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THE WILSON JOURNAL OF ORNITHOLOGY • Vol 124. No. 3. September 20/2 
OBSERVATIONS 
One ot us (RLP), during eight previous visits to 
the island (from 1985 to 2003), had occasionally 
noticed Masked Booby chicks with deformed 
wings, but during March 2005 the number was 
much larger. This condition prevents affected 
chicks from ever flying or leaving the island. 
We surveyed the entire atoll during 11-12 
March 2005 and counted 10.375 individual 
Masked Booby chicks. Clipperton is a low, flat 
atoll with almost no vegetation and the entire 
population was counted easily. We subsampled 
the chick population on 11-12 March to ascertain 
the percentage of individuals with deformed 
wings. We used a stretched, 22-m length of poly¬ 
propylene line and made a series of strip transects 
between pre-determined landmarks, zigzagging 
between the lagoon side and the ocean side 
around the entire island. We had one person 
olding each end ot the line and one person in the 
middle, which allowed us to count every chick 
wuhm the transect, dead or alive, and note those 
w.th deformed wings. We sampled 1,019 live 
chicks using this method, corresponding to 9.8% 
of ‘he total chick population; of those. 50 (4.9%) 
individuals had deformed wings, including 45 
(4.4%) hatching-year (HY) and five (0.5%) after¬ 
hatching-year (AMY) individuals. We extrapolat¬ 
ed to the total population and estimated that 
508 chicks on the island at the time had wing 
deformities, including 456 HYs and 52 AHYs. 
DISCUSSION 
Birds with deformed wings exhibited three 
modal plumages and. because Masked Boobies 
breed synchronously at Clipperton Island (Wei- 
merskirch et al. 2008; RLP. pers. obs.). we 
inferred these modes represented at least three 
separate year classes (plumage descriptions in 
Nelson 2005). HY had dark backs, heads, and 
necks rFig. 2A), often with some downs plumage. 
Many HYs were flying around the colony during 
the daytime by the end of our stay (27 Mar), but 
still returning in the afternoon or evening to be fed 
by the parents. Some AHYs had white heads, 
necks, and upper backs, but still had residual dark 
flecking on the rump, lower back, and on the 
greater coverts of the upper wing (Fig. 2B); these 
