A neotropical forest bird can measure the slight
changes in tropical photoperiod
Michaela Hau
1*,2
, Martin Wikelski
1,2
and John C. Wing?eld
1
1
Department of Zoology, University ofWashington, Seattle,WA 98195, USA
2
SmithsonianTropical Research Institute, Balboa, Republic of Panama
?
Many tropical birds breed seasonally, but it is largely unknown which environmental cues they use to time
reproduction. Changes in tropical photoperiod have been regarded as too small to be used as a proximate
environmental cue. This hypothesis, however, has never been rigorously tested. Here, we report on experi-
mental evidence that photoperiodic changes characteristic of tropical latitudes stimulate reproductive
activity in a neotropical bird from the forest understory. In the central Republic of Panama
?
(98N), photo-
period varies annually between 12 hours (December) and 13 hours (June). Free-living spotted antbirds
(Hylophylax n. naevioides) had regressed gonads in December, but increased gonads ahead of the rainy (the
breeding) season in May. Captive spotted antbirds exposed to a `long' photoperiod of 13 hours increased
gonadal size eight-fold and song activity six-fold over that of control birds remaining on a simulated s`hort'
photoperiod of 12 hours of daylight. Moreover, even a photoperiod of 12 hours 17 minutes was su?cient to
stimulate gonadal growth in photostimulated birds over that of controls. The dramatic changes in gonadal
development were not accompanied by similar changes in hormone titres such as luteinizing hormone and
testosterone as expected from temperate zone birds.We propose a more general role of the tropical photo-
period in the regulation of seasonal events in tropical organisms, or in temperate zone species migrating to
the tropics.
Keywords: neotropical bird, seasonal breeding, tropical photoperiod, avian reproduction, Hyloph
naevioides
1. INTRODUCTION
Evidence is accumulating that many, if not most, tropical
birds show a distinct seasonal pattern in life history para-
meters such as breeding activity (for overviews, see
Moreau 1950; Skutch 1950; Voous 1950; Snow & Snow
1964; Fogden 1972; Stiles 1980; Bell 1982; Dittami &
Gwinner 1990; Tye 1991). Di?erences in food availability
and in the degree of nest predation have been suggested
to act as ultimate factors, concentrating avian breeding in
the tropics to certain parts of the year (e.g. Skutch 1950;
Morton 1971; Fogden 1972; Snow 1976; Sinclair 1978;
Grant & Boag 1980; Worthington 1982; Poulin et al. 1992;
Young 1994; Komdeur 1996). In contrast, the proximate
environmental factors that regulate breeding events in
tropical birds are still largely obscure. How do birds
accomplish seasonal breeding in the tropics, an environ-
ment regarded as having a comparatively low seasonality?
In the temperate zones, most birds respond hierarchi-
cally to the two main types of proximate environmental
information to regulate breeding on a seasonal basis:
long-term cues such as photoperiod (`initial predictive
information', Wing?eld 1980) govern the gross hormonal
and gonadal changes necessary for the transition between
states of reproductive inactivity and activity (e.g. Murton
& Westwood 1977; Farner & Follett 1979; Ball 1993;
Wing?eld & Farner 1993), whereas short-term cues such
as temperature, food, nest site availability, etc., mediate
the more subtle physiological changes that lead to the
actual reproductive events (`supplementary information',
Wing?eld 1980; Wing?eld et al. 1992; Ball 1993; Nager &
van Noordwijk 1995).
In view of the slight photoperiodic changes in the
tropics (and their weak temporal association with seasonal
climatic changes), it has been suggested that tropical birds
might not be able to use daylength as a long-term cue to
control breeding (e.g. Voous 1950; Miller 1959, 1965;
Cockrem 1995). Instead, in many tropical bird species
investigated so far breeding activity has been linked to
short-term changes in food availability, possibly signalled
by rainfall (Ward 1969; Jones & Ward 1976; Fogden &
Fogden 1979). In this scenario, tropical birds would use
fundamentally di?erent environmental signals from many
of their temperate zone counterparts (see alsoWing?eld et
al. 1992).
However, the dismissal of the tropical photoperiod as a
proximate cue for timing life history events in tropical
birds has been based on a few and sometimes inconclusive
experiments. Some tropical bird species have been tested
for their capability to react to changes in daylength, but
these studies often involved an exposure of tropical birds
to exaggerated subtropical or even temperate zone photo-
periods (Rollo & Domm 1943; Miller 1959, 1965; Wolfson
& Winchester 1959; Disney et al. 1961; Epple et al. 1972;
Chandola & Chakravorty 1982; Gwinner & Dittami
Proc. R. Soc. Lond. B (1998) 265, 89^95 89 & 1998 The Royal Society
Received 5 September 1997 Accepted 6 October 1997
*
Author and address for correspondence
(hau@zoology.washington.edu).
1985; Tewary & Dixit 1986) and so did not investigate the
possible role of the tropical photoperiod. Other experiments
had very small samples sizes (Rollo & Domm 1943), were
lacking a control group for a non-intentional experiment
on the e?ect of the tropical photoperiod (Disney et al.
1961), or yielded similar reactions in experimental and
control birds (Rollo & Domm 1943; Marshall & Disney
1956; Lofts 1962).
Therefore, the present experiments were designed to test
whether a neotropical bird species, the spotted antbird
(Hylophylax n. naevioides) from a lowland moist forest of
central Panama (98 N 798 W), can perceive the small
changes in photoperiod of its natural habitat. Spotted
antbirds (Family: Thamnophilidae) are small (17 g) subos-
cine passerines with a presumably purely neotropical
phylogenetic history (Sibley & Monroe 1990). They are
facultative followers of army ant swarms in the forest
understory, where pairs defend year-round all-purpose
territories (Willis 1972). In central Panama? , spotted
antbirds breed within the rainy season (May
^
October;
Willis 1972; Sieving 1992), which lasts on average from 19
December to 4 May. To investigate the photoperiodic
sensitivity of spotted antbirds, we monitored two physiolo-
gical measures that are direct indicators of breeding state
(e.g. Murton & Westwood 1977; Wilson & Donham 1988;
Ball 1993;Wing?eld & Farner 1993): (i) gonadal size, and
(ii) plasma levels of the reproductive hormones, lutei-
nizing hormone (LH) and testosterone (T). In addition,
as a behavioural parameter, song activity was recorded.
To put the experimental data into the perspective of the
natural situation, we studied the gonadal development in
a free-living population of spotted antbirds.
2. METHODS
In February 1996, we mist-netted 28 spotted antbirds and
randomly divided them into two groups, each of eight males and
six females. Following capture, the birds were immediately
housed in individual cages (35 cm630 cm630 cm) with water
and food provided ad libitum. Experimental rooms (two adjacent
wooden rooms in the basement of an apartment building) were
built light-tight.
Photoperiodic conditions were simulated as closely as possible
to the natural situation and adjusted to measurements taken at
our ?eld site on Pipeline Road in Soberania National Park,
Republic of Panama
?
. For this, a light sensor (photoresistor,
Conrad Electronic, M?nchen, calibrated for lux measurements)
with a lower sensitivity threshold of about 0.2 lux was mounted
in the forest understory onto a horizontal liana stem (1m from a
25 cm DBH tree) at a height of about 1.5m. Data were stored on-
site by a HOBO logger (Onset Inc., Pocasset) and expressed as
seven-day running means (?gure 1).
Experimental photoperiods included two twilight periods and
light illumination was always indirect (all light sources were
mounted behind partly perforated cardboard shields). Two
broad spectrum ?uorescent light tubes (Phillips 40W, 1m long)
lighted each experimental room with about 170 lux (measured at
cage level) during the day. No light was given during the night.
Twilight of 15min duration was provided in the morning and in
the evening by one incandescent light bulb (Phillips 25W, about
0.2 lux). Temperature inside the rooms averaged 28 8C, which
approximates to the mean temperature of the birds' natural
habitat. Fans continuously ventilated the air. Fresh food was
o?ered twice a day and consisted of freshly made egg food
(following Gwinner et al. 1987), live mealworms, and juvenile
crickets. Spare mealworms and crickets were kept in dark
containers in a distant room (and thus could not convey environ-
mental information to the birds).Water was renewed daily, cages
were cleaned every 3^4 days.
After being initially kept under a s`hort' photoperiod of 12 h for
at least eight days (maximally 18 days, depending on time of
capture), one group was transferred to a `long' photoperiod of
13 h on 20 February. The other group remained on 12 h of light
per day. Gonad sizes of all birds were determined before, and
then four and seven weeks after photostimulation (see below). At
these times, birds were also weighed and had a blood sample
taken for subsequent analysis of plasma titres of LH and T.
Another small blood sample for LH analysis only was taken on
29 February. For a second experiment, the 12 h group was
randomly divided into two groups on 5 April. Five birds were
exposed to a photoperiod of 12.28 h (a 17min increase in photo-
period), the others remained on 12 h light per day. After three
weeks, gonadal development of all birds was examined.
Throughout the experiment, 30min of song activity were
measured at regular intervals in the morning (between 06:00
and 07:00 h) with Dictaphones. The birds were released after
termination of the experiment.
Due to a delayed shipment of live crickets to Panama
?
, the birds
could not be given crickets during a four-day period (in March,
see arrow in ?gure 2c) in experiment 1. To replicate and experi-
mentally induce the observed drop in song activity, in the second
experiment the birds were deprived of crickets for a day at the
times indicated by arrows in ?gure 3c.
At approximately monthly intervals, free-living spotted antbirds
were caught in mist nets within their territories, laparotomized
under anaesthesia (see below), and released again. Many indivi-
duals were caught and measured repeatedly.
Length and width of the left testis and diameter of the largest
follicle were measured to the nearest 0.1mm below 1mm length
and to the nearest 0.2mm above 1mm length by unilateral lapar-
otomy under Iso?urane anaesthesia (for details on standard
laparotomy procedures, see Wing?eld & Farner 1976). Testis
volume was calculated using a formula for ellipsoid cylinders (4/
3 pa
2
b, where a is half the testis length and b is half the testis
90 M. Hau and others Neotropical bird can measure tropical photoperiod
Proc R. Soc. Lond. B (1998)
Figure 1. Weekly running means of climate parameters at the
study site in Soberania National Park, Republic of Panama? ,
from December 1995 through July 1996. (a) Daylength; (b)
light intensity.
width). Blood samples were obtained by puncturing a super?cial
wing vein with a 26-gauge needle. Blood was collected in hepar-
inized microcapillaries and kept cool until centrifugation.
Plasma was separated, stored at 720 8C and transported to
Seattle on ice for hormone analysis.
LH was measured using the post-precipitation, double-anti-
body radioimmunoassay (RIA) for avian LH developed by
Follett et al. (1972) and Sharp et al. (1987). Plasma levels of T
were measured with an indirect RIA. Aliquots of about 100 ml
plasma were equilibrated with 2000 cpm of
3
H-testosterone over-
night at 4 8C for the determination of extraction e?ciency.
Samples were then extracted with 4ml of dichloromethane,
dried down in a 40 8C water bath under nitrogen gas, and redis-
solved in 550 ml of bu?er. Samples were allowed to equilibrate
with bu?er overnight at 4 8C. 200 ml fractions were taken for
duplicates used in the RIA as described previously (e.g. Wing-
?eld & Farner 1975). Fractions (100 ml) were directly counted for
the determination of recovery. Mean (+s.d.) recovery was
77.2+7.5%. Two 400 ml aliquots of distilled water (water blanks)
and a total of four 400 ml aliquots containing either 0.15, 0.25, or
0.5 ng of non-radioactive T standards were taken through the
whole assay procedure to estimate non-speci?c interference,
assay accuracy and intra-assay variation. Blanks were usually
below detection limit, accuracy of the T standards was
10+0.8%, and intra-assay variation was 13%. Assay sensitivity
was at 0.15 ngml
71
(2 s.d. from lowest dilution). Since many
samples in the Tassay were below the detection limit, they were
set at 0.15 ng ml
71
as the highest possible value for a conservative
estimate for statistical comparisons.
We obtained continuous gonadal data from 22 experimental
birds (6/8 male, 5/3 female, control/photostimulated birds,
Neotropical bird can measure tropical photoperiod M. Hau and others 91
Proc R. Soc. Lond. B (1998)
Figure 2. Changes in (a) testis volume (mean + s.e.), (b)
follicle diameter (mean+s.e.) and (c) song activity of spotted
antbirds during the ?rst experiment. In panels (a) and (b)
gonadal development of experimental birds is compared to
that of a free-living population (mean+s.e., s.e. given only
when sample size 53). Open symbols, 12 h group; closed
symbols, 13 h group; half-closed symbols, free-living popula-
tion. Vertical solid line indicates time of photostimulation of
the 13 h group. The arrow indicates lack of crickets in the
birds' diet.
Figure 3. Changes in (a) testis volume, (b) follicle diameter
and (c) song activity of individual spotted antbirds before and
after photostimulation of the 12.28 h group during the second
experiment. Closed symbols, solid lines, 12.28 h individuals;
open symbols, broken lines, 12 h individuals. Arrows indicate
lack of crickets in the birds' diet.
respectively). Sample sizes for other measurements di?er because
not all variables could be measured in all individuals at all times
(body mass, 6/8 males, 5/6 females; LH, 5/8 males, 3/4 females;
T, 6/8 males, 5/6 females, respectively). Data were processed and
analysed with SPSS for Windows (SPSS Inc., Chicago), using a
repeated measures ANOVA after testing for normal distribution
with a Kolmogorov
^
Smirnov test, unless stated otherwise. Data
from male and female birds were always analysed in separate
statistical tests. Gonadal data from the second experiment were
analysed using a one-tailed Sign test. For free-living birds, a
repeated measures ANOVA was conducted on the data from
seven males that were caught repeatedly (at least three, and up
to ?ve times) during December 1995 to July 1996 (the signi?-
cance level was adjusted by the lower-bound epsilon to correct
for an apparent violation of the sphericity assumption). If an
individual could not be caught during one period, the missing
data point was substituted by the mean of the population during
the respective capturing period (since the population was closely
synchronized, see below).
3. RESULTS
A minimal daylength of 11.94 h was measured at the
beginning of January 1996, whereas maximal daylength
in the second week of June 13 was 12.96 h (?gure 1a). It is
noteworthy here that seasonal changes in photoperiod in
the deep tropics do not vary symmetrically around 12 h.
Highest light intensity levels (6.1E d
71
) were recorded at
the beginning of January 1996, and lowest light intensities
(1.0 E d
71
) in June and July 1996 (?gure 1b).
Gonads of all birds were small following capture from
the wild (?gure 2a,b). However, after photostimulation,
the testis volume of 13 h-males increased eightfold over
that of 12 h-males, an e?ect clearly visible already four
weeks after photostimulation (?gure 2a). Follicular
diameter of 13 h-females increased more than threefold
over that of 12 h-females (?gure 2b). In both males and
females, gonads of photostimulated birds were larger than
those of controls (males, F
1,12
?85.92, p?0.003; females,
F
1,6
?6.4, p50.05). Only gonadal sizes of photostimulated
birds increased over time (males, F
2,24
?13.7, p50.001;
females, F
2,12
?13.12, p?0.001), and di?ered from initial
sizes already after four weeks (least signi?cant di?erence
post hoc test, both males and females p50.01). Both males
and females sang in captivity. Song activity of all birds in
the 13 h-group increased sixfold, whereas no change was
observed in the 12 h-group (?gure 2c).
Gonadal sizes of free-living spotted antbirds were deter-
mined during the periods (with sample sizes for males/
females, respectively) from 20 December 1995
^
11 January
1996 (14/2), 12 February
^
4 March (13/4), 20 March
^
4
April (7/6), 7 April
^
24 April (8/2) (?gure 2a,b). In free-
living males, minimal gonadal sizes were found in
December 1995, maximal gonadal sizes were reached in
April 1996 (F
1,30
?127.42, p?0.011). Gonadal sizes were
monitored throughout July 1996, but no further increase
beyond the April values was detected. Active nests of
spotted antbirds were ?rst found at the beginning of May
1996 at the start of the rainy season (T. R. Robinson,
personal communication and observations). Male photo-
stimulated birds reached larger maximal gonadal sizes
than free-living conspeci?cs during the breeding season
(t-test, p50.01, see ?gure 2a).
In the second experiment, the size of gonads in all
12.28 h-birds increased (p?0.02; ?gure 3), as did song
activity as a response to an increase in photoperiod of
only 17 min. The control group showed no changes
(p40.05).
The body mass of all birds increased during the ?rst
experiment (F
2,42
?38.22, p50.001; table 1). There was no
di?erence in male body mass between the two treatment
groups (F
1,12
?0.23, p40.6), but there was a trend in photo-
stimulated females to gain more weight than control
females (F
1,9
?5.15, p?0.05).We did not obtain continuous
data on body mass during the second experiment. We
could not detect signi?cant di?erences in male LH levels
between the two groups (table 1; F
1,11
?4.38, p?0.06), nor
within the two experimental groups (for all tests p40.4).
Likewise, there were no di?erence in LH levels in females
in any of the tests (for all tests p40.1). We did not obtain
enough samples for the analysis of the LH orT pattern in
experiment 2. T levels of all birds were low, most samples
being below or around the detection limit (table 1). There
were no di?erences inTover time nor within groups (table
1; males, for all tests p40.2; females, for all tests p40.3).
4. DISCUSSION
We conclude that spotted antbirds are able to perceive
the one-hour di?erence between the longest and shortest
photoperiod of their tropical habitat. After only four
weeks of exposure to an increase in daylength of one
hour, both males and females showed dramatically
increased gonadal sizes and song activity over their initial
values and over those of control and free-living birds. Even
more surprising, these birds were able to respond physio-
logically and behaviourally to an increase in photoperiod
of as little as 17 minutes. Such a sensitivity is extremely
high and has, to our knowledge, not been reported in
tropical species before. These results suggest that a bird
species from the forest understory in the deep tropics
possesses both the sensitivity and the physiological
mechanism to exploit natural changes in tropical photo-
period as long-term seasonal information.
Gonadal size changed more dramatically after the one-
hour prolongation of the experimental photoperiod as
compared to the 17 minutes increase in daylength.
However, the duration of photostimulation was also
shorter in the second than in the ?rst experiment (three
versus seven weeks). Nevertheless, these data provide
strong evidence that spotted antbirds can respond to very
small photoperiodic changes because all photostimulated
birds increased gonadal size and song activity. Despite
the incomplete acoustic isolation of our experimental
groups from each other or from the outside, no social (e.g.
song) or other non-photoperiodic stimulation of the birds
could be detected.
Our ?eld data show that the remarkable photoperiodic
capacity of spotted antbirds observed in an experimental
situation does not remain a hypothetical mechanism. If
the birds measure photoperiodic changes in the wild, we
would expect them to undergo regular seasonal changes
in gonadal state, i.e. to regress gonads during the non-
breeding season as well as grow them again in advance of
the breeding season, in a similar way to that of many
temperate zone birds. Indeed, free-living spotted antbirds
92 M. Hau and others Neotropical bird can measure tropical photoperiod
Proc R. Soc. Lond. B (1998)
had uniformly regressed gonads during the non-breeding
season and enlarged gonads ahead of the breeding season
(see ?gure 2a,b). The ?nding that spotted antbirds started
to increase gonadal sizes 1^2 months before the rainy (i.e.
the breeding) season provides an additional indication that
they make use of long-term environmental information in
the wild.
Three observations from the present experiments
suggest that food stimuli may serve as supplementary
information for spotted antbirds, ?ne-tuning breeding
events with local environmental circumstances (Wing?eld
1980;Wing?eld et al. 1992). (1) There was a consistent drop
in song activity in both experiments when crickets were
omitted from the birds' diet (see arrows in ?gures 2c, 3c).
(2) Maximal gonadal sizes of birds experiencing a 13-
hour photoperiod exceeded those of free-living conspeci-
?cs (?gure 2a,b). (3) The slight gonadal development of
control birds might have been caused by improved food
availability in captivity (since body mass of all birds
increased throughout the ?rst experiment; table 1). In
contrast, gonadal growth of 12-hour birds could have also
been caused by endogenous annual factors (Gwinner
1986). These hypotheses are testable and will be subject to
further studies.
The strong gonadal responses to photostimulation were
not re?ected by similar changes in plasma hormone titres
of spotted antbirds. At present, we are not able to fully
interpret these hormonal results, but four physiological
mechanisms are possible: (1) captivity suppresses
hormonal responses in spotted antbirds, as has been
shown for some temperate zone birds (Wing?eld &
Moore 1987; Wing?eld et al. 1992); (2) tropical birds have
evolved di?erent pathways for the transduction of environ-
mental information (especially photoperiod) into
physiological signals
?
these pathways might not involve
the hormones identi?ed in temperate zone birds; (3)
spotted antbirds possess similar endocrinological pathways
to those of temperate zone birds but the speci?c hormones
exert di?erent e?ects and functions (see, also, Levin &
Wing?eld 1992); and (4) spotted antbirds employ similar
physiological mechanisms to those of temperate zone
birds, but changes in hormone levels occur at lower
magnitudes. Previous ?eld studies on hormonal processes
in tropical birds indeed found similarities between tempe-
rate zone and tropical birds, withT being lower in tropical
than in temperate zone species (see summaries in Dittami
& Gwinner 1990; Levin & Wing?eld 1992). Further
studies are needed to understand the hormonal processes
that transduce environmental information (such as photo-
periodic changes) into physiological signals in tropical
birds. For example, follicle-stimulating hormone (FSH)
might be a better predictor of gonadal development than
LH in spotted antbirds.
The high sensitivity of tropical birds to small-scale
changes in photoperiod allows a precise seasonal time
measurement and appears advantageous for organisms
that live in seasonal environments. Other tropical organ-
isms probably use similar strategies. A sensitivity to the
slight changes in the tropical photoperiod seems to
underlie the sexual maturation in juveniles of a tropical
mammal (Wayne & Rissman 1991), the gonadal develop-
ment of an insect (Tanaka et al. 1987), and the reproductive
activity in two poecilid ?sh species (Burns 1985). Although
in a di?erent experimental context, African stonechats
(Saxicola torquata axillaris) are capable to distinguish photo-
periods di?ering by 0.55 hours, as they only express
circannual rhythmicity under 12.25 hours, but not under
12.8 hours (Gwinner 1996).
It is so far not generally appreciated that even temperate
zone organisms can show high sensitivities towards changes
in daylength. Such has been reported for circadian responses
of temperate zone passerines (Wever 1967), photorefractori-
ness in house sparrows (Passer domesticus; Dawson 1991), the
photoperiodic threshold for the expression of annual
rhythms in European starlings (Sturnus vulgaris, e.g. Hamner
1971; Schwab 1971) and photoresponsiveness in Syrian
hamsters (Mesocricetus auratus; Heideman & Bronson 1993).
For temperate zone organisms, precise photoperiodic time
measurement could be equally valuable for an exact timing
of reproduction as well as for other life history events. For
example, photoperiod has been reported to be the only
environmental factor that a?ects the time-course of the
Neotropical bird can measure tropical photoperiod M. Hau and others 93
Proc R. Soc. Lond. B (1998)
Table 1. Body mass (g, mean+s.e.), and plasma titres of LH and T (ngml
71
), median and inter-quartile ranges (in brackets) of
control and photostimulated spotted antbirds
control photostimulated
before 10 days after 4 weeks after 7 weeks after before 10 days after 4 weeks after 7 weeks after
(a) males
body mass (g) 16.72+0.41
?
16.42+0.62 18.72+1.04 16.25+0.31
?
16.52+0.33 18.09+0.55
LH (ng ml
71
) 1.28
(0.91/2.33)
1.07
(0.87/1.47)
2.02
(1.47/2.74)
1.4
(0.64/1.73)
1.58
(1.07/3.56)
2.32
(2.0/2.78)
1.91
(1.78/2.35)
1.91
(1.55/2.39)
T (ng ml
71
) 0.15
(0.15/0.18)
?
0.15
(0.15/0.15)
0.15
(0.15/0.15)
0.17
(0.15/0.28)
?
0.15
(0.15/0.31)
0.15
(0.15/0.18)
(b) females
body mass (g) 16.8+0.35 15.88+0.28 17.43+0.56
?
16.7+0.41 16.14+0.52 19.22+0.42
?
LH (ng ml
71
) 0.99
(0.95/1.18)
1.34
(1.05/1.65)
1.16
(0.79/1.6)
1.14
(0.69/3.22)
2.18
(1.06/3.68)
1.46
(2.0/2.18)
1.3
(1.1/1.58)
1.4
(1.01/1.43)
T (ng ml
71
) 0.15
(0.15/0.15)
?
0.15
(0.15/0.15)
0.15
(0.15/0.15)
0.15
(0.15/0.15)
?
0.15
(0.15/0.15)
0.15
(0.15/0.15)
endogenous migratory programme of long-distance
migrants such as garden warblers (Gwinner 1996). If the
tropical photoperiod could be perceived by migratory
birds, it would be a valuable mechanism to ?ne-tune initia-
tion and termination of migration in tropical latitudes. The
measurement of the tropical photoperiod could enable a bird
to truncate its endogenous migratory time programme once
it reached the appropriate latitude of its wintering site. Like-
wise, birds that happened to have wintered too far south in
the tropics could use the local photoperiod to accelerate the
initiation of spring migration (Gwinner 1996).
We thank E. Gwinner,W. D. Robinson,T. R. Robinson, J. Haber-
setzer, A. S. Rand, N. G. Smith, family C. Carrasco, B. Poulin,
G. Lefebvre, L. Erckmann, E. S. Morton and the sta? at STRI
for invaluable help during the study, and P. R. Grant for the idea
of using a 15 minute light increase. Four anonymous referees
provided helpful critiques on the manuscript. IN.RE.NA.RE.
kindly permitted our work in Soberan|
?
a National Park, Republic
of Panama? . This study was supported by grants from the
Deutsche Forschungsgemeinschaft, the Deutsche Ornithologen-
Gesellschaft and the Max-Planck-Gesellschaft to M.H., by the
Alexander-von-Humboldt Society and the SmithsonianTropical
Research Institute to M.W., and the National Science Foundation
to J.C.W.
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