Influences of Larus gulls and nocturnal environmental condition on Leach's storm-petrel activity patterns at the breeding colony

Bryant, Shelley L. (1993) Influences of Larus gulls and nocturnal environmental condition on Leach's storm-petrel activity patterns at the breeding colony. Masters thesis, Memorial University of Newfoundland.

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Abstract

The risk of predation is perhaps the most serious pressure an animal must contend with during its lifetime. Predation risk has played a strong selective force in many aspects of life history, including the activity patterns of prey species. Leach's Storm-Petrels (Oceanodroma leucothoa) arrive and depart from breeding colonies only at night. Anecdotal reports suggest that storm-petrels return to the colony later on bright nights than on overcast or foggy ones, and that fewer birds are seen at the colony on bright nights. These are considered predator avoidance behaviours as diurnal gulls prey on storm-petrels, especially on bright moonlit nights. -- This study examined the effects of the presence of predators on the activity and parental care patterns at two Leach's Storm-Petrel colonies - one with gulls (Gull Island) and one without (Green Island). Data were collected on storm-petrel flight and vocalization activity, reproductive chronology, chick interfeed interval, and gull activity and predation on Leach's Storm-Petrels. A model gull experiment was conducted on Green Island to determine storm-petrel responsiveness to gulls at this colony. In addition, fledging mass and winglength measurements were collected from five colonies; four in Newfoundland and one in Maine. -- Differences were found in storm-petrel responses to nocturnal environmental condition both between the colonies and between the reproductive phases of incubation and chick rearing. At both colonies, and over the entire reproductive season, storm-petrels arrived and began to vocalize later on bright evenings than on intermediate or dark ones, and under clear versus cloudy or foggy skies. Leach's Storm-Petrels on Gull Island were also quiet on arrival for a longer period than those on Green Island. Once at the colony, the behaviour of the storm-petrels was similar under a variety of nocturnal environmental conditions at Gull and Green Islands, although the storm-petrels at Green Island were apparently less affected by nocturnal environmental condition. Storm-petrels tended to be most affected by nocturnal environmental condition during incubation on Gull Island and during chick rearing on Green Island. The activity of Leach's Storm-Petrels at both colonies was affected by the presence of gulls (model gulls on Green Island). Gulls had a higher level of activity under those conditions that were more conducive to nocturnal hunting (i.e. brighter nights), and more storm-petrel remains were found after bright or moonlit nights. The reproductive season was somewhat attenuated at Gull Island compared to Green Island, and fledging mass and winglength tended to be lower at colonies with gulls, and at larger colonies compared to smaller ones. The results of this study indicate that many factors contribute to the ultimate decision an individual storm-petrel makes to return to and land at the colony: a number of these factors are outlined in a model of offshore, colony, and underlying influences.

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