ArticlePDF Available

Using molt cycles to categorize the age of tropical birds: An integrative new system

Authors:

Abstract and Figures

Accurately differentiating age classes is essential for the long-term monitoring of resident New World tropical bird species. Molt and plumage criteria have long been used to accurately age temperate birds, but application of temperate age-classification models to the Neotropics has been hindered because annual life-cycle events of tropical birds do not always correspond with temperate age-classification nomenclature. However, recent studies have shown that similar molt and plumage criteria can be used to categorize tropical birds into age classes. We propose a categorical age-classification system for tropical birds based on identification of molt cycles and their inserted plumages. This approach allows determination of the age ranges (in months) of birds throughout plumage succession. Although our proposed cycle-based system is an improvement over temperate calendar-based models, we believe that combining both systems provides the most accurate means of categorizing age and preserving age-related data. Our proposed cycle-based age-classification system can be used for all birds, including temperate species, and provides a framework for investigating molt and population dynamics that could ultimately influence management decisions. ©2010 The Author(s). Journal compilation
Content may be subject to copyright.
J. Field Ornithol. 81(2):186–194, 2010 DOI: 10.1111/j.1557-9263.2010.00276.x
Using molt cycles to categorize the age of tropical birds:
an integrative new system
Jared D. Wolfe,1,2,5Thomas B. Ryder,3and Peter Pyle4
1U.S.D.A. Forest Service, Pacific Southwest Research Station, Redwood Sciences Laboratory, 1700 Bayview Drive,
Arcata, California 95521, USA
2School of Renewable Natural Resources, Louisiana State University and Louisiana State University AgCenter, Baton
Rouge, Louisiana 70803-6202, USA
3Smithsonian Migratory Bird Center, National Zoological Park, P.O. Box 37012-MRC 5503, Washington, D.C.
20013, USA
4Institute for Bird Populations, P.O. Box 1346, Point Reyes Station, California 94956, USA
Received 9 November 2009; accepted 15 January 2010
ABSTRACT. Accurately differentiating age classes is essential for the long-term monitoring of resident New
World tropical bird species. Molt and plumage criteria have long been used to accurately age temperate birds, but
application of temperate age-classification models to the Neotropics has been hindered because annual life-cycle
events of tropical birds do not always correspond with temperate age-classification nomenclature. However, recent
studies have shown that similar molt and plumage criteria can be used to categorize tropical birds into age classes.
We propose a categorical age-classification system for tropical birds based on identification of molt cycles and their
inserted plumages. This approach allows determination of the age ranges (in months) of birds throughout plumage
succession. Although our proposed cycle-based system is an improvement over temperate calendar-based models, we
believe that combining both systems provides the most accurate means of categorizing age and preserving age-related
data. Our proposed cycle-based age-classification system can be used for all birds, including temperate species, and
provides a framework for investigating molt and population dynamics that could ultimately influence management
decisions.
RESUMEN. Usando ciclos de mudas para categorizar la edad de aves tropicales: un nuevo
sistema integral
Diferenciar las clases de edades con precisi´
on es esencial para monitoreos a largo plazo de especies de aves
residentes del nuevo mundo. Criterios de muda y plumas han sido utilizados para estimar con precisi´
on la edad
de aves de la zona temperada, pero aplicaciones de este modelo de clasificaci´
on de la edad de la zona temperada
en el neotropico se ha retrasado. Debido a que los eventos de los ciclos de vida de las aves tropicales no siempre
corresponden con la nomenclatura de clasificaci´
on de edades de la zona temperada. Sin embargo, estudios recientes
han mostrado que criterios similares pueden ser utilizados para categorizar las clases de edades en aves tropicales.
Nosotros proponemos un sistema de clasificaci´
on de edades categ´
orico para aves tropicales basado en la identificaci´
on
de ciclos de muda y los plumajes insertados. Esta aproximaci´
on permite una determinaci´
on de rango de edades
(en meses) para aves desde el principio hasta el fin de la sucesi´
on de plumas. Aunque el sistema base del ciclo que
proponemos es un mejoramiento del modelo base de calendario de la zona temperada, creemos la combinaci´
on
de ambos sistemas provee una manera mas precisa para categorizar la edad y conservar datos relacionados con
la edad. Nuestro sistema de ciclo base de clasificaci´
on de edad propuesto puede ser utilizado para todas las aves,
incluyendo las especies de zona temperada y provee un marco para investigar muda y din´
amica poblacional que
pueden finalmente influir decisiones de manejo relacionadas con aves tropicales.
Key words: age classification, molt, molt cycle, plumage, tropical birds
Plumage characteristics have been thoroughly
incorporated for use in classifying the age of
temperate birds (Dwight 1900, Mulvihill 1993,
Jenni and Winkler 1994, Pyle 1997a). However,
current temperate models for classifying the
age of birds do not always conform to the
life cycles of tropical taxa. As a result, molt
5Corresponding author. Email: jwolfe5@lsu.edu
strategies, plumage characteristics, and age-class
differences of most Neotropical species remain
either undocumented or have been studied in
a preliminary manner (Snow and Snow 1964,
Wolf 1969, Diamond 1974, Foster 1975, Prys-
Jones 1982). Recent advances in our under-
standing of the molt patterns of tropical birds
provideaframeworkforagedeterminationusing
boundaries between retained and replaced wing
coverts termed “molt limits” (Pyle et al. 2004,
C
2010 The Author(s). Journal compilation C
2010 Association of Field Ornithologists
186
Journal of Field Ornithology
Vol. 81, No. 2 Molt-Cycle Age-Categorization System 187
Fig. 1. A temperate-tropical comparison of annual life history events for two seedeaters in the genus
Sporophila (Aves: Emberizidae) exemplifying the prolonged tropical breeding season, sometimes overlapping
1 January, characteristic of some passerines at southern latitudes (Pyle 1997b, Wolfe et al. 2009).
Ryder and Dur˜
aes 2005, Ryder and Wolfe 2009,
Wolfe et al. 2009). Many tropical land bird
species have incomplete or partial molts imme-
diately following the prejuvenile molt, resulting
in distinguishable molt limits and thereby facil-
itating age recognition.
A popular system for age-classification of
temperate birds relies on a calendar-based age-
classification system that uses hatching date
relative to 1 January (Pyle 1997b). With this
system, an individual in its calendar year of
hatching is termed “hatching year” (HY) and
older birds are termed “after hatching year”
(AHY) until 1 January, when these individuals
become “second year” (SY) and “after second-
year” (ASY), respectively.
Importantly, the calendar-based age-
classification system cannot be used to
categorize the age of species that breed across
1 January. When breeding seasons overlap 1
January, the calendar-based age-classification
system cannot accurately discriminate cohorts
(Snow 1976, Wolfe et al. 2009; Fig. 1). Here we
refer to this inherent problem as the “calendar
dilemma.”
To mitigate the calendar dilemma, we have
considered two modifications to the calendar-
based age-classification system in tropical lati-
tudes: (1) determining, on a species-by-species
basis, whether or not the breeding season
peaks before or after 1 January and, depending
on when most individuals breed, categorically
188 J. D. Wolfe et al. J. Field Ornithol.
placing all birds of each species in the same age
class, or (2) redefining a separate calendar date
(other than 1 January) on which all age codes
of a given species changes. These two proposed
solutions do not solve the calendar dilemma
for species with biannual breeding distributions
(Wolfe et al. 2009), eruptive breeders (Snow and
Snow 1964, Diamond 1974), or those species in
lowland equatorial regions that show little or no
seasonality in breeding. Our proposed solution
is to use molt cycles and their inserted plumages,
which are assumed to be homologous across all
taxa, as a means of classifying age. Our objective
was to provide a coding system based on molts
and plumages that, when combined with other
information, can be used to accurately desig-
nate cohorts, thereby eliminating the calendar
dilemma.
DEFINING AND IDENTIFYING MOLT
CYCLES
Following Humphrey and Parkes (1959; here-
after, H-P), molt and plumage cycles are based
on presumably ancestral prebasic molts and
evolved inserted molts. Prebasic molts are reg-
ular (often annual) events that typically adhere
to well-defined periods, even for tropical species
with prolonged breeding seasons (Foster 1975,
Prys-Jones 1982, Stutchbury and Morton 2001,
Pyle et al. 2004). Humphrey and Parkes (1959)
were implicit in associating plumage with age.
Here, we explicitly anchor plumage to age using
updated terminology (Howell et al. 2003).
The assumption of plumage homology fol-
lowing the H-P system, and recent proposed re-
visions (Howell and Corben 2000, Howell et al.
2003), have resulted in useful nomenclature
that can be incorporated into age-classification
systems. Howell et al.’s (2003) revision was
rooted in the assumption that juvenal plumage
is homologous to later basic plumages. Howell
et al. (2003) thus redefined the “prejuvenal
molt” as the “first prebasic molt” and replaced
what was formerly considered the “first prebasic
molt” of most species with the “preformative
molt” (Fig. 2). This inserted molt produces
the “formative plumage” and lacks homologous
counterparts in later age groups.
Using this terminology, molt cycles are de-
fined based on prebasic molts. Therefore, the
“first molt cycle” can be defined as the pe-
riod from the beginning of the first prebasic
(prejuvenal) molt until the beginning of the
second prebasic molt, and subsequent cycles
are similarly determined through the “definitive
molt cycle” when plumage no longer changes
with successive homologous molts. The benefits
of Howell et al.’s (2003) revision of the H-P
system include the assumed homologous initi-
ation of the first molt and plumage cycles, an
important step for establishing nonambiguous
nomenclature.
Familiarity with molt limits, retained juvenal
plumage, feather shape, feather wear, and other
essential plumage characteristics (cf. Mulvihill
1993; Pyle 1997a, b) facilitates precise molt-
cycle determination. For example, many tropical
oscines and suboscines in their first molt cycle
(i.e., in juvenal, formative, or first alternate
plumages) can be distinguished from older birds
in definitive molt cycles (i.e., definitive basic
or alternate plumages). However, correctly dif-
ferentiating a formative plumage following a
complete molt from subsequent basic plumages
is usually not possible (cf. Pyle 1997b). For
example, several tropical genera in the family
Thamnophilidae (e.g., Gymnopithys,Rhegma-
torhina,Sclateria,Pyriglena,andPhlegopsis)in-
clude species that undergo complete preforma-
tive molts (Ryder and Wolfe 2009). In addition,
there are species that undergo eccentric prefor-
mative molts where inner primaries are replaced,
for example, genera within Thamnophilidae
(Cymbiliamus,Pygptila,andMicrophias), and
some species of Myrmoborus,Formicivora,and
Cercromacra (Ryder and Wolfe 2009). With-
out knowledge of molt extent, biologists could
potentially mistake an eccentric preformative
for a definitive prebasic molt. Distinguishing
plumages in later molt cycles (e.g., second basic
from definitive basic plumages) is also not pos-
sible for most oscines and suboscines, although
identification of some second-cycle, third-cycle,
and fourth-cycle nonpasserines, such as gulls
and certain raptors, is possible by examining
plumage and flight-feather molt patterns and
using other criteria (Pyle 2008).
USING MOLT CYCLES TO AGE TROPICAL
BIRDS
With our cycle-based age-classification sys-
tem, initiation of prebasic primary molt is the
definitive marker that indicates advancement
in molt cycle. Thus, for most oscines and
Vol. 81, No. 2 Molt-Cycle Age-Categorization System 189
Fig. 2. (A) Comparison of the standard Humphrey and Parkes (H-P; 1959) system and the Complex
Basic Strategy (CBS), as described by Howell et al. (2003). (B) Comparison of the standard H-P system
and the Complex Alternate Strategy (CAS) as described by Howell et al. (2003). Appropriate cycle-based
age-classification codes are placed next to their corresponding molt and plumage categories in the Howell
et al. (2003) schematic.
190 J. D. Wolfe et al. J. Field Ornithol.
Fig. 2. Continued.
suboscines, one molt cycle ends and the succeed-
ing cycle is initiated when the first primary (P1)
is shed (molt can initiate with other primaries in
larger birds with alternate remigial-replacement
strategies; cf. Pyle 2006, 2008, Rohwer et al.
2009). Even though initiation of body-feather
replacement may precede that of flight-feather
replacement, determining if body-feather
Vol. 81, No. 2 Molt-Cycle Age-Categorization System 191
Table 1. Comparison of common equivalent age codes in the cycle-based age-classification system and the
calendar-based age-classification system for the first, second, and definitive molt cycles. The calendar-based
age codes are currently recognized by the Bird Banding Laboratory (USGS).
Cycle-based age-classification system Calendar-based age-classification system
UCU Unknown molt cycle, unknown plumage U or AHY Unknown or after hatching year
UCB Unknown molt cycle, basic plumage U or AHY Unknown or after hatching year
UCA Unknown molt cycle, alternate plumage U or AHY Unknown or after hatching year
UCS Unknown molt cycle, supplemental plumage U or AHY Unknown or after hatching year
FCJ First molt cycle, juvenal plumage HY or SY Hatch year or second year
FCF First molt cycle, formative plumage HY or SY Hatch year or second year
FCA First molt cycle, alternate plumage SY Second year
FCS First molt cycle, supplemental plumage SY Second year
SCB Second molt cycle, basic plumage SY or TY Second year or third year
SCA Second molt cycle, alternate plumage TY Third year
SCS Second molt cycle, supplemental plumage TY Third year
DCB Definitive molt cycle, basic plumage TY or ATY Third year or after third year
DCA Definitive molt cycle, alternate plumage ATY After third year
DCS Definitive molt cycle, supplemental plumage ATY After third year
replacement is representative of a prebasic molt,
an inserted molt, or replacement of acciden-
tally lost feathers is often difficult. Thus, the
symmetrical shedding of P1 (or other primaries
during certain molts in larger birds) represents
an unambiguous marker for the succession of
molt cycles. Within molt cycles, however, the
initiation of body-feather replacement (e.g., as
part of preformative, prealternate, or presup-
plemental molts) is treated as a marker for
succeeding plumages.
The first step in using our cycle-based age-
classification system is to define the molt cycle
as either the first (FC), second (SC), third (TC),
fourth (4C), and so on, or definitive (DC)
cycle. For many oscines and suboscines, the
second basic plumage equates to definitive basic
plumage. However, it can be coded as SC during
the period of the second prebasic molt, provided
that this molt can be recognized as such.
Once a bird has been identified as being in
its first cycle, its plumage can then be defined as
juvenal (J), formative (F), alternate (A), or sup-
plemental (S). We use “juvenal” as opposed to
“first basic” for this initial plumage, as suggested
by Howell et al. (2003), due to the familiarity
and wide use of the term juvenal. Thus, a first-
cycle individual in complete juvenal plumage is
coded FCJ and, once a preformative molt begins,
is coded FCF. Other possible plumages within
the first cycle include first alternate (FCA) and
first supplemental (FCS), although these are not
commonly encountered in tropical oscines and
suboscines. Individuals in their second cycle can
be recorded as basic (SCB), alternate (SCA), or
supplemental (SCS), and the same plumages can
be found in the third (TCB, TCA, or TCS),
fourth (4CB, 4CA, or 4CS), and so on, and
definitive (DCB, DCA, or DCS) cycles.
As with the calendar-based age-classification
system, acknowledging uncertainty when using
the cycle-based age-classification system will
be important. The unknown code “UCU” is
proposed for cases where both the cycle and the
plumage within the cycle are unknown (Table 1).
If the plumage is known (i.e., basic, alternate,
or supplemental), but the molt cycle is undeter-
mined, plumage-specific unknown codes (UCB,
UCA, or UCS) can be used. Alternatively, when
the molt cycle is known (i.e., first, second, or
third), but the plumage is undetermined, cycle-
specific unknown codes (FCU, SCU, DCU, and
so on) can be used.
An age bracket, or age in months during
which individuals may start or end a particular
molt, can be coupled with each determined
cycle-based age code for each species, thereby
providing an estimation of age in months for
each individual. Age brackets, especially for
tropical birds, will be perpetually refined as
the results of more studies of bird molt be-
come available. Due to intraspecific temporal
variation in the duration of juvenal plumage,
age brackets of juvenal and formative plumages
typically overlap to encompass a margin of error.
In addition, species with inserted molts (e.g.,
192 J. D. Wolfe et al. J. Field Ornithol.
prealternate or supplemental molt) provide
greater refinement in the cycle-based age-
classification system due to the greater num-
ber of plumages within a cycle, which refines
age brackets. Similarly, the cycle-based age-
classification system expands upon the utility of
“unknown” codes by utilizing specific unknown
plumage or molt cycle codes (i.e., FCU, SCU,
and TCU, or UCB, UCA, and UCS; Table 1).
As such, molt cycles and associated age brackets
provide a robust, noncalendar-based age classi-
fication system for tropical birds.
CASE STUDY
As an example of the utility of integrating
plumage cycles and age classification systems,
consider five Variable Seedeaters (Sporophila
americana) captured in Tortugero, Costa Rica.
Four individuals were captured between 19 and
28 February 2005, and the remaining individual
on 30 April 2005. All five individuals had mixed
juvenal and formative feathering, indicating that
they were in their first cycle. The four birds
captured in February had no alternate feathers,
but the individual captured in April was under-
going the first prealternate molt (Wolfe et al.
2009).
Using the calendar-based age-classification
system, classifying the correct age of the five
Variable Seedeaters was difficult because it was
impossible to determine if they hatched before
or after 1 January 2005. Given the variability
in the duration of the juvenal, formative and
alternate plumages, determining if they were
hatching-year or a second-year birds was difficult
so they were classified as “unknown.”
Recognizing that the four seedeaters captured
in February were in formative plumage, they
could be classified as FCF using the cycle-based
age-classification system. Because the formative
phase occurs prior to the first prealternate molt
in the Complex Alternate Strategy (Fig. 2), the
associated age bracket is more refined for species
adhering to the Complex Basic Strategy (Fig 2).
Thus, the associated age bracket for this species
indicates that the four seedeaters captured in
February were between 1 and 8 mo old. The
seedeater undergoing its first prealternate molt
when captured in April had passed the formative
and entered the alternate phase of the first molt
cycle. Thus, it could be classified as FCA and
the associated age bracket indicates that it was
between 7 and 12 mo old.
Several groups of temperate birds are also
subject to the calendar dilemma. For example,
a Red Crossbill (Loxia curvirostra)wascaptured
by Klamath Demographic Monitoring Network
cooperators in northern California on 1 July
2005. This crossbill had mixed juvenal and
formative feathering, indicating that it was in
formative plumage. Red Crossbills can breed
across 1 January (Pyle 1997b) so, not knowing
whether it had hatched before or after 1 January
2005, determining the correct age of the cross-
bill was difficult using the calendar-based age-
classification system. As a result, the crossbill’s
age was classified as “unknown.”
However, recognizing that the captured cross-
bill was in formative plumage, it could be
classified as FCF using the cycle-based age-
classification system. Because the formative
phase occurs prior to the definitive basic molt
in the Complex Basic Strategy (Fig. 2), the
associated age bracket is generally less refined
for species adhering to the Complex Alternate
Strategy (Fig 2). Thus, the associated age bracket
for Red Crossbills indicates that an individual
captured in formative plumage is between one
and 12 mo old.
DISCUSSION
Despite a growing interest in the popula-
tion dynamics of Neotropical landbirds, no
robust and widely applicable technique has been
used for age categorization of tropical taxa.
Here, we have presented a cycle-based age-
classification system that derives conclusions
through plumage and molt patterns, while pro-
viding repeatable assignment of age classes.
The accuracy and ultimate value of the cycle-
based age-classification system is dependent on
the knowledge of the practitioner and available
molt data for tropical birds. Recent studies
indicate that molt strategies of tropical birds
are similar to those of temperate birds (Pyle
et al. 2004, Ryder and Dur˜
aes 2005, Ryder and
Wolfe 2009, Wolfe et al. 2009) and that many
tropical oscines and suboscines, like many tem-
perate oscines and suboscines, have incomplete
or partial preformative molts (Dickey and van
Rossem 1938, Wolfe et al. 2009). Thus, plumage
criteria can be useful for classifying age for most
tropical species.
Vol. 81, No. 2 Molt-Cycle Age-Categorization System 193
Although our cycle-based age-classification
system is an improvement over the calendar-
based age-classification system when assigning
tropical birds to age classes, several problems
need to be addressed. Specifically, the novel na-
ture of the cycle-based age-classification system
coupled with the potential for complex and
protracted molt strategies in tropical birds will
present initial challenges. For example, some
tropical species can potentially undergo an an-
nual molt lasting up to 4 to 6 mo and can exhibit
individual variation in timing (both start and
completion dates) of up to 2 mo, even within
a population, leaving a 2 mo margin of error
within each molt cycle. These situations would
result in individuals of the same age being placed
into different molt cycles, or birds of different
ages being given the same cycle-based age-class
code. For example, an individual that is 1-mo old
and has just begun the preformative molt and
an individual over a-year-old that has not begun
the second prebasic molt would both be coded
“FCF” even though they belonged to different
cohorts. If age brackets include the necessary
margins of error for such cases, this problem can
be alleviated. Conversely, codes and associated
age brackets reflecting individuals undergoing
molt can be generated for species with protracted
molts. Another solution to the above situation
includes using the cycle-based age-classification
system in combination with the calendar-based
age-classification system. In the above exam-
ple, the calendar-based age-classification system
might age the first bird as an HY individual and
the second bird as an SY individual, thus preserv-
ing cohort information, whereas the addition
of the cycle-based age-classification system code
(FCF) indicates that both individuals were in
the same plumage. Combining the two systems
ultimately provides the most accurate means
of categorizing age and preserving age-related
data. Additionally, cycle-based age-classification
system codes can be combined with information
concerning skull ossification, molt status, and
primary wear to further alleviate this problem
and narrow assigned age brackets.
To date, our ability to age birds in tropi-
cal regions has been hindered by our inability
to accurately apply temperate age-classification
systems to tropical birds. Our cycle-based age-
classification system provides the theoretical
frame-work necessary to assign birds into age
classes that are reflective of their life-cycle, not
a calendar date. As additional studies of the
duration and timing of molt of tropical birds are
conducted, the age brackets and utilitarian value
of the cycle-based age-classification system will
likely improve. These modifications to the tradi-
tional temperate classification models should be
applied to population-level studies in the tropics
while they are still in their initial stages. By
increasing the accuracy of age categorization, we
enhance our ability to understand demographics
and that will ultimately improve our ability to
effectively manage populations. We also believe
that our system will further refine age classi-
fication of temperate species and enhance our
understanding of avian molt patterns.
ACKNOWLEDGMENTS
Special thanks to L. Wolfe and D. Wilde for their help
with figure formatting. Thanks also to personnel at the
Klamath Bird Observatory, Institute for Bird Populations,
Point Reyes Bird Observatory, and Redwood Sciences
Laboratory. Special thanks to C.J. Ralph, D. and C.
Romo, K. Swing, J. Guerra, E. Johnson and all the staff
at the Tiputini Biodiversity Station and Caribbean Con-
servation Corporation for their tireless logistical and field
support. IACUC protocol number 5-12-20. This research
was conducted in accordance with permit number 13-
IC-FAUDFN, Ministerio de Ambiente, Distrito Forestal
Napo, Tena, Ecuador. We thank them for allowing us to
conduct our research at the Tiputini Biodiversity Station.
Funding was provided by the International Center for
Tropical Ecology, AFO’s Alexander Bergstrom Award,
National Geographic Society (7113-01), and National
Science Foundation (IBN 0235141, IOB 0508189, OISE
0513341). This is contribution #386 of the Institute for
Bird Populations. This is also a contribution of Land
Bird Monitoring of the Americas (LaMNA) and the
Tortuguero Integrated Bird Monitoring Project (TIBMP).
LITERATURE CITED
DICKEY,D.R.,AND A. J. VAN ROSSEM. 1938. The birds
of El Salvador. Field Museum of Natural History
Zoological Series 23: 1–609.
DIAMOND, A. W. 1974. Annual cycles in Jamaican forest
birds. Journal of Zoology 173: 277–301.
DWIGHT, J. 1900. The sequence of plumages and moults
of the passerine birds of New York. Annals of the
New York Academy of Sciences 13: 73–360.
FOSTER, M. S. 1975. The overlap of molt and breeding
in some tropical birds. Condor 77: 304–314.
HOWEL L,S.N.G.,AND C. CORBEN. 2000. A commentary
on molt and plumage terminology: implications from
the Western Gull. Western Birds 31: 50–56.
HOWEL L,S.N.G.,C.CORBEN,P.PYLE,AND D. I.
ROGERS. 2003. The first basic problem: a review of
molt and plumage homologies. Condor 105: 635–
653.
194 J. D. Wolfe et al. J. Field Ornithol.
HUMPHREY,P.S.,AND K. C. PARKES. 1959. An approach
to the study of molts and plumages. AUK 76: 1–
31.
JENNI,L.,AND R. WINKLER. 1994. Moult and ageing of
European passerines. Academic Press, New York, NY.
MULVIHILL, R. S. 1993. Using wing molt to age passerines.
North American Bird Bander 18: 1–10.
PRYS-JONES, R. P. 1982. Molt and weight of some land-
birds on Dominica, West Indies. Journal of Field
Ornithology 53: 352–362.
PYLE, P. 1997a. Molt limits in North American passerines.
North American Bird Bander 22: 49–90.
———. 1997b. Identification guide to North American
birds. Part 1. Slate Creek Press, Bolinas, CA.
———. 2006. Staffelmauser and other adaptive strategies
for wing molt in larger birds. Western Birds 37: 179–
185.
———. 2008. Identification guide to North American
birds. Part 2. Slate Creek Press, Point Reyes Station,
CA.
PYLE,P.,A.MCANDREWS,P.VEL´
EZ,R.L.WILKERSON,R.
B. SIEGEL,AND D. F. DESANTE. 2004. Molt patterns
and age and sex determination of selected southeast-
ern Cuban landbirds. Journal of Field Ornithology
75: 136–145.
ROHWER, S., R. E. RICKLEFS,V.G.ROHWER,AND M. M.
COPPLE. 2009. Allometry of the duration of flight
feather molt in birds. PLoS Biology 7: 1–9.
RYDER,T.B.,AND R. DUR˜
AES. 2005. It’s not easy being
green: using molt limits to age and sex green plumage
manakins (Aves: Pipridae). Ornitologia Neotropical
16: 481–491.
RYDER,T.B.,AND J. D. WOLFE. 2009. The current state
of knowledge on molt and plumage sequences in se-
lected Neotropical bird families: a review. Ornitologia
Neotropical 20: 1–18.
SNOW,D.W.,AND B. K. SNOW. 1964. Breeding seasons
and annual cycles of Trinidad landbirds. Zoologica
49: 1–39.
SNOW, D. W. 1976. The relationship between climate
and annual cycles in the Cotingidae. IBIS 118: 366–
401.
STUTCHBURY,B.J.M.,AND E. S. MORTON. 2001.
Behavioral ecology of tropical birds. Academic Press,
San Diego, CA.
WOLF, L. L. 1969. Breeding and molting periods in
a Costa Rican population of the Andean Sparrow.
Condor 71: 212–219.
WOLFE,J.D.,P.,PYLE,AND C. J. RALPH. 2009. Breeding
seasons, molt patterns, and gender and age criteria for
selected northeastern Costa Rican resident landbirds.
Wilson Journal of Ornithology 121: 556–567.
... The reproductive condition and sexual maturity of birds have been determined based on some aspects of their behavior, molt changes (Illera and Atienza 2002;Wolfe et al. 2010;Silveira and Marini 2012), external morphology, and morphometry (Gandini et al. 1992;Batista-Silveira and Marini 2012). In some cases, the use of morphometric characters is effective, but this effectiveness decreases when differences in body size between sexes or among individuals of the same species are minimal (Fletcher and Hamer 2003). ...
... On the other hand, the association between features of plumage, molting, and sex and reproductive status is complex. There may exist overlap between the molting and the breeding seasons, and several species have no sexual dimorphism or polymorphism (Wolfe et al. 2010). Hence, the use of classification criteria using plumage and molt widely used in temperate zones has proven inadequate for many Neotropical bird species (Wolfe et al. 2010). ...
... There may exist overlap between the molting and the breeding seasons, and several species have no sexual dimorphism or polymorphism (Wolfe et al. 2010). Hence, the use of classification criteria using plumage and molt widely used in temperate zones has proven inadequate for many Neotropical bird species (Wolfe et al. 2010). ...
Article
The Carib Grackle (Quiscalus lugubris) has recently extended its distribution to montane habitats in the Colombian Andes, including urban areas. Little is known about its reproductive biology in both natural and urban environments. We analyzed the relationship between morphological and morphometric external characters with sex, reproductive condition, and the annual reproductive activity of two groups of individuals that inhabit the city of Bucaramanga, Colombia. We aimed to know if there is a clear association between external features (including plumage/molting) and sex, maturity, and reproductive stage. We recorded the external morphology of these individuals (molt, iris color, and brood patch presence) as well as standard morphometric traits and classified these birds in reproductive stages according to morphology and histological analyses of their reproductive tracts. We found a clear sexual dimorphism between adults in morphometric features and plumage color. However, neither morphometric features nor iris and plumage color/molt pattern clearly indicates sexual maturity; some immatures can be mistakenly taken as adults due to the morphological characteristics obtained after they complete their pre-basic molt. Females reach maturity at different body masses and could reproduce asynchronically; therefore, the presence and type of brood patch is the only useful feature for the identification of female reproductive stages. Quiscalus lugubris has an extended breeding season throughout the year and a seasonal molting activity; at the end of the second rainy season (November) and during the driest time of the year (December–January), adults exhibited reproductive tracts in regression and were found in active molt.
... In the southern hemisphere, furthermore, opposite seasonality to that of the northern hemisphere leads to conflicting use of seasonally based codes for trans-equatorial migrants and, as many resident species in the southern hemisphere breed across December and January, calendar-based systems are also rendered inoperable (Lowe 1989, de Beer et al. 2001, Jackson 2005, Melville 2011. To address these issues, Wolfe et al. (2010) devised an age-coding system based on the molt-cycle nomenclature of Humphrey and Parkes (1959) and Howell et al. (2003). Since 2010, this moltcycle aging system has been increasingly used by banders and ornithologists throughout the Americas and in some other parts of the world (Wolfe et al. 2012, Smith et al. 2015, Tórrez and Arendt 2016, Johnson and Wolfe 2017, Diaz et al. 2021, and it has subsequently become known as the "WRP system" after the authors of Wolfe et al. (2010). ...
... To address these issues, Wolfe et al. (2010) devised an age-coding system based on the molt-cycle nomenclature of Humphrey and Parkes (1959) and Howell et al. (2003). Since 2010, this moltcycle aging system has been increasingly used by banders and ornithologists throughout the Americas and in some other parts of the world (Wolfe et al. 2012, Smith et al. 2015, Tórrez and Arendt 2016, Johnson and Wolfe 2017, Diaz et al. 2021, and it has subsequently become known as the "WRP system" after the authors of Wolfe et al. (2010). For continuity, we retain the "WRP" acronym to refer to this system, although we also suggest "Molt-cycle Ageing System (MCAS)" as an alternative designation. ...
... The H-P nomenclature provides a clear and globally consistent chronological progression of molts and plumages to which the WRP age-designation system adheres. As detailed by Wolfe et al. (2010) and subsequent publications (Johnson et al. 2011, Wolfe et al. 2012, Johnson and Wolfe 2017), the WRP system employs a threeletter alpha code in which each letter describes a different aspect of H-P terminology (Table 1). Here we recommend the continued use of most original coding. ...
Article
Full-text available
Determination of a bird’s age or cohort is critical for studies on avian demography, occurrence patterns, behavior, and conservation management. Age designations have largely been developed in north-temperate regions and utilize calendar-based or seasonally based codes; however, in tropical regions and in the southern hemisphere, these coding systems have limited utility at best. To address these issues, we had previously devised the “WRP system,” based on the nomenclature of Humphrey and Parkes (H–P) and Howell et al., which defines molts in an evolutionary context applicable to birds globally. Here we refine and build upon core concepts and definitions of the WRP coding system, resolving key limitations that were identified during its first decade of use. The WRP system employs a three-letter alpha code in which each letter describes a different aspect of H–P terminology: the molt cycle (which informs a bird’s age) and molt and plumage status within the cycle (each of which can also inform age). Here we recommend the continued use of most of the original (“core”) WRP coding while augmenting the system with an optional adjunct-code entry for comprehensiveness, clarity, and flexibility, and we clarify a few additional codes to cover less common molting and plumage strategies. For most users, from 7 to 13 core and 1 adjunct WRP code will be sufficient to describe all plumages and provide molt status and ages for demographic studies or other purposes. The revised WRP system is flexible enough to be adapted to the specific goals of programs while also providing core codes that can facilitate the comparison of avian age, molt, and plumage status on a global basis. We anticipate that our revised and standardized version of the WRP system will be easily adopted and could eventually replace calendar-based and seasonally based coding.
... Chats were caught in mist nets often with the aid of callplayback and taxidermied or wooden painted decoys. Birds were removed from nets, sexed according to (Pyle 1997), placed into age categories defined by Wolfe et al. (2010), and banded. Birds that could not clearly be distinguished as male or female were classified as unknown sex. ...
... Chats have a complex-basic moult strategy. They undergo an incomplete preformative moult during their first annual cycle and a complete definitive prebasic moult during subsequent annual cycles (Pyle 1997;Wolfe et al. 2010). Chats likely complete preformative and definitive prebasic moults on their natal/breeding grounds prior to fall migration (Pyle 1997;pers. ...
Article
Full-text available
Mercury (Hg) is an environmental contaminant that can negatively impact human and wildlife health. For songbirds, Hg risk may be elevated near riparian habitats due to the transfer of methylmercury (MeHg) from aquatic to terrestrial food webs. We measured Hg levels in tail feathers sampled across the breeding range of the Yellow-breasted Chat (Icteria virens), a riparian songbird species of conservation concern. We assessed the risk of Hg toxicity based on published benchmarks. Simultaneously, we measured corticosterone, a hormone implicated in the stress response system, released via the hypothalamus-pituitary-adrenal axis. To better understand range-wide trends in Hg and corticosterone, we examined whether age, sex, subspecies, or range position were important predictors. Lastly, we examined whether Hg and corticosterone were correlated. Hg levels in chats were relatively low: 0.30 ± 0.02 µg/g dry weight. 148 out of 150 (98.6%) had Hg levels considered background, and 2 (1.6%) had levels considered low toxicity risk. Hg levels were similar between sexes and subspecies. Younger chats (<1 year) had higher Hg levels than older chats (>1 year). Hg levels were lowest in the northern and central portion of the eastern subspecies’ range. Corticosterone concentrations in feathers averaged 3.68 ± 0.23 pg/mm. Corticosterone levels were similar between ages and sexes. Western chats had higher levels of corticosterone than eastern chats. Hg and corticosterone were not correlated, suggesting these low Hg burdens did not affect the activity of the hypothalamus-pituitary-adrenal axis. Altogether, the chat has low Hg toxicity risk across its breeding range, despite living in riparian habitats.
... Age and sex were assessed in the field based on the Wolfe-Ryder-Pyle system (Wolfe et al., 2010). For the eight species with statistically significant relationships between morphology and elevation, we performed MANOVAs assessing the relationship between TA B L E 1 Study species. ...
Article
Patterns across species of intraspecific phenotypic variation with environment can shed light on the underlying drivers of adaptive evolution. Phenotypic variation within a species along tropical elevational gradients is of particular interest because species with narrow elevational ranges may still experience considerably varied environmental conditions. Here, we examine morphological variation in 27 tropical bird species, spanning 11 families and 3 orders, across a 675 m elevational gradient in Western Ecuador. We analyzed a data set of six morphological variables in 3263 individual birds using multivariate analyses of variance (MANOVAs) and canonical correlation analyses (CCAs). We found that morphology varies significantly with elevation in 8 species, and that spatial segregation by age or sex was apparently not responsible for this result. The phenotypic traits that varied with elevation varied strongly by species. To the best of our knowledge, morphological variation over equally short elevational and horizontal distances across a diverse suite of vertebrate species has not previously been demonstrated. Abstract in Spanish is available with online material. Patterns across species of intraspecific phenotypic variation with environment can shed light on the underlying drivers of adaptive evolution. Here, we examine morphological variation in 27 tropical bird species along a 675 m elevational gradient in Western Ecuador using multivariate analyses. We found that morphology varies significantly with elevation in 8 species, and that spatial segregation by age or sex was apparently not responsible for this result.
... In many cases polarization comes from long-term observation of a population. However, other methods exist to group individuals into cohorts based on single sampling events; for example, tooth ageing (Hewison et al. 1999;Gipson et al. 2000;Blundell and Pendleton 2008), counting horn annuli in wild sheep (Geist 1966;Hemming 1969), or using molt patterns in birds (Mulvihill 1993;Wolfe et al. 2010;Johnson et al. 2011). These covariates assist in determining reproductive tenure, but additional information such as spatial proximity of candidate parents can also help in pedigree construction (e.g. ...
Article
Full-text available
Pedigrees have a long history in classical genetics, agronomics, evolutionary ecology, and ex situ captive breeding. Use of molecular techniques has expanded the variety of species for which pedigrees can be constructed. However, molecular pedigrees almost exclusively consider microsatellite loci, despite advances in high-throughput sequencing allowing development of genomic marker sets in nearly any organism. Here we generate a novel set of genomic SNPs derived from ddRAD sequencing in two populations of Weddell seals (Leptonychotes weddellii) and describe the diversity and differentiation between them. We then compare and contrast parentage assignment rates and accuracy in one population that has been the subject of long-term monitoring. Specifically, we consider pedigrees constructed using two sets of markers (microsatellites and SNPs), two pedigree construction software (CERVUS than Sequoia), as well as varying the groupings of candidate parents (either all individuals simultaneously, only individuals born before a focal year, or only individuals known to have survived to a focal year). ddRAD sequencing returned between 1568 and 3240 loci depending on whether both populations were considered simultaneously or individually. Parentage assignment rates were always higher using CERVUS than Sequoia, with the latter at times either not assigning parentage or creating “inferred parents”. In all cases, “polarizing” the datasets (e.g., including year of birth) significantly improved assignments. This represents one of the first direct comparisons of pedigree construction using different markers in the same set of individuals, and the SNPs described here will be a resource for continued pedigree construction, and future research in Weddell seals.
... To estimate breeding phenology, we used active nests from the incubation through chick phases (non-flying), as well as photographic records of young before their first prejuvenile moult (FPJ, sensu Wolfe et al. 2010). We used primary data collected from 2019 to 2020 (n = 17) complemented with photo records from Lagoa do Peixe NP in the WikiAves (www.wikiaves.com.br) ...
Article
en The central-peripheral hypothesis states that the demographic performance of a species decreases from the centre to the edge of its range. Peripheral populations are often smaller and tend to occur under different and suboptimal conditions from those of core populations. Peripheral populations can also coexist during part of their annual cycle with populations from the core of the species’ range. Studies on peripheral populations are thus valuable for broadly understanding ecological and evolutionary processes. The Two-banded Plover (TWBP, Charadrius falklandicus, Charadriidae) is an endemic South American shorebird that breeds in Argentine and Chilean Patagonia and migrates northward during the Austral winter. There are breeding records, however, from Lagoa do Peixe National Park in southern Brazil. In this study, we (i) mapped TWBP nests, (ii) characterised their reproductive biology and nesting habitats, (iii) colour-marked birds and evaluated their seasonal occurrence patterns and (iv) estimated the size of the Brazilian population by combining supervised habitat classification analyses and generalised additive models. We estimated that the Brazilian population has 55 (95% CI: 44.1–66.6) breeding pairs and found that the length of their breeding season was roughly 5 months, spanning the Austral spring and summer. The population’s nesting habitat differed, and their apparent reproductive success was lower than that of core populations. Unlike more southerly populations, the results of our mark-resighting efforts demonstrate that the Brazilian population is sedentary. Taken together, these results indicate that the Brazilian TWBP population seems geographically isolated from the species’ southernmost core populations, resulting in a heteropatric distribution. Furthermore, differences in nesting habitat and year-round residency indicate that this peripheral population is ecologically distinct. The marked behavioural and ecological differences combined with the small population at the northern edge of the TWBP distribution support the central-peripheral hypothesis in a Neotropical system. Portuguese fr A hipótese centro-periferia estabelece que a performance demográfica de uma espécie diminui do centro para os extremos de sua distribuição. Populações periféricas são frequentemente menores e tendem a ocorrer em condições subótimas quando comparadas com populações centrais. Populações periféricas podem ainda coexistir durante parte do seu ciclo anual com populações do centro da distribuição da espécie. Estudos com populações periféricas são, portanto, cruciais para uma ampla compreensão de processos ecológicos e evolutivos. A Batuíra-de-coleira-dupla (Charadrius falklandicus, Charadriidae) é uma espécie de ave costeira endêmica da América do Sul, que se reproduz na Patagônia argentina e chilena, e migra para o norte durante o verão austral. No entanto, existem registros de sua reprodução no Parque Nacional da Lagoa do Peixe, no sul do Brasil. Nesse estudo, nós (i) mapeamos ninhos da espécie, (ii) caracterizamos sua biologia reprodutiva e habitat de nidificação, (iii) marcamos individualmente aves e avaliamos seu padrão de ocorrência sazonal, e (iv) estimamos o tamanho da população brasileira através da combinação de análises de classificação supervisionada de habitat e modelos aditivos generalizados. Estimamos que a população brasileira possui 55 pares reprodutivos (IC95%: 44,1 – 66,6) e que o período de reprodução foi de, aproximadamente, 5 meses, abrangendo a primavera e verão austrais. O habitat de nidificação da população diferiu e o sucesso reprodutivo aparente foi menor quando comparado às populações centrais. Diferentemente das populações mais austrais, os resultados de esforços de marcação e recaptura demonstraram que a população brasileira é sedentária. A combinação desses resultados sugere que a população brasileira da Batuíra-de-coleira-dupla está geograficamente isolada das populações austrais, resultando em distribuição heteropátrica. Além disso, o habitat de nidificação e o padrão sedentário indicam que essa população periférica é ecologicamente distinta. As diferenças comportamentais e ecológicas, combinadas com o pequeno tamanho populacional na Lagoa do Peixe, no limite norte da distribuição da Batuíra-de-coleira-dupla, suportam a hipótese centro-periferia em um sistema Neotropical.
Article
We developed aging criteria for 7 species of manakins (Pipridae) from the Manu Biosphere Reserve, Peru, based on patterns of plumage maturation and wing-feather replacement following their preformative molt, and summarize information on their morphological characteristics. Each species underwent a partial preformative molt, which could be identified using the presence of molt limits in the greater coverts. Some male Band-tailed Manakin (Pipra fasciicauda), Round-tailed Manakin (Ceratopipra chloromeros), Cerulean-capped Manakin (Lepidothrix coeruleocapilla), and Yungas Manakin (Chiroxiphia boliviana) showed evidence of delayed plumage maturation, allowing for age classification up to the third annual cycle, whereas Blue-crowned Manakin (L. coronata caelestipileata), Fiery-capped Manakin (Machaeropterus pyrocephalus), and Green Manakin (Cryptopipo holochlora viridor) appeared to attain definitive plumage after their second molt cycle. Morphometrics showed strong overlap and were less useful for separation of age and sex classes. Our findings add to the growing list of studies that suggest many tropical passerines can be aged using primarily molt limits. Data on molt and plumage maturation are still needed for the vast majority of tropical birds in order to inform conservation-based research and studies of avian life history.
Article
Full-text available
The analysis of metal concentrations in bird feathers and genotoxicity tests are tools used to evaluate anthropogenic impacts on ecosystems. We investigated the response of birds, used as bioindicators, to disturbances observed in three areas with distinctive environmental characteristics (natural, agricultural, and urban) in southern Brazil. For this purpose, we quantified metals (Mn, Cu, Cr, and Zn) in feathers and determined the number of micronuclei (MN) and other nuclear abnormalities (NA) in 108 birds from 25 species and 17 families captured in the study area. No significant differences was found in the metal concentrations and the number of MN and NA between the sampling areas. Zn and Cu concentrations were significantly higher in insectivorous than those in omnivorous birds. The Zn concentration was significantly different between some species, and the Cu concentration was significantly higher in juveniles than that in adults. The best generalized linear models showed that omnivorous birds had more MN and NA and that juveniles and birds with better body condition index had increased NA numbers. This study demonstrates that the analyzed variables contribute in different ways to the result of each biomarker, mainly due to particular ecological and physiological characteristics of each species. We conclude that wild birds have the potential to be used as environmental bioindicators in the study area, but future studies should focus on one or a few species whose ecological and physiological habits are well known.
Article
Age-specific patterns of primary molt facilitate age classification of native North American upland gamebirds, a critical step in understanding their ecology, behavior, life history, population dynamics and harvest. However, deviations from typical molt patterns can create confusing plumages that complicate age classification. We examined data from live-captured greater sage-grouse Centrocercus urophasianus across seven studies in five U.S. states and wings from harvested birds in Oregon and Colorado for evidence of atypical primary molt. We documented atypical replacement through primary nine during preformative molt, atypical retention of juvenile primary 10 during second prebasic molt, and atypical retention of basic outer primaries during definitive prebasic molt. Atypical primary molts were observed more often in live-captured females (3.2%, n = 561) than males (0.8%, n = 494). Many individuals with atypical primary patterns, especially females, are difficult or impossible to reliably age by plumage or morphology and may bias research and harvest data.
Article
Full-text available
All birds have fundamentally similar patterns of plumage succession. Thus Humphrey and Parkes (1959) proposed a system of nomenclature (the H-P system), based on homologies, that has become standard for molt studies in North America. However, presumably analogous similarities in pattern between first basic and definitive basic plumages have obscured homologies. Many plumages conventionally known as “first basic” are better considered as novel first-cycle plumages that lack homologous counterparts in subsequent cycles. Consequently, current nomenclature does not consistently reflect between-species homologies. Howell and Corben (2000b) proposed that traditional juvenal plumage can be considered an unambiguous starting point for a terminology that better reflects presumed homologies in basic plumages; alternate and other nonbasic plumages may not necessarily be homologous between species. Four underlying strategies of increasing complexity incorporate all known patterns of plumage succession: the Simple Basic Strategy, the Complex Basic Strategy, the Simple Alternate Strategy, and the Complex Alternate Strategy. We review inconsistency in the H-P system; explain the four underlying strategies; and discuss how one can identify homologies (if any) between plumages in first and subsequent cycles and among taxa. Many species have novel plumages added into their first plumage cycle; we argue that existing terminology for these plumages is unsuitable and we term them formative plumages attained by preformative molts. Finally, we provide examples of how this modified H-P system can be applied to diverse taxa of birds while reflecting the homology underlying all basic plumage cycles. Our revision validates the flexibility and utility of the H-P system. El Problema del Primer Plumaje Básico: Una Revisión de las Homologías de la Muda y del Plumaje Resumen. Todas las aves tienen patrones de sucesión del plumaje fundamentalmente similares. De este modo, Humphrey y Parkes (1959) propusieron un sistema de nomenclatura (el sistema H-P), basado en homologías, el cual ha sido de uso común en estudios de muda de plumaje en Norte América. Sin embargo, supuestas similitudes análogas entre el primer plumaje básico y el plumaje definitivo básico han confundido las homologías. Muchos plumajes convencionalmente conocidos como “primer básico” son considerados mejor como plumajes originales del primer ciclo que carecen de contrapartes homólogas en los ciclos siguientes. Consecuentemente, la nomenclatura actual no refleja las homologías entre especies. Howell y Corben (2000b) propusieron que el tradicional plumaje juvenil puede ser considerado como un punto de partida inequívoco para una terminología que refleje mejor las homologías presuntas en los plumajes básicos; los plumajes alternos y otros plumajes no básicos pudieran no ser homólogos entre especies. Cuatro estrategias de creciente complejidad incorporan todos los patrones conocidos de sucesión de plumajes: La Estrategia Básica Simple, La Estrategia Básica Compleja, La Estrategia Alterna Simple, y La Estrategia Alterna Compleja. Examinamos ciertas inconsistencias en el sistema H-P; explicamos las cuatro estrategias subyacentes, y discutimos cómo se pueden identificar homologías (cuando existen) entre los plumajes del primer ciclo y de los ciclos siguientes, y entre taxa diferentes. Muchas especies tienen plumajes originales adicionales en su primer ciclo de plumaje; sostenemos que la terminología actual para estos plumajes es inadecuada y los denominamos como plumajes formativos, logrados por mudas preformativas. Finalmente, damos ejemplos de como este sistema H-P modificado puede ser aplicado a diversos tipos de aves y al mismo tiempo reflejar la homología subyacente a todos los ciclos de plumajes básicos. Nuestra revisión valida la flexibilidad y utilidad del sistema H-P.
Article
Data from a banding station at Guantanamo Bay and the examination of 830 specimens were used to study molt patterns and criteria for determination of age and sex in 15 resident species of Cuban landbirds. All 15 species undergo their prebasic molt in August to November, with a few species commencing as early as May and/or completing as late as December. This timing corresponded with that of related taxa found in North America. With few exceptions, the extent of the first prebasic molts was also comparable to those of related taxa found at northern latitudes, being partial to incomplete in all nine passerines and four of six nonpasserines. Three species showed evidence of eccentric primary molt patterns during the first prebasic molt, to be expected in birds residing in scrubby or exposed environments such as those found at Guantanamo Bay. Determination of age is possible in most Cuban landbirds based on molt limits and the shape and condition of the primary coverts and rectrices, criteria very similar to those found in North American species of related taxa.
Article
Tropical birds offer unique opportunities to test ecological and evolutionary theory because their life history traits are so diverse and different from temperate zone models upon which most empirical studies are based. We review recent studies on the behavioral ecology of tropical birds, studies that explore new advances in this field. Life histories and their evolution remain the focus of research on tropical birds. Clutch size manipulations in two species showed that food limitation does not explain small clutch size. In antbirds, enlarged clutches decreased post-fledging survival whereas in thrushes, enlarged broods were costly due to high nest predation. Small clutches may be favored via different ultimate selective forces and shared underlying tradeoffs between the immune, metabolic, and endocrine systems in the body may account for the commonly observed ‘slow pace of life’ in tropical birds. The physiological tradeoff between testosterone and immunocom- petence may explain the evolution of low testosterone levels in tropical passerines where adult survival is paramount. In contrast to life history theory, few studies have explored temperate-tropical differences in terri- toriality, mating systems, and song function. The idea that low breeding synchrony in tropical birds is associated with low levels of extra-pair fertilizations was supported by several new paternity studies conducted on tropical passerines. Seasonally breeding tropical birds have higher testosterone levels than tropical birds with prolonged breeding seasons, although it is unclear if this pattern is driven by mating systems per se or selection from pathogens. Recent work on relations between pair members in permanently paired tropical passerines focuses on the question of mate defense versus territorial defense and the extent of cooperation versus selfish interests in inter-sexual relations.