The Wilson Journal of Ornithology 123(3):588-594, 2011 
MOLT PATTERNS, BIOMETRICS, AND AGE AND GENDER 
CLASSIFICATION OF LANDBIRDS ON SAIPAN, NORTHERN 
MARIANA ISLANDS 
PAUL RADLEY, 1 3 ANDREA L. CRARY, 2 JAMES BRADLEY, 2 
CHRISTINA CARTER, 2 AND PETER PYLE 2 
ABSTRACT.—Molt strategies and plumage development by age and gender are poorly understood for resident tropical 
landbirds. We used data from six banding stations on Saipan. Northern Mariana Islands, and examination of 267 museum 
specimens to describe patterns of molt and to establish criteria for assessing age and gender of nine native resident landbird 
species on the island. Feather replacement sequences in the majority of Saipan’s resident landbirds were typical of related 
species. Prcformative molts occurred in at least eight of the nine species: these and definitive prchasic molts were 
incomplete to complete, and prealtemate molt appeared to be absent for all species. We developed criteria for classifying 
gender ol sexually dimorphic species using plumage cues, presence of brood patch or cloacal protuberance, and biometrics. 
We confirmed whether or not both brood patch and cloacal protuberance were reliable indicators of gender during breeding 
m monumurphic species. Distinct periods or seasons of molt arc not well defined and can vary between years. Age 
classification ol first-cycle birds based on moll limits, feather shape and condition, and extent of skull pneumittKat.cn is 
possible for most landbird species sampled on Saipan. Received 16 September 2010. Accepted 12 January 2011 
Data concerning molt and plumage develop¬ 
ment indicate most tropical resident landbird 
species undergo a single annual prebasic molt 
that generally follows or partially overlaps their 
breeding seasons, whereas prealternate molts 
appear to be rare (Prys-Jones 1982; Avery 1985; 
Pyle et al. 2004; Ryder and Wolfe 2009; Wolfe et 
al. 2009a, b). Most available data relate to species 
of the Neotropics, but six species in the Hawaiian 
Archipelago exhibit a similar trend in seasonal 
molt strategies (e.g.. Banks and Laybourne 1977, 
Fancy et al. 1993, Jeffery et al. 1993, Pratt et al’ 
1994. Ralph and Fancy 1994, Simon 1998. 
VanderWerf 2001). However, comparable litera¬ 
ture regarding molt for avian species on oceanic 
islands in other tropical regions of the Pacific is 
entirely lacking. 
Understanding a population's demographic pa¬ 
rameters is critical when developing and imple¬ 
menting species conservation and management 
strategies (Anders and Marshall 2005, VanderWeif 
2008, Saracco et al. 2009). Accurate assignment of 
age and gender of captured individuals, both of 
which may be based upon molt strategies, plumage 
development, and reproductive status is necessary to 
‘Commonwealth of the Northern Mariana Islands, 
Division of Fish and Wildlife, Department of Lands and 
Natural Resources, P. O. Box 10007. Saipan, MP 96950 
USA. 
‘The Institute for Bird Populations, P. O. Box 1346. 
Point Reyes Station. CA 94956, USA. 
1 Corresponding author; 
e-mail: paulradleycnmidfw@gmail.com 
estimate vital rates of avian species. Vital rates are 
presently unknown for landbird species on Saipan, 
an oceanic island in the Mariana Archipelago of Ihe 
western tropical Pacific Ocean. The Commonwealth 
of the Northern Mariana Islands’ Division of Fish 
and Wildlife (CNMI-DFW) began collaboration 
with the Institute for Bird Populations (IBP) in 2007 
to initiate the Tropical Monitoring of Avian 
Productivity and Survivorship (TMAPS) project 
on the island. 
We established six TMAPS banding stations on 
Saipan in spring 2008 to collect baseline popula¬ 
tion data for landbirds in several cover types. The 
ultimate intent of the TMAPS Project on Saipan is 
to accrue data necessary to guide pre-emptive 
avian conservation efforts on the island, which 
focus on several species potentially threatened 
with extinction or extirpation by potential intro¬ 
duction of the brown tree snake (Boiga irrdgu 
laris). This introduced snake was responsible for 
extinction or extirpation of nine of 12 species ol 
native forest landbirds on Guam within the Iasi 
half-century (Savidge 1987. Rodda and Savidge 
2007), and is thought to be the single greatest 
threat to terrestrial ecosystems in the Northern 
Mariana Islands (Colvin et al. 2005). 
We present information on molt and establish 
criteria for assignment of age and gender based on 
plumage, skull condition, eye color, wing chord, 
breeding condition, and relevant morphometrii. 
data lor nine resident species of landbirds 
common to Saipan and other islands in the near 
vicinity. We update preliminary information 
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