Proc. R. Soc. B (2012) 279, 3527–3534

* Autho

doi:10.1098/rspb.2012.0743

Published online 30 May 2012

ReceivedAccepted

A tropical bird can use the equatorialchange in sunrise and sunset times to

synchronize its circannual clockWolfgang Goymann1,*, Barbara Helm2,3, Willi Jensen1,

Ingrid Schwabl1 and Ignacio T. Moore4

1Max-Planck-Institut fur Ornithologie, Abteilung fur Verhaltensneurobiologie,

Eberhard-Gwinner-Straße 6A, 82319 Seewiesen, Germany2Fachbereich Biologie, Universitat Konstanz, Postfach 616, 78457 Konstanz, Germany

3Institute of Biodiversity, Animal Health and Comparative Medicine, University of Glasgow,

Graham Kerr Building, Glasgow G12 8QQ, UK4Department of Biological Sciences, Virginia Tech, 2119 Derring Hall, Blacksburg, VA 24060, USA

At higher latitudes, most organisms use the periodic changes in day length to time their annual life cycle.

At the equator, changes in day length are minimal, and it is unknown which cues organisms use to syn-

chronize their underlying circannual rhythms to environmental conditions. Here, we demonstrate that the

African stonechat (Saxicola torquatus axillaris)—an equatorial songbird—can use subtle solar cues for the

annual timing of postnuptial moult, a reliable marker of the circannual cycle. We compared four groups

that were kept over more than 3 years: (i) a control group maintained under constant equatorial day

length, (ii) a 12-month solar time group maintained under equatorial day length, but including a simu-

lation of the annual periodic change in sunrise and sunset times (solar time), (iii) a 14-month solar time

group similar to the previous group but with an extended solar time cycle and (iv) a group maintained

under a European temperate photoperiod. Within all 3 years, 12-month solar time birds were significantly

more synchronized than controls and 14-month solar time birds. Furthermore, the moult of 12-month

solar time birds occurred during the same time of the year as that of free-living Kenyan conspecifics.

Thus, our data indicate that stonechats may use the subtle periodic pattern of sunrise and sunset at

the equator to synchronize their circannual clock.

Keywords: circannual rhythm; equation of time; photoperiod; moult Zeitgeber; life cycle

1. INTRODUCTIONFor seasonal organisms, the annual change in day length

(photoperiod) represents the most reliable predictive cue

for the timing of life-cycle stages. For example, many

birds at temperate and high latitudes show circannual

rhythms that are synchronized by photoperiodic infor-

mation as the main Zeitgeber (a synchronizing external

cue; [1]). This synchronization is crucial for the proper

timing of life-history stages and activities such as

migration, breeding and moult (the annual loss and repla-

cement of feathers that is an essential part of the life cycle

of birds) [2–5]. The strength of photoperiod as a Zeitgeber

is demonstrated by the fact that some species can entrain

to photoperiodic cycles that differ greatly from the

12-month period they are naturally exposed to. For

example, sika deer (Cervus nippon) can entrain antler

growth to photoperiodic cycles of only four-month dur-

ation [6] and European starlings (Sturnus vulgaris) are

capable of adjusting their annual life cycle to photo-

periods with a duration of just 2.4 months [3,7].

Further, organisms can be very sensitive to even subtle

photoperiodic cues: some tropical plants and birds have

been shown to respond to slight and gradual changes in

day length with an amplitude of just 15–17 min [8,9].

r for correspondence (goymann@orn.mpg.de).

3 April 20123 May 2012 3527

However, close to the equator, photoperiod is practically

constant (figure 1) and yet, many local organisms show

strictly annual cycles in breeding and other activities

such as moult [10–13]. It is unknown which environ-

mental cues they use to time and synchronize their life

cycle. Gwinner & Scheuerlein [14] addressed this topic

in a tropical songbird, the African stonechat (Saxicola tor-

quatus axillaris). Stonechats show robust endogenous

circannual rhythms [15], but in nature, their life-cycle

stages occur annually. In Kenya, breeding is limited to

the rains around March to June and is followed by post-

nuptial moult that starts at the end of June and

continues until mid-November [16] (see also phenology

data in figure 1). Gwinner & Scheuerlein [14] demon-

strated experimentally that changes in day light intensity

(i.e. mimicking the annual pattern in cloud cover during

the rainy and dry seasons) may provide synchronizing

cues. However, cloud cover is very variable within and

between years and hence may not be precise. There is,

however, an alternative photic cue that is more predictive

and could be used as Zeitgeber: while day length is con-

stant at the equator, the times of sunrise and sunset

fluctuate in parallel by about 30 min over the year with

two recurring annual maxima and minima (referred to

in astronomy as the ‘equation of time’; figure 1 and sup-

plementary discussion 1 in Borchert et al. [12]). This

annual fluctuation is caused by the elliptic shape of the

This journal is q 2012 The Royal Society

3528 W. Goymann et al. Solar time synchronizes circannual clock

Earth’s orbit around the sun and the tilt of the Earth’s

axis relative to its orbit and corresponds to the difference

between apparent solar time and the mean time. As a

slight but reliable environmental cue, it is consistent

from year to year and independent of climate, but so far

it is unknown whether animals can use this cue to time

their life cycle. It has been suggested that—close to the

equator—tropical trees synchronize flowering according

to the annual change in the time of sunrise and sunset

or associated changes in insolation [12,17]. In view of

the sensitivity of birds to subtle photic cues [9,18], we

hypothesized that they may be able to entrain to the

annual pattern of sunrise and sunset times at the equator

(hereafter called ‘solar time’).

To test this hypothesis, we examined synchronicity in

four experimental groups of African stonechats of equator-

ial origin that were kept under different photic conditions.

We exposed one group to a constant equatorial photo-

period (constant day length) and simulated the annual

variation in sunrise and sunset time at the equator (12-

month solar time group; upper graph in figure 2b). As a

control to assess the birds’ behaviour in the absence of

any Zeitgeber, a second group was kept under the same

constant equatorial photoperiod but without simulat-

ing the annual change in sunrise and sunset times

(constant time group; upper graph in figure 2a). To

further investigate the ability of stonechats to synchronize

to solar time, we exposed a third group to this poten-

tial Zeitgeber but weakened its strength by extending

the Zeitgeber period to 14 months (14-month solar time

group; upper graph in figure 2c). Because postnuptial

moult was previously found to be a reliable marker

for the annual cycle of stonechats [19–21], we used

individual timing patterns of moult as a measure of syn-

chronization. We further compared the synchronicity of

postnuptial moult of these three groups with that of a

fourth group, consisting of African stonechats studied ear-

lier by our team under a naturally changing European

photoperiod (upper graph in figure 3). The published

studies had shown that this photoperiodic condition acts

as strong Zeitgeber [20,22,23] and are therefore used

here as a positive reference for tight synchronization (via

extraction of information on synchronicity from the

original data).

We predicted that whether variation in sunrise and

sunset functioned as a Zeitgeber, moult of birds in the

12-month solar time group should cycle with the Zeitgeber

period length of 12 months and should be synchronous

between individuals as measured by a synchronicity

index (defined as the per cent overlap in the timing of

moult between individuals). Moult in this group should

occur at a similar time relative to the Zeitgeber as it does

in free-living Kenyan conspecifics (i.e. show a similar

phase-angle difference (c) relative to the periodic change

in sunrise and sunset times as in free-living conspecifics

[16] (see also figure 1). In contrast, birds in the constant

time group should free run with respect to each other

and to external time, i.e. there should be little synchroni-

city among birds and no synchronicity to external time.

The 14-month solar time group was used to further exam-

ine possible Zeitgeber characteristics of solar time. If birds

are able to entrain to this extended and weaker cycle (with

smaller changes in solar time per unit of time when com-

pared with the 12-month solar time group), then they

Proc. R. Soc. B (2012)

should synchronize to each other and to a 14-month

cycle, or else, show individual free-running rhythms.

Differences between the three solar time groups should

develop over time, as free-running birds would follow their

individual schedules, so that the last recorded cycles

should yield the clearest contrasts. Furthermore, moult

in birds can proceed independently for body plumage

and flight feathers [24]. In the absence of a Zeitgeber, the

two processes may dissociate, and flight feather moult in

particular can be omitted [14,25,26]. We therefore use

dissociation of the two moult processes as further evidence

of weak or absent entrainment.

2. METHODS(a) Study animals

Originally, we used 24 African stonechats, all of which were

descendants of a resident population imported from

Nakuru, Kenya (08140 S, 36800 E; 2500 m a.s.l.) in 1999.

Six birds hatched in 1999 were collected as nestlings in

Kenya, the other birds were bred and raised in captivity in

2001 (n ¼ 1), 2002 (n ¼ 3), 2003 (n ¼ 7), 2004 (n ¼ 1)

and 2005 (n ¼ 6). Before the experiment, all birds were

maintained and synchronized under the local photoperiod

of Erling-Andechs, Germany (47.58 N). On 5 August

2005, during postnuptial moult, the birds were moved to

three identical indoor rooms. Birds of different sex and age

classes and siblings were equally distributed among rooms

so that these factors would not bias results. Three additional

stonechats entered the experiment in October and Decem-

ber 2005 and in March 2006 (hatched in 1999 and 2004)

to replace birds that had died. The birds received our stan-

dard diet [27] and were kept in individual cages (60 �35 � 45 cm) where they could hear but not see each other

until January 2009. In addition, we used data from 13

African stonechats collected in Kenya that were maintained

under a European photoperiod in the same temperature-

controlled indoor rooms from 1982 to 1984. These African

stonechats were studied from fledging until the completion

of their second adult moult. We include data from five

females and eight males (for details on these birds see earlier

studies [20,22]).

(b) Experimental rooms and treatment

Birds were kept in three adjacent sound-proof and tempera-

ture-controlled rooms connected to the same ventilation

system [18]. Each room was equipped with two halogen

lamps with dimmable 150 W Osram Haloline 646 959

bulbs (colour rendering index 100, colour temperature

3000 K). The lamps were mounted in line with the cages,

so that the birds received light only indirectly from a white

reflecting wall opposite the light source, i.e. birds could not

get any ‘celestial cues’ from the two light sources. Light inten-

sity was approximately 400 lux at the front of each cage.

During the night, several small light bulbs ensured indirect

‘moonlight’ illumination of 0.01 lux. Room temperature in

all rooms ranged between 188C and 208C during the night

and from 22 to 258C during the day. The photoperiod in all

three rooms was 12.07 h of light plus two 25 min periods of

dawn and dusk during which light intensity gradually

increased or decreased from 0.01 (night level) to 400 lux

(day level). These conditions simulated the natural day

length including dusk and dawn (based on civil twilight) at

an elevation of 2500 m a.s.l. at the equator, corresponding

Table 1. Per cent of synchronicity in body moult and flight feather (wing) moult during three consecutive years. Values are

means+95% CIs; values printed in bold are significantly different from the constant time and/or the 14-month solar timegroup (for details, see text and figure 4). Asterisk represents not included, as during juvenile moult flight feathers show noregular moult pattern.

year moult Constant 12-month solar time 14-month solar time European photoperiod

first body 22.0+12.0; n ¼ 7 31.9+11.3; n ¼ 7 15.5+11.6; n ¼ 8 88.9+3.6; n ¼ 13

wing 7.7+7.6; n ¼ 7 43.1+13.7; n ¼ 7 6.6+6.4; n ¼ 5 *

second body 15.0+12.3; n ¼ 6 22.2+19.4; n ¼ 5 25.8+29.1; n ¼ 4 86.5+2.9; n ¼ 13

wing 17.5+14.6; n ¼ 6 38.8+19.4; n ¼ 5 10.2+19.6; n ¼ 4 85.2+4.2; n ¼ 13

third body 18.0+8.4; n ¼ 5 50.4+12.7; n ¼ 5 19.5+23.6; n ¼ 4 84.0+5.0; n ¼ 10

Wing 14.4+9.9; n ¼ 5 53.6+24.7; n ¼ 5 18.5+16.5; n ¼ 4 86.5+4.7; n ¼ 10

Solar time synchronizes circannual clock W. Goymann et al. 3529

to the altitude at Nakuru, Kenya. The first room housed the

constant time group. Here, the timing of sunrise and sunset

was kept constant throughout the year with dawn starting at

06.05 (full daylight at 06.30) and dusk starting at 18.37

(full night at 18.02; figure 2a). Day length and sunrise

times were calculated with Astronomy Lab 2 (Personal

MicroCosms, Greenwood Village, CO 80112, USA). The

second room housed the 12-month solar time group. This

group received a simulation of the annual variation in sunrise

and sunset time at the equator: the onset of dawn started

between 05.51 and 06.22 (full day light between 06.16 and

06.47) and onset of dusk between 18.23 and 18.54 (night

between 18.48–19.19; figure 2b). In the third room, birds

received a simulation of the annual variation in sunrise/

sunset time, but the natural cycle of 12 months was length-

ened to 14 months (14-month solar time group; figure 2c).

The changes in sunrise and sunset times were implemented

in weekly intervals in steps ranging from 0 to 3 min depend-

ing on the steepness of the cycle (figures 1 and 2). In

addition, we compared the data from these three groups

with a fourth group of 13 African stonechats that had been

maintained under the photoperiodic conditions of migratory

European stonechats at 47.58 N during summer and 408 N

during winter in a previous study (see earlier studies

[20,22,23]). In these birds, day length was adjusted weekly

assuming that they reach 458 on 4 October 42.58 on 11 Octo-

ber and 408 on 18 October (figure 3). In spring, birds were

assumed to reach 42.58 on 28 February, 458 on 7 March

and 47.58 on 14 March. In these birds, there was no simu-

lation of dawn and dusk, but light intensity was similar to

the other three groups with 0.01 lux at night and 400 lux

during the day.

To examine entrainment to the change in sunrise and

sunset time, we quantified the phase-angle difference (c) of

moult onset to the putative Zeitgeber cycle as follows.

As reference phase, we chose the date on which sunrise

and sunset were timed the latest (Julian date 41, i.e. 10

February; figure 1). Based on published field data from

free-living Kenyan stonechats, moult started on 30 June (cal-

culated as the mean date between captures of free-living

stonechats that did not yet moult and the first captures that

showed substantial moult (see our fig. 1 and fig. 4d,e in

Dittami & Gwinner [16])). This yielded a phase-delayed

and, hence negative, phase-angle difference c of 2141 days.

(c) Moult parameters and statistical analysis

For the duration of the study, every two to three weeks we

recorded—for each bird—the state of moult for body and

flight feathers separately. We assessed the state of body

Proc. R. Soc. B (2012)

moult by noting feather growth in any of the 19 different

feather tracts on the bird’s body [28]. We defined the onset

of body moult as the mean date between the last recording

without moult and the first day on which moult was detected

in at least four of the 19 feather tracts. End of moult was

defined as the mean date between the last recording with

moult finished in at least 15 of the 19 feather tracts and the

first day without moult. Flight feather moult was scored fol-

lowing [29]. All 10 primaries and all nine secondary wing

feathers were scored on a scale from 0 (old) to 5 (fully

grown new). Scores were averaged over the two wings result-

ing in values ranging from 0 (all feathers old) to 95 (all

feathers new). Onset of flight feather moult was defined as

the mean date between the last recording without flight

feather moult and the first day on which at least one primary

or secondary moulted, and completion was defined as the

mean date between the last recording with moult and the

first recording without moult. For each year and within each

group, we then measured the proportion of overlap in moult

of each individual with each other individual. An example

illustrates how the proportion of overlap in moult was calcu-

lated: let us assume we have two individuals A and B. A

moults from day 50 to 100, and B moults from day 75 to

150. This means that the moult of individual A overlaps

during 50 per cent of its total duration (days 75–100) with

the moult of individual B. However, the moult of individual

B overlaps only 33.3 per cent of its total duration (days 75–

100) with that of individual A. From these individual

values, we then calculated the mean for each individual,

which gives the moult synchronicity index of each individual

within its group (figure 4). This moult synchronicity index

was then compared between groups using a non-parametric

Kruskal–Wallis test and Conover post hoc tests [30]. Non-

parametric tests were used because they are more conservative

and with small sample sizes normality tests are not very

reliable. However, parametric tests (generalized-linear

mixed model) rendered similar results. For the calculation,

we used data only for individuals that had entered the exper-

iment at the latest in April 2006 and that had completed all

cycles from the beginning of the experiment until the period

of analysis (first, second and third year). The sample sizes

for the first, second and third year were 7, 7 and 6 for the con-

stant time group, 7, 7 and 5 for the 12-month solar time

group, and 5, 5 and 4 for the 14-month solar time group

(see also table 1). We also created mean body and flight

feather moult curves of individuals within each treatment

group by taking the mean score of a 20-day period. Smooth-

ing was performed by calculating a moving average over five

adjacent 20-day periods. A regular sinusoidal curve with

06.49

06.44

06.39

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of s

unri

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day

leng

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h)

06.34

06.29

06.24

06.190 60 120 180 240 300 360

Julian date (days)

sunrise and day length

y = –141 days

12.11

12.09

12.07

12.05

12.03

Figure 1. The annual change in the time of sunrise (solid redline) and subsequent sunset (not shown) in combination witha constant photoperiod (horizontal dotted black line) atNakuru, Kenya (08000 N, 368000 E). This annual change in

sunrise and sunset is referred to as the ‘equation of time’ inastronomy and is caused by the elliptic shape of the Earth’sorbit around the sun and the tilt of the Earth’s axis relativeto its orbit. The horizontal bars indicate the period during

3530 W. Goymann et al. Solar time synchronizes circannual clock

one peak per year indicates synchronization to an external cue

with the magnitude of the amplitude reflecting the degree of

synchronization among birds within a group: a high ampli-

tude indicates that the birds of one group are synchronous

to each other whereas a low amplitude indicates low synchro-

nicity among birds. In contrast, an irregular curve with

various peaks per year indicates free-running rhythms, i.e.

each bird follows its own endogenous schedule without syn-

chronization to an external Zeitgeber cue.

To calculate the period length t of moult cycles between

the second and third year, we used body moult only, because

several birds of the constant time and 14-month solar time

group showed irregular patterns or skipped flight feather

moult. Period length t for each bird was measured by calcu-

lating the difference between moult completion date in the

third year and moult completion date in the second year.

Statistical comparisons were conducted with SYSTAT 13 at

an a-level of 0.05 (two-tailed). To minimize the loss of

birds along the experiment, we did not measure gonad size

that requires invasive surgery, nor puncture the birds’ veins

for repeated blood sampling.

which free-living African stonechats at Nakuru, Kenya,breed (black bar) and moult their body- (blue bar) andflight feathers (orange bar) [16]. The arrow between thetwo vertical dashed lines indicates the phase-angle difference(c) between the day on which sunrise is latest (Julian day 41)

and the mean date with the first occasion of moult (Julian day182) in free-living African stonechats.

3. RESULTSWe found that during the first cycle, for which we had pre-

dicted weak differences between treatments, the groups

differed significantly in synchronicity of body moult

(Kruskal–Wallis test Hbody moult ¼ 25.80, p , 0.0001).

The body moult was significantly more synchronous

between birds exposed to a European photoperiod

than between birds of all other groups (post hoc Conover

tests: all ps , 0.0001). Birds of the 12-month solar time

group were significantly more synchronous to each other

than birds of the 14-month solar time group (p ¼ 0.01)

and tended to be more synchronous than birds in the con-

stant control group (p ¼ 0.08; table 1, figures 2 and 3).

Flight feather moult also differed significantly in synchro-

nicity between groups (H ¼ 13.03, p , 0.002), being

significantly higher in the 12-month solar time group

than in the control (p , 0.001) and the 14-month solar

time group (p , 0.001; table 1, figures 2 and 3). Birds

maintained under a European photoperiod were not

included in this comparison because their first cycle,

post-juvenile moult, did not involve replacement of flight

feathers (figure 3).

During the second cycle, synchronicity of body and

flight feather moult also differed significantly between

the groups (Hbody moult ¼ 10.55, p ¼ 0.0001; Hflight feather

moult ¼ 22.20, p , 0.0001; figures 2 and 3). Body and

flight feather moult of birds under a European photo-

period was more synchronous than those of birds in all

other groups (all ps , 0.0001). Furthermore, synchroni-

city of flight feather moult was significantly higher in

birds of the 12-month solar treatment than in birds of

the constant control group (p ¼ 0.02) and the 14-month

solar time group (p ¼ 0.007; table 1, figures 2 and 3).

In the third and last year, the synchronicity of both

body and flight feather moult differed significantly

between treatments (Hbody moult ¼ 20.45, p , 0.0002,

Hflight feather moult ¼ 19.59, p ¼ 0.0002): body and flight

feather moults of birds in the 12-month solar time group

were significantly more synchronous than those of the con-

stant time and the 14-month solar time group (figures

2–4, table 1), but less so than in the group of African

Proc. R. Soc. B (2012)

stonechats entrained to the European photoperiod

(figures 3 and 4, table 1).

Furthermore, in contrast to birds of the constant time

and the 14-month solar time group, birds of the 12-

month solar time group showed coupled patterns of

body and flight feather moult in all years (figure 2), indi-

cating a common Zeitgeber for the different kinds of

moult. Another indicator of a common Zeitgeber were

the smoothed averages of body and flight feather moult

in the 12-month solar time group (blue and orange

curves in figure 2b): similar to the European photoperiod

group (figure 3), the 12-month solar time group showed a

regular pattern with one clear sinusoidal moulting peak

per year (figure 2b), indicating synchronization among

birds and within each year. The sinus amplitude of the

European photoperiod group was higher than the ampli-

tude of the 12-month solar time group, demonstrating

that photoperiod is a stronger Zeitgeber than solar time.

In contrast to these two groups, the constant control

group and the 14-month solar time group showed irregu-

lar smoothed average curves without a clear annual peak

in combination with low amplitudes (figure 2a,c), thus

indicating low synchronicity among birds, suggesting

that each bird followed its own free-running rhythm.

Period length of the body moult cycle between the

second and third year was closest to the expected 365

days in birds maintained under a European photoperiod

and in the 12-month solar time group (figure 5). The coef-

ficient of variation (CV) in period length was lowest in the

group maintained under a European photoperiod (CV ¼

0.05), intermediate for the 12-month solar time group

(CV ¼ 0.16) and highest for the groups maintained

under 14-month solar time (CV ¼ 0.34) and constant

time conditions (CV ¼ 0.33). For flight feather moult,

we could not calculate the period length between

06.36

06.5106.4306.3506.2706.19

06.5106.4306.3506.2706.19

m 395f 347f 392f 393f 360

m 355m 323m 327

f 372f 334

m 367m 389f 365f 391

m 358

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first year second year third year

first year second year third year

first cycle second cycle third cycle

(a)

(b)

(c)

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m 375m 390f 364

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200 400 600 800time (days)

1000 1200 1400

Figure 2. Body moult (blue bars) and flight feather moult (orange bars) of African stonechats maintained under an equatorialphotoperiod for almost 31

2years. The thin (brown) lines indicate the total time each individual bird stayed in the experiment.

The time axis represents consecutive days (1 denotes 1 January 2005). The experiment started in August 2005 (day 215) andended in January 2009 (day 1500). Depicted are three groups of birds which either received (a) no change in the timing ofsunrise and sunset (constant time conditions; upper graph in (a), (b) a simulation of the natural, 12-month cycle in the

change of sunrise and sunset times at the equator (12-month solar time conditions, upper graph in (b); only change in sunriseshown, but sunset changes accordingly) and (c) a simulation of the annual pattern in sunrise and sunset times at the equatorstretched to 14 months (14-month solar time conditions; upper graph in c; only sunrise shown). The blue and orange curvesrepresent smoothed averages of body moult and flight feather moult scores of all individuals within one group (represented onthe right y-axis scale; see methods for the details of how the smoothed averages were calculated), respectively. A sinusoidal

curve with one peak per year indicates synchronization to an external annual Zeitgeber with the magnitude of the amplitudereflecting synchronicity among birds within one group. A curve with several peaks per year indicates the absence of synchro-nization to an external Zeitgeber. Please note that owing to differences in scoring method the amplitude of body moult (blue) isnaturally smaller than the amplitude of flight feather moult (orange). The dashed vertical lines indicate the external years in

(a,b) and the prolonged 14 month ‘year’ in (c). (b) Black vertical arrows in the upper graphs indicate the onset of moult infree-living stonechats at Nakuru, Kenya. (c) Vertical arrows indicate the expected onset of moult in case the birds wouldhave synchronized to a 14-month solar time period. Both the vertical dashed lines and the arrows serve as orientation howrhythms should be arranged if they were entrained. Note that all birds were initially synchronized to a European photoperiodbefore they entered the experiment (with most birds still moulting when entering the experiment around day 215). Bird IDs

starting with ‘f ’ represent females, those starting with ‘m’ represent males.

Solar time synchronizes circannual clock W. Goymann et al. 3531

the second and third year for the constant time and the

14-month solar time group, because some birds showed

irregular moult patterns. By contrast, all birds of the

Proc. R. Soc. B (2012)

12-month solar time group showed a consistent pattern

of flight feather moult with a period close to one year

(mean period +95% CI: 360.8+60.3 days, CV ¼ 0.13).

1816141210da

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(h)

indi

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m gr79f gr76

m gr75

m gr69m gr66m gr62m gr60f gr59f gr56

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600 800 1000

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first year second year third year

Figure 3. Body moult (blue bar) and flight feather moult(orange bar) of African stonechats maintained under a Euro-pean photoperiod (upper graph with the red line indicatingthe length of the daily period with lights-on) from hatching(approx. day 115) until the end of their third year of life.

The time axis represents consecutive days (1 denotes 1 Jan-uary 1982). The thin brown line indicates the total timeeach individual bird stayed in the experiment. The blueand orange curves represent smoothed averages of bodymoult and flight feather moult scores (represented on the

right y-axis scale; see methods for the details of how thesmoothed averages were calculated), respectively. A sinusoi-dal curve with one peak per year indicates synchronizationto an external annual Zeitgeber with the magnitude of the

amplitude reflecting synchronicity among birds. A curvewith several peaks per year indicates the absence of synchro-nization to an external Zeitgeber. Please note that owing todifferences in scoring method the amplitude of body moult(blue) is naturally smaller than the amplitude of flight feather

moult (orange). Bird IDs starting with ‘f ’ represent females,those starting with ‘m’ represent males.

(a)

(b)

100p = 0.0005

p = 0.012p < 0.002

p < 0.0001

p< 0.002

constanttime

12-monthsolar time

14-monthsolar time

Europeanphotoperiod

p = 0.004

80

60

% o

verl

ap in

mou

lt

40

20

0

100

80

60%

ove

rlap

in m

oult

40

20

0

Figure 4. (a) Synchronicity of flight feather and (b) bodymoult of equatorial stonechats during the third year of theexperiment (median and interquartile range). Birds main-tained under a simulation of the natural 12-month changein sunrise and sunset times at the equator (12-month solar

time group) showed an intermediate pattern of moult syn-chronicity. They were significantly more synchronous thanbirds kept under equatorial conditions with a constant timeof sunrise and sunset (constant time group), and birds main-tained under a simulation of the change in sunrise and sunset

times extended to a 14-month period (14-month solar timegroup). At the same time, they were less synchronous thanequatorial stonechats maintained under a northern temper-ate European photoperiod (Kruskal–Wallis test with posthoc comparisons following Conover [30]).

3532 W. Goymann et al. Solar time synchronizes circannual clock

Together, these results indicate that birds maintained under

the 12-month solar time condition were more closely syn-

chronized to the 365 day cycle than the birds of the

constant and 14-month solar time period, but less so than

birds maintained under a European photoperiod.

Onset of moult in birds of the 12-month solar time

group was timed similarly as in free-living conspecifics.

For free-living Kenyan stonechats, flight feather and

body moult was observed to start 141 days after the

chosen reference phase (i.e. the day with the latest sunrise

and sunset; figure 1). This negative phase-angle difference

was closely matched by the timing of flight feather moult

in the 12-month solar time group (mean c+95%

CI ¼ 2141+28 days in the first year, c ¼ 2144+57

days in the second year and c ¼ 2138+64 days in the

third year). Onset of body moult occurred slightly earlier

with c ¼ 2129+63 days in the first year, c ¼ 2142+63 days in the second year and c ¼ 2109+43 days

in the third year. The smoothed averages (figure 2b)

indicate that both types of moult were highly synchronized

in birds of the 12-month solar time group. These data

strongly suggest that the photic cues of the 12-month

solar time simulation acted as a Zeitgeber to which the

birds could synchronize.

4. DISCUSSIONOur findings support the hypothesis that the annual

change in the timing of sunrise and sunset is a potential

Zeitgeber for circannual rhythms of equatorial birds. In

the absence of any change in day length, stonechats in

Proc. R. Soc. B (2012)

the 12-month solar time group entrained to the subtle

shift of the solar day and assumed a phase relationship

to the Zeitgeber that was similar to the situation observed

in free-living birds [16]. Compared with the photoperi-

odic cues of a northern temperate photoperiod this

Zeitgeber was relatively weak, potentially allowing other

environmental cues (e.g. cloud cover [14] or slight

changes in the duration of dawn and dusk [18,31]) to

further modify the circannual rhythm. When this subtle

Zeitgeber was further reduced in strength by changing its

period length from a natural 12-month solar time cycle

to a 14-month solar time cycle (and consequently attenu-

ated changes in sunrise and sunset times), entrainment

was absent. Because the stonechats’ endogenous circann-

ual rhythm is usually 9–10 months [15] and entrainment

gets weaker the further the Zeitgeber period differs from

the endogenous period [7] birds in the 14-month solar

800pe

riod

of

body

mou

lt (d

ays) 700

600

500

400

300

200

100

365 days

constanttime

12-monthsolar time

14-monthsolar time

Europeanphotoperiod

0

Figure 5. Mean period length t (+95% CI) of body moult

between the second and the third year of the experiment.The variance around the mean was significantly lower inthe 12-month solar time group than those maintainedunder constant time conditions (for details, see text).

Solar time synchronizes circannual clock W. Goymann et al. 3533

time group may not have been able to adjust their clocks

to this particular Zeitgeber. In fact, the 14-month cycle

even seemed to disrupt the circannual rhythm as the indi-

vidual moult patterns of birds appeared to be less regular

in the 14-month solar time group compared with the con-

stant time group (figure 2). This is intriguing, as it has

been demonstrated before that a circannual cycle can

develop only under permissive conditions, i.e. African

stonechats showed circannual cycles under a constant

photoperiod with 12.25 hrs of light, but 70 per cent of

them ceased to cycle with a constant photoperiod of

12.8 hrs of light without simulated dawn and dusk [25].

Similar to many other longer-lived tropical organisms,

African stonechats at the equator express a highly seasonal

annual life cycle with reproduction during the rainy season

and subsequent moult when the young become indepen-

dent [16,32]. In captivity, African stonechats show

strong and robust endogenous circannual cycles under

permissive conditions [19,21,25], but how these cycles

are synchronized in nature had remained unclear although

changes in day light intensity owing to annual cycles of

cloud cover may play a role [14]. Our data suggest that

the annual pattern of sunrise and sunset time at the

equator may be a sufficient environmental cue to function

as a Zeitgeber. We are aware of only one other experimental

study that examined entrainment to an annual cycle of

sunrise and sunset times. The study species, Anoura geof-

froyi, a tropical bat, failed to synchronize its circannual

rhythm to a nine-month cycle, or in fact to any kind of

photoperiodic simulation [33].

It is highly unlikely that the higher synchronicity of the

12-month solar time group was caused by an external

stimulus other than the change in the timing of sunrise

and sunset. All three experimental rooms were adjacent

to each other, were connected to the same ventilation,

electrical and light-control systems, sex and age classes

were similar between the rooms, and all birds were fed

and cages cleaned at the same time of the day. Thus, any

disturbance would have affected all three rooms equally.

Furthermore, any synchronization owing to vocal inter-

actions (the birds could not see each other!) should have

been similar in all experimental rooms. Before the exper-

iment, all birds were maintained under a European

photoperiod, which represents a very strong Zeitgeber

Proc. R. Soc. B (2012)

cue, potentially explaining why it took the 12-month

solar time birds three cycles to fully synchronize to the

weaker Zeitgeber.

The mechanistic details of circannual rhythms and

their entrainment to Zeitgeber are still poorly understood.

However, in a number of organisms from beetles to mam-

mals [34–36], it has become clear that circannual clocks

respond to photic cues in a phase-dependent way. This

may explain how African stonechats could breed annually

despite the biannual changes in sunrise and sunset times.

Stonechats are highly responsive to slight differences in

photic cues during specific times of year [37], while

during other periods of the year their reproductive

system shows little or no response to stimulatory light

conditions [14]. If accommodated to equatorial light con-

ditions, then circannual phase responses could function

as a filter that enables the birds to respond to only one

annual recurrence of inductive light conditions. Such

putative filter effects of circannual rhythms would fit

well with current thinking of their adaptive significance

[38,39]. Recent research, most visibly in the context of

rapid global change, has documented the fitness costs of

mismatches between seasonal behaviours and environ-

mental cycles. The ability of organisms to avoid such

mismatches hinges on their ability to correctly predict

and anticipate environmental seasonality, not only at

high latitudes but also in seasonal tropical habitats such

as those used by African stonechats [15,16]. On their

Kenyan breeding grounds, the occurrence of rainy and

dry seasons is predictable over a range of several years,

and timely preparation for breeding may give birds a valu-

able head start for maximizing reproductive output

during the short time of high insect availability. Com-

pared with photoperiodically predictable high-latitude

habitats, it is more difficult at the equator to anticipate

the time of highest likelihood of rains and to buffer against

breeding after spurious, short rains. Being able to use

subtle solar cues to predict correctly upcoming seasons

could therefore provide survival and reproductive value,

and thus enhance fitness [39]. As a consequence, selec-

tion should favour animals that are able to predict

periods of favourable conditions to time life cycles such

as reproduction or moult [38].

To our knowledge, this is the first experimental

study demonstrating that a tropical organism can use

the subtle celestial cue of the timing in sunrise and

sunset to set its circannual clock. Because the annual

variation in sunrise and sunset at the equator has pre-

viously been suggested to time the synchronicity in

flowering and bud-breaking in tropical plants [12, 17],

we suggest that the use of solar time may be widespread

among taxonomic groups with strong seasonality within

the tropics.

We thank Andrea Peter and Sonja Bauer for bird care andassistance with the measurements, and Monika Trappschuhfor assistance with data entry. Furthermore, we thankNicole Geberzahn, Michaela Hau and AlexanderScheuerlein for discussions and comments. This study wasfunded by the Max-Planck Gesellschaft, B.H. is supportedby the European Social Fund in Baden-Wurttemberg.I.T.M. acknowledges support from NSF grant no. IOS-0545735. This paper is dedicated to the memory of the lateEbo Gwinner, who encouraged this kind of experiment in1985 [31], and the famous chronobiology institute inErling-Andechs.

3534 W. Goymann et al. Solar time synchronizes circannual clock

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