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THE WILSON JOURNAL OF ORNITHOLOGY • Vol 124. No. 3. September 2012 
were 1-2 year old birds. Flying birds in this 
plumage are normally rare at Clipperton because 
most fledglings begin a nomadic phase and do not 
return to the colony until they are full adults 
(Kepler 1969. Nelson 2005). The third plumage 
was the lull adult stage: all white except for black 
flight feathers and tail (Fig. 2C); birds in this 
Plumage were in their third year or older. 
The degree of wing deformity varied among 
birds, from relatively slight in one wing (Fig. 2C), 
to major deformities in both wings (Fig. 2B). The 
majority of affected birds had only one deformed 
wing; relatively few had both. The outward 
appearance ot the deformity was also variable. 
Some birds had the classic outward rotation of the 
primaries when the wing was folded against the 
body, presenting the angel wing appearance 
(Fig. 2B), while others (the majority) displayed 
only drooping primaries; the so-called slipped or 
dropped wing condition (Fig. 2A). 
We saw at least seven Masked Boobies in adult 
plumage during our stay that, because of wing 
deformities, were flightless and still being fed by 
their presumed parents (Fig. 2C). This is. to our 
knowledge, the longest period of time (min - 
2 yrs) that parents ot any bird species have been 
recorded feeding dependent young. The only 
remotely comparable situation of which we are 
aware involves another seabird, the Great Frigate- 
bird ( Fregata minor), which has a fledging period 
of up to 169 days with post-fledging feeding by 
the parents for up to an additional 428 days (total 
587 days; Schreiber and Burger 2002:670). Our 
observations also suggest the time and age at 
which Masked Boobies terminate parental care is, 
at least in some cases, affected by the chick and 
not by the adults. 
The etiology of angel wing is unknown but 
some of the suggested causes have included 
vitamin or nutrient deficiency or imbalance 
(Zsivanovits et al. 2006). elevated protein con¬ 
centration in the diet (Kear 1973), elevated levels 
of contaminants (polychlorinated biphenyls, poly¬ 
chlorinated dibenzo-p-dioxins. and polychlori¬ 
nated dibenzofurans; Thompson et al. 2006), or 
a genetic effect, perhaps due to inbreeding after a 
bottleneck event (Kreeger and Walser 1984. Drew 
and Kreeger 1986). Some specifics about the 
outbreak at Clipperton shed some light on these 
role in the occurrence of the observed angel wing. 
Under normal conditions during the chick-feeding 
stage, foraging adults depart the island in the 
morning, feed and return to the nest by dusk; on 
average they range only 103 km off the island 
with a maximum of 242 km (Weimerskirch et al. 
2008). Thus, it seems unlikely that chicks raised at 
Clipperton were exposed to any significant 
sources of anthropogenic contamination. 
A direct genetic effect is a possibility but also 
seems unlikely (Kear 1973), at least in part 
because none of the afflicted individuals would 
ever successfully breed. Kreeger and Water 
(1984) documented nine cases of angel wing in 
a population of Giant Canada Geese \Branm 
canadensis maxima) breeding in and around 
Minncapolis-St. Paul, Minnesota. That subspecies 
was once thought to be extinct and. because 
current stocks were derived from very small 
populations (as few as 1 pair in some casest. 
Kreeger and Walser (1984) suggested the angel 
wing condition may have been a genetic disorder 
resulting from inbreeding. Masked Boobies at 
(. lipperton Island also experienced a bottleneck 
event, although perhaps not as extreme as in the 
case ol Giant Canada Geese. Pigs were introduced 
on Clipperton in 1917 and by 1958 the once 
massive Masked Booby population had collapsed 
to an estimated 150 individuals (Stager 1964). The 
pigs were eliminated, and by 2003 the population 
had rebounded to >100.000 individuals (Pitman 
et al. 2005). The prevalence of angel wing at 
Clipperton based on our visits to the island seems 
to vary considerably from year to year, which 
suggests it is more likely linked to some factors) 
other than genetics. Wc also saw at least one 
Brown Booby (S. leucogaster ) chick with this 
condition in 2005, perhaps further evidence the 
condition is not inherited. 
Excess protein in the diet has often been cited 
as a possible cause of angel wing (Kear 1973. 
Serafin 1982, Zsivanovits et al. 2006. Meredith 
and Keehle 2011). However, Masked Boobies at 
Clipperton normally feed on a high protein diet 
comprised almost exclusively of flyingfish ( Exo- 
coetidae), and smaller amounts of ommastrephid 
squid ( Ommastrephidae ) and other fish (Wei¬ 
merskirch et al. 2008: R. L. Pitman and L. T. 
Ballance in prep.). Angel wing has also been 
The sheer isolation (Fig. 1) and lack of 
inhabitants at Clipperton almost certainl 
dudes the possibility of contaminants ha : 
reported in wild populations of Double-crested 
Cormorant and White Pelicans (Drew r and Kree¬ 
ger 1986. Kuiken et al. 1999), two species that 
also feed almost exclusively on fish. The only 
