending experimental nests did not respond to increased food demand, and chick mass decreased with increasing chickequivalent per nest. After rotations were terminated, the mass of most chicks increased quickly to normal levels. These results suggest that the average amount of food delivered daily by each parent is determined independently of food demand and that chicks attain a mass that balances a fixed food intake against food requirement, which varies in direct relation to mass. Received 29 September 1986, accepted 17 June 1987. PELAGIC seabirds typically rear a single, slowly growing chick. This pattern of reproductive performance characterizes species in several taxonomic orders and in a wide variety of marine environments from the tropics to high latitudes. Two types of explanations have been offered for this low productivity. Lack (1968), Nelson (1977), and others have argued that breeding productivity is limited by the availability of food in the environment or by the ability of adults to transport food between the feeding and nesting sites. Alternatively, some authors have suggested that pelagic seabirds reduce breeding productivity below the level they are capable of supporting either to reduce the stress of reproduction and thereby prolong life (Goodman 1974, Dunn 1979) or to prevent overpopulation (Wynne-Edwards 1962). These two classes of hypotheses can be distinguished by the ability of adults to increase their rate of food delivery to the nest in response to increased demand. Accordingly, if feeding rate were maximized at a level imposed by resource availability or ability to transport food, then feeding rate could not be increased. If, however, feeding rate were optimized at a lower level, parents would be able to respond to increased food demand at the nest. Several investigators have attempted to increase the food demand of the brood by adding a second chick. Such experiments have met with mixed success, generally failing to elicit marked responses in Procellariiformes (Rice and Kenyon 1962; Huntington 1963; Harris 1966, 1969) but leading to increased feeding rate and successful rearing of twins in Swallow-tailed Gulls (Creagrusfurcatus, Charadriiformes; Harris 1970) and gannets (Sula spp., Pelecaniformes; Nelson 1964, Jarvis 1974). Cross fostering the chicks of a larger species into the nests of a smaller, related species also has stimulated increased food provisioning by parents in Gray-backed Terns (Sterna lunata; Shea and Ricklefs 1985). The failure of petrels (Procellariiformes) to respond to the presence of twins is difficult to interpret with respect to the contrasting hypotheses of maximized vs. optimized feeding rate. Although the results are consistent with resource limitation, twinned chicks may interact so as to interfere with proper communication with adults. Recent studies have demonstrated the influence of chick-chick interactions within broods on feeding rates (Henderson 1975; Bengtsson and Ryden 1981, 1983; Stamps et al. 1985). Furthermore, storm-petrel parents may not be suitably responsive to the novel family situation created by the twinning experiment. Finally, the feeding rates of parents may be fixed to match average food demand, and not to respond to variation in the nutritional level of the chick. Petrel chicks vocalize when a parent is at the nest, and actively solicit food, but it is not known whether solicitation varies with respect to level of nutrition. The experiment reported here was designed to chronically increase the food demand of chick rearing for storm-petrels without there being more than one chick in the nest at any time. By 750 The Auk 104: 750-756. October 1987 This content downloaded from 157.55.39.132 on Thu, 15 Sep 2016 05:05:30 UTC All use subject to http://about.jstor.org/terms October 1987] Parental Feeding in Leach's Storm-Petrel 751 rotating a number of chicks among a smaller number of nests on a daily basis, I created a graded food demand, ranging from 1 to 2 chickequivalents. This experimental design circumvented some of the potential problems of twinning experiments but raised others. A positive parental response to increased food demand could mean that foraging effort is normally regulated below the maximum possible. But a failure to respond would be consistent with several alternative explanations. Among these are: (1) parents cannot deliver more food, (2) chicks do not increase solicitation when deprived, and (3) the presence of different chicks in the nest from night to night so confuses the parents that they cannot respond appropriately.