Feeding Ecology of Cory's Shearwater in the Portuguese Atlantic

• Main Author: Granadeiro, Jose Pedro
• Format: Thesis
• Language: English
• Published: ProQuest Dissertations & Theses 1998
• Subjects: North East Atlantic
• Online Access: http://theses.gla.ac.uk/75242/; http://theses.gla.ac.uk/75242/1/13815406.pdf

This thesis considers aspects of the morphology, diet and feeding ecology of Cory's shearwater Calonectris diomedea in the Portuguese Atlantic (sub-tropical and temperate north east Atlantic). Morphological variability among colonies in measurements of adult Cory's shearwater in the Mediterranean, Atlantic and Cape Verde is described. Discriminant methods are employed to enable identification of each sex from bill measurements, in some Atlantic colonies. Relationships between the size and weight of several species of prey fish and the dimensions of diagnostic hard structures (vertebrae, otoliths) are presented, to enable back-calculation of size of prey from remains found in diet samples. The importance of using information from all hard remains present in food samples is discussed in the context of biases in dietary studies of piscivorous animals. The diet of Cory's shearwater in the archipelago of the Azores is described, and comparisons are made with the diet of other marine predators occurring in this area. The use of shearwaters as monitors of pelagic environments is briefly discussed. An interspecific study is described to test the existence of a causal relationship linking mercury levels in the plumage of seabirds with the levels found in the organisms that constitute their prey. The implications for the use of seabirds as monitors of mercury contamination are examined. The patterns of activity and burrow attendance of breeding Cory's shearwaters are analysed, using a novel logging system. The system records data on date, time, identity, of marked birds entering and leaving their nests. Data on the reliability of the system are presented, and its potential use in studies of other burrow-nesting animals is discussed. Existing methods developed to assess the natural patterns of food delivery to nestling Procellariiformes are critically examined, using independent information on parental behaviour provided by the logging system. Some implications of errors in the underlying assumptions of these methods on the understanding of the adaptive significance of reproductive traits of Procellariiformes are discussed. A flexible foraging behaviour of Cory's shearwater in a year of presumed low food availability is documented, and this behaviour is interpreted in relation to a situation of "normal" abundance of food resources. The ability of Cory's shearwaters to adopt alternative long and short foraging trips is discussed in the context of the life-history strategies of long-lived seabirds. An experimental study testing the influence of the nutritional status of the chicks on the provisioning behaviour of Cory's shearwater is presented. Results are discussed in light of current hypotheses explaining regulation of food delivery in Procellariiformes.