Short Communications 
The Wilson Journal of Ornithology 124(3):597-602. 2012 
Incidence of Wing Deformities (‘Angel Wing’) Among Masked Boobies at 
Clipperton Island: Life History Consequences and Insight into Etiology 
Robert L. Pitman , 13 Lisa T. Ballance , 1 and Charles A. Bost- 
ABSTRACT,—‘Angel wing’ is a developmental 
wing deformity among birds that can cause flightless¬ 
ness: it is mostly known from domestic birds, especially 
waterfowl, and has only rarely been reported among 
wild bird populations. We estimated ihat 508 (4.9%) 
Masked Booby (Sula ( lucivlatra ) chicks on Clipperton 
Island (10 18' N. 109 13' W) in the eastern tropical 
Pacific Ocean exhibited angel wing during March 2005. 
Both hatching-year birds and after-hatching-year birds 
exhibited the condition; the latter included seven 
flightless birds in adult plumage (i.e.. minimum 2 yrs 
of age) which were still being fed by their presumed 
parents. The angel wing outbreak coincided in time with 
high nestling mortality, apparently related to food 
shortage, and we speculate on causal linkages. Received 
8 December 2011. Accepted 10 March 2012. 
’Angel wing’ is a musculoskeletal disorder that 
can result in permanent wing delormity and 
flightlessness in birds (Rear 1973). The proximate 
cause is a deformity of the distal end ot the 
carpometacarpus, which at times causes the 
primary flight feathers to droop when the wing 
is folded next to the body, or it can result in a 
dorsolateral rotation of the primaries, causing 
them to twist and project outward (Rear 1973, 
Zsivanovits et al. 2006). The resulting appearance 
gives rise to the 20 or more common names tor 
this condition, depending upon whether the 
primaries twist (e.g., flip. tilt, airplane, or angel 
wing), or droop (e.g.. slipped, dropped, or drooped 
wing). Symptoms begin during the chick stage, 
apparently as primary feather growth exceeds 
the development of the supporting tissue ot the 
carpus. The condition can occur unilaterally or. 
less commonly, bilaterally; unilaterally, it occurs 
much more commonly on the leli than the right 
wing, and more commonly among males than 
females. It can be successfully treated in captive 
'Protected Resources Division. Southwest Fisheries 
Science Center. National Marine Fisheries Service. NOAA. 
La Jolla Shores Drive. La Jolla. CA 92037, USA. 
J Centre d'Etudes Biologiqucs de Chizc. CNRS. 79360 
Villiers en Bois. France. 
3 Corresponding author; e-mail: robert.pitman@noaa.gov 
birds (Zsivanovits et al. 2006). but is probably 
mostly fatal among birds in the wild due to the 
consequences of flightlessness. 
Angel wing has been reported far more 
commonly among domesticated birds or wild 
birds raised in captivity than among birds in the 
wild. The vast majority of reported cases have 
been of waterfowl, but it has also occurred among 
psittacines, raptors, bustards, herons, and cranes 
(Rear 1973, Serufin 1982. Naldo et al. 1998. 
Thompson et al. 2006. Zsivanovits et al. 2006). 
Other wild waterbirds diagnosed with angel wing 
have included Double-crested Cormorants (Pha- 
lacrocorax cturitus ) nesting in Canada (Ruiken 
et al, 1999) and American White Pelicans 
(P elec anus erythrorhynchos ) nesting in Minne¬ 
sota (Drew and Rreeger 1986). To our knowledge, 
angel wing has not been reported among wild 
populations of any marine birds. We document a 
high incidence of angel wing among Masked 
Boobies (Sula dacytylatra) at Clipperton Island 
in the eastern Pacific Ocean, comment on its 
etiology, and discuss some lite history conse¬ 
quences of its occurrence. 
METHODS 
Study Area .—Clipperton Island (10 18' N, 
109 13’ W) is an isolated, uninhabited, French- 
owned atoll in the middle of the eastern tropical 
Pacific Ocean. -1,280 km w'est of the coast of 
Mexico (Fig. 1). It is ~4 km long and 3 km wide 
with a large central lagoon: it is tiny (1.7 km 2 of 
total exposed surface area), but is home to the 
largest Masked Booby colony in the world 
(Pitman et al. 2005). 
Procedures— We participated in a private 
French scientific expedition to Clipperton Island 
(Charpy 2009) where we studied the diet of 
Masked Boobies nesting there during 3-27 March 
2005. Five w'eeks prior to our visit (3-28 Jan), and 
as part of the same expedition. H. Weimerskirch 
and M. Le Corre also conducted booby research 
on the island (Weimerskirch et al. 2008. 2009). 
597 
