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Age and sex of common murres Uria aalge recovered during the 1997-98 Point Reyes tarball incidents in Central California
Nevins, H.M., & Carter, H.R. (2003) Marine Ornithology 31:51-581
Summary
We examined 1,138 Common Murres Uria aalge carcasses recovered along the coast of central California coast from November 1997 through March 1998 during the Point Reyes Tarball Incidents, a prolonged oiling event traced to the sunken vessel S.S. Jacob Luckenbach. We used head plumage, supraorbital ridge, and bursa of Fabricius, to classify age among carcasses as hatch-year (HY), or after-hatch year (AHY). We then separated AHY birds into two maturity categories based on gonad development: subadult and adult. The observed age class composition (14.6% HY, 37.6% subadult, and 47.8% adult) was not different from expected values generated with a stage-based matrix model that assumed a year-round resident population. The sex-ratio for HY birds was equal (1.2:1), indicating little difference in at-sea distribution among male and female HY birds during winter. We found male-biased sex ratios in subadult (1.6:1) and adult (1.5:1) age classes. Sex-based differences in winter dispersal or colony attendance may have caused different risks to oiling among AHY birds. Mortality of mostly AHY murres during the Point Reyes Tarball Incidents and other oil spills has contributed to slow recovery of the central California population following historic and recent declines.

Keywords: age, California, Common Murre, dispersal, oil pollution, sex, Uria aalge

http://www.marineornithology.org/PDF/31_1/31_1_7_nevins.pdf


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