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Diet and foraging movements of sympatric-nesting Arctic Terns (Sterna paradisaea) and Common Terns (Sterna hirundo) on Country Island, Nova Scotia

Document
Call Number
LE3 .A278 2025
Date Issued
2025
Supervisor
Degree Name
Master of Science
Degree Level
Masters
Degree Discipline
Affiliation
Abstract

Sympatrically-nesting seabirds may experience resource competition in various dimensions of the ecological niche, and in response often exhibit niche partitioning as either spatial segregation, dietary segregation, or some combination of the two. Furthermore, as central-place foragers seabirds must efficiently exploit resources away from the colony while spatially constrained by incubation or chick-rearing. Arctic Terns (Sterna paradisaea) and Common Terns (Sterna hirundo) are two ecologically similar small seabirds whose breeding range overlaps in the northwest Atlantic. I tracked 26 Arctic and 25 Common Terns with GPS loggers during the incubation period on Country Island, Nova Scotia, Canada in 2018 and 2019 to evaluate spatial niche partitioning and compare foraging patterns between the two species while nesting sympatrically. I used stable isotope analysis of δ15N and δ13C from blood plasma of captured birds to investigate isotopic niche, dietary partitioning and diet composition of incubating adults, and compared diet between sexes. Arctic Terns had depleted δ15N and δ13C signatures relative to Common Terns and a more restricted isotopic niche, however the isotopic niche of both species overlapped by an average of 55%. I observed greater intra-annual variation in isotopic signatures within than between species or between years, and did not detect any difference in isotopic signatures between sexes. Results of a Bayesian model of tern diet suggested Common Terns consumed a greater proportion and variety of fish and Arctic Terns consumed more marine invertebrates early in the breeding season and more terrestrial invertebrates later in the season. In contrast to the large overlap in dietary niche displayed by both species, I found clear spatial segregation between foraging adults consistent with species-specific habitat preferences observed elsewhere in their range; Arctic Terns tended to forage in more pelagic waters while Common Terns frequented coastal and estuarine areas. Foraging Arctic Terns traveled shorter mean linear distances from the colony and made more frequent and briefer foraging trips than Common Terns, which traveled greater mean and maximum distances when foraging and made longer and fewer trips per day. Mean maximum foraging distances reported in this study are greater than previously estimated for Arctic and Common Terns breeding at Country Island, confirming that this tern colony relies on coastal and estuarine habitat along a > 60 km stretch of coastline along Nova Scotia’s Eastern Shore and on pelagic areas up to 25 km offshore. I also examined impacts of tagging on adult body condition, reproductive success and chick growth, and found no significant impact of tagging on tern reproduction but marginally non-significant impacts on adult body condition. This study presents the first comprehensive analysis of dietary and spatial resource use and niche partitioning among sympatric-breeding Arctic and Common Terns in eastern Canada, providing valuable data to inform conservation measures within the region.

Rights
The author retains copyright in this thesis. Any substantial copying or any other actions that exceed fair dealing or other exceptions in the Copyright Act require the permission of the author.
Publisher
Acadia University

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