Sarah Trefry

Sexual Dimorphism and Sex Ratios in Magnificent Frigatebirds

Ph.D. Candidate
Department of Biology
University of New Brunswick, Fredericton


Sarah Trefry in Mangrove with Magnificient Frigatebird chick. (photo by James Hudson)

Research Description

Several distinct morphological and behavioural characteristics set Magnificent Frigatebirds (Fregata magnificens) apart from other seabirds, presenting an exciting opportunity to test hypotheses about their ecology and evolution. The most notable features that distinguish frigatebirds from other seabirds are the extreme differences between sexes; females are larger while males are more ornate, and sport a bright red inflatable throat sac when breeding. The behaviour of males is also unique; they display communally to females flying overhead, and abandon the female and young before the chick becomes independent.

While Magnificent Frigatebirds are considered one of the most threatened species in the West Indies, large gaps remain in our knowledge of their ecology, including the timing of their breeding cycle, where they go in the non-breeding season, information on their moult ecology, and where they feed. My research, designed to fill in some of these gaps, is taking place on Barbuda Island, in the Lesser Antilles.
This project presents exciting opportunities to test current models of the evolution of sexual dimorphism, sex ratios, and breeding behaviour. The main theoretical objectives of my PhD project are to: 1) test predictions of natural and sexual selection mechanisms that may lead to size differences between the sexes, 2) test mechanisms that could lead to skewed offspring sex ratios, and 3) to establish clearly aspects of frigatebird ecology such as their breeding periodicity, where and on what they feed, and where they go to moult after breeding.
These objectives will be achieved through behavioral observations, capture and tagging birds with yellow wing tags and GPS loggers, and collecting blood and feather samples for stable isotope analysis of diet and genetic sexing.

Unraveling the proximate correlates of the patterns exhibited by Magnificent Frigatebirds should generate explanatory causal hypotheses of significance to the interpretation of mating systems in birds in general, further advancing our understanding of their evolutionary ecology. An equally important goal of this project is to collaborate with local interest groups, to promote awareness and protection of the birds and the lagoon, and to advance conservation efforts of frigatebirds and other seabirds.  For more photos of Magnificent Frigatebird research follow this link.

Map of banded Magnificent Frigatebirds resights.


Society for the Conservation and Study of Caribbean Birds

Environmental Awareness Group of Antigua & Barbuda

National Parks Antigua

Official website of Antigua and Barbuda

Beaverhill Bird Observatory

Funding and Collaborators

NSERC LogoDr. Stephen S. Lewis Fellowship, UNB
Barbuda Council
Antigua & Barbuda National Parks
Environmental Awareness Group, Antigua
Beaverhill Bird Observatory


  • Myers-Smith IH, Trefry SA and Swarbrick V. 2012. Resilience: Easy to use but hard to define. Ideas in Ecology and Evolution 5:44-53. doi: 10.4033/iee.2012.5.11.c 
  • Trefry SA, AW Diamond and LK Jesson. 2012. Wing marker woes: a case study and meta-analysis of the impacts of wing and patagial tags. Journal of Ornithology. doi: 10.1007/s10336-012-0862-y.
  • Trefry SA, B Freedman, JMG Hudson and GHR Henry. 2010. Breeding bird surveys at Alexandra Fiord, Ellesmere Island, Nunavut (1980-2008). Arctic 63:308-314.
  • Trefry SA and DS Hik. 2010. Variation in pika (Ochotona collaris, O. princeps) vocalizations within and between populations. Ecography 33:784-795. doi: 10.1111/j.1600-0587.2009.05589.x
  • Trefry SA and DS Hik. 2009. Eavesdropping on the neighbourhood: collared pika (Ochotona collaris) responses to playback calls of conspecifics and heterospecifics. Ethology 115:928-938.
  • Trefry SA, DL Dickson, and AK Hoover. 2007. A common eider X king eider hybrid captured on the Kent Peninsula, Nunavut.  Arctic 60:251-254.

Other Refereed Contributions

  • Trefry SA, AW Diamond and LK Jesson. Aug. 18, 2012. Wing marker woes: a case study and meta-analysis of the impacts of wing and patagial tags. North American Ornithology Conference, Vancouver, British Columbia. Best oral presentation student award, Waterbird Society.
  • Trefry SA and AW Diamond. Aug. 5, 2011. Wing tag woes: impact of individual markers on nest success in Magnificent Frigatebirds (La Frégate superbe: l’impact des marquages à l’aile sur le succès des nids). Society of Canadian Ornithologists (Moncton).
  • Trefry SA and AW Diamond. Sept. 11, 2010. Reversed sexual size dimorphism in Magnificent Frigatebirds. World Seabird Conference (Victoria).
  • Trefry SA and AW Diamond. May 13, 2009; July 15, 2009; August 21, 2009. Sexual size dimorphism and sex ratios in Magnificent Frigatebirds. Canadian Society for Ecology and Evolution (Halifax); Society for the Conservation and Study of Caribbean Birds (Antigua); Society of Canadian Ornithologists (Edmonton).
  • Trefry SA, B Freedman, G Henry and J Hudson. May 13, 2009. Breeding birds of a polar oasis in a warming climate. Canadian Society for Ecology and Evolution (Halifax). 
  • Trefry SA and DS Hik. October 21, 2007. Collared pika vocalizations: influences of sympatric animals and geographic variation. Association of Canadian Universities for Northern Studies international student conference (Saskatoon).