Comparative Biochemistry and Physiology, Part A 152 (2009) 593–598

Contents lists available at ScienceDirect

Comparative Biochemistry and Physiology, Part A

j ourna l homepage: www.e lsev ie r.com/ locate /cbpa

Physiological variation in Amethyst Sunbirds (Chalcomitra amethystina) over analtitudinal gradient: A seasonal comparison

Claire Lindsay, Colleen Downs ⁎, Mark BrownSchool of Biological and Conservation Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville, Pietermaritzburg, 3209, South Africa

⁎ Corresponding author. Tel.: +27 33 2605127; fax: +2E-mail address: downs@ukzn.ac.za (C. Downs).

1095-6433/$ – see front matter © 2009 Elsevier Inc. Aldoi:10.1016/j.cbpa.2009.01.009

a b s t r a c tD

a r t i c l e i n f o

Article history:

Southern Africa is characte Received 20 November 2008Received in revised form 14 January 2009Accepted 14 January 2009Available online 22 January 2009

Keywords:Altitudinal variationAmethyst SunbirdMetabolic ratesPhenotypic flexibilitySeasonal acclimatization

rised by an unpredictable environment with daily and seasonal temperaturefluctuations, thus posing challenging thermal conditions and increased energetic stress for endothermicvertebrates. Amethyst Sunbirds (Chalcomitra amethystina) are relatively large African sunbirds (15 g). Theyare considered non-migratory and thus have to cope with the temperature changes and physiologicalstresses a new season brings. This study compared altitudinal subpopulations and the seasonal shifts inmetabolic parameters between and within the subpopulations in metabolic parameters. Amethyst Sunbirdswere caught in summer and winter at three different altitudinal subpopulations; Underberg (1555 m asl),Howick (1075 m asl) and Oribi Gorge (541 m asl). Upon capture, metabolic rates of the sunbirds weremeasured indirectly by quantifying oxygen consumption (V̇O2) using flow through respirometry, at 5 and25 °C. Birds then underwent a 6-week acclimation period at 25 °C on a 12 L: 12D cycle. V̇O2 was measuredpost-acclimation at 8 different temperatures (15, 5, 10, 20, 30, 28, 25 and 33 °C), which were orderedrandomly in the experimental protocol to avoid acclimation bias. Experiments were repeated for a winterand summer season. In general, Amethyst Sunbird subpopulations from Underberg and Howick showedhigher post-acclimation resting metabolic rates per temperature in winter than in summer trials. Underbergand Howick subpopulations respectively showed a significant difference between summer and winter V̇O2 at5 and 10 °C. Thermal neutral zones of all of the subpopulations of sunbirds shifted between winter andsummer. Post-acclimation basal metabolic rate of sunbirds decreased significantly by 38.8% from winter tosummer for the Underberg subpopulation, increased by 44.8% for the Howick subpopulation and did notchange significantly for the Oribi Gorge subpopulation (5.8% decrease).

© 2009 Elsevier Inc. All rights reserved.

ACTE

R

1. Introduction

Southern Africa is characterised by an unpredictable environmentwith daily and seasonal temperature variation (Schultz, 1997). Thesefluctuations pose challenging thermal conditions and increasedenergetic stress for vertebrates, especially for endothermic mammalsand birds. Climate can affect birds directly or physiologically, throughits impact on energy maintenance and water balance, and/orindirectly (i.e. ecologically), through its influence on vegetation, foodavailability, photoperiod and thus available foraging time (Carey et al.,1978; Weathers and Caccamise, 1978; Weathers and van Riper, 1982;Cooper, 2000; Cooper, 2002; Lovegrove and Smith, 2003; Crick, 2004).It would be a mistake to regard organisms that are exposed to theexternal environment as being passive to change, since they oftenalleviate the effects of seasonal environment changes as well asexperimental conditions using physiological adjustment or plasticity(Dawson, 2003; Rezende et al., 2004; Cossins et al., 2006). The ability

RET

7 33 2605105.

l rights reserved.

to change is most pronounced in species living in fluctuatingenvironments (Cavieres and Sabat, 2008), which in this case couldinvolve seasonal changes in ambient temperature, or alternativelychanges in mean temperatures over an altitudinal gradient. Suchspecies typically facilitate thermoregulatory homeostasis by someform of seasonal acclimatization (Cooper, 2000; Arens and Cooper,2005; Cossins et al., 2006). As early as 1962, Hart noticed that a strikingfeature of small birdswas their ability towithstand changes in ambienttemperature with very little protection but a greater metabolic cost.Similarly, Swanson and Weinacht (1997) noted that seasonal differ-ences in metabolism are common in small passerine birds. Weathersand van Riper (1982) found that birds show a remarkable degree ofphysiological adjustment to differing climates. Small diurnal birds thatundertake only local movements or are non-migratory, such asAmethyst Sunbirds (Chalcomitra amethystina) (Tree, 2005), must dealwith seasonal changes in ambient temperature and thuswould have toemploy seasonal adjustments in their physiology if they are to reducethe thermal stress placed on them (Withers, 1992; Maddocks andGeiser, 2000). This problem is intensified because small size restrictsthe capacities of these small animals for seasonal insulative acclima-tization (Clemens, 1988).

Fig. 1. Comparison of pre (a) and post-acclimation (b) body masses of Amethyst Sunbirdsubpopulations between sites and seasons.

594 C. Lindsay et al. / Comparative Biochemistry and Physiology, Part A 152 (2009) 593–598

ACTED

Avian basal metabolic rate (BMR) is beginning to be viewed as ahighly flexible physiological trait influenced by environmentalfluctuations, and, in particular, changes in ambient temperatures. Itis obvious from citations that many non-migrants adjust their BMRseasonally, and winter-acclimatized and cold-acclimated birds havebeen known to exhibit higher basal metabolic rates, than thosesummer-acclimatised or warm acclimated birds (Klaasen et al., 2004;McKechnie, 2008; Smit et al., 2008). Recent evidence suggests thatduring winter, BMR of species resident in highly seasonal environ-ments reflects the prevailing conditions immediately beforemetabolicmeasurements (McKechnie, 2008). Summer responses may be lessvaried due to temperature fluctuations remaining within the thermalneutral zone (TNZ). It was predicted that BMR and TNZ would varybetween bird subpopulations from different altitudinal localities, andbetween seasons. In addition, it was predicted that birds fromdifferentaltitudes would respond similarly post-acclimation to a particulartemperature over a period of six weeks. Differences in altitudinalsubpopulations post-acclimation were expected to indicate a differ-ence in acclimation strategies, most likely as a result of adapting to livein different altitudinal and thus thermal environments.

The ecological significance of seasonal acclimatization foranimals living in a non-migratory manner in a changing environ-ment is obvious (Southwick, 1980) and consequently, knowledgeof the plasticity in ecophysiological parameters and the strategiesemployed to cope with variability in food and water availabilityand extreme ambient fluctuations (in terms of seasonal and dailyfluctuations) is important in understanding the survival of birds insouthern Africa. Klaasen et al. (2004) questioned whether physio-logical flexibility in response to ambient temperature variationwas a general feature of the metabolic properties of birds, irres-pective of whether seasonal and daily temperature fluctuationswere extreme. Thus, as most of the studies on small passerineshave been on Holarctic species (McKechnie, 2008), substantiallymore research is required on the seasonal metabolic changes insmall subtropical passerines. The necessity of further research onthe phenotypic flexibility of metabolic rates (in particular BMR),as well as examination of altitudinal differences between sub-populations of the same species, is highlighted elsewhere (Lindsayet al., 2009; in press). However, seasonal differences and possibleplasticity/flexibility of populations in physiological responses be-tween seasons necessitates comparison between winter and summerstudies. Here physiological data fromAmethyst Sunbird subpopulationsfrom KwaZulu-Natal (KZN), South Africa from winter and summerstudies, andpre- andpost-acclimation, in termsof restingmetabolic rate(RMR), BMR, and TNZ were compared.

2. Materials and methods

2.1. Study site, bird capture and maintenance

Amethyst Sunbirds were captured in summer (November 2006–December 2006) and winter (May 2006–June 2006) at three differentlocations in KZN, South Africa, under permit from Ezemvelo KZNWildlife, using mist-nets. Number of sunbirds caught, capture co-ordinates, and capture site altitudes and are shown in Table 1. Studyindividuals were weighed and colour banded to allow for capturelocation and individual identification (Downs and Brown, 2002). None

RETR

Table 1Location and number of Amethyst Sunbirds captured in winter and summer

Location Altitude(m)

GPS co-ordinates No. of sunbirds caught

Latitude Longitude Winter Summer

Underberg 1553 29°47.614S 29°30.319E 9 10Howick 1075 29°28.203S 30°13.316E 6 10Oribi Gorge 541 30°40.067S 30°15.316E 8 10

of the birds used in the experiments showed any indication ofbreeding activity, as indicated by the absence of brood patches.Experimental protocol for metabolic measurements was kept stan-dard between winter and summer trials, and these methods aredescribed in elsewhere (Downs and Brown, 2002; Lindsay et al.,2009). Numbers of sunbirds used in a single trial at each temperaturein winter and summer respectively are described elsewhere (Lindsayet al., 2009; in press).

Weather data was obtained from the South African WeatherService (SAWS) for Shaleburn (representing Underberg), Cedara(representing Howick) and Paddock (representing Oribi Gorge) forJanuary 2004–May 2007.

2.2. Statistical analyses

Descriptive statistics were calculated in STATISTICA (Statsoft, Tulsa,USA) for each subpopulation. Theminimum RMR (Lindsay et al., 2009)at each of the temperatures studied for each individual was used inanalysis to determine change with temperature using GeneralizedLinear Models (commonly called GLM) Repeated Measures Analysis ofVariance (RMANOVA). BMR was calculated by taking the lowest meanRMR per subpopulation. GLM RMANOVA was further used for thecomparison of seasonal V̇O2 measurements between populations atdifferent altitudes, between pre- and post-acclimation data and forbody mass comparison Post-hoc Scheffé tests were done to determinewhere significant differences occurred (pb0.05). Pre-acclimation V̇O2

data and weather data was analyzed using Factorial ANOVA. Post-hocScheffé tests were done to determine significance. Data are presented

595C. Lindsay et al. / Comparative Biochemistry and Physiology, Part A 152 (2009) 593–598

ED

as mean±SE of the individuals measured (n). Percentage change inPre-acclimation MR and BMR between winter and summer wasdetermined using the following equation: (Winter MR−Summer MR)/Winter MR)⁎100).

2.3. Release

Birds were weighed and released at the original capture site uponcompletion of respirometry trials.

3. Results

3.1. Body mass

Body mass (g) of Amethyst Sunbirds was compared betweensummer and winter, pre-acclimation and post-acclimation (Fig. 1). Nosignificant differences existed between pre- or post-acclimationmasses between seasons or between altitudinal subpopulationswithin seasons (Fig. 1, RMANOVA, F(2, 10)=1.106, p=0.368).

3.2. Pre-acclimation summer vs. winter

Pre-acclimation V̇O2 of Amethyst Sunbirds was compared betweenaltitudinal sites and between seasons for 5 and 25 °C (Fig. 2a and b).At 5 °C, all subpopulations showed a significant decrease in pre-acclimation V̇O2 from winter to summer (Fig. 2a, RMANOVA, F(1, 4)=102.19, p=0.001). The high altitude Underberg showed a 47.0%

Fig. 2. Seasonal and altitudinal differences in pre-acclimation V̇O2 values of AmethystSunbird subpopulations at (a) 5 °C and (b) 25 °C.

Fig. 3. Seasonal differences in post-acclimation resting metabolic rates of the altitudinalsubpopulations of Amethyst Sunbirds from highest to lowest altitude with (a) Underberg,(b) Howick and (c) Oribi Gorge.

RETRACT

decrease from 8.44±0.170 mL O2g−1h−1 or 0.047 W to 4.47±0.263 mL O2g−1h−1 or 0.025 W (Post-hoc Scheffé, pb0.05), theintermediate altitude Howick a 61.9% decrease from V̇O2 values of15.94±0.426 mL O2g−1h−1 or 0.089 W to 6.08±0.350 mL O2g−1h−1 or0.034 W (Post-hoc Scheffé, pb0.05). The low altitude Oribi Gorgesubpopulations showed the smallest seasonal difference among thealtitudes with a decrease in V̇O2 fromwinter to summer trials of 41.6%,

Fig. 5.Mean monthly temperatures for each of the three capture locations from January2004–May 2007 (Solid bar indicates winter months, hollow bar indicates summermonths).

596 C. Lindsay et al. / Comparative Biochemistry and Physiology, Part A 152 (2009) 593–598

A

D

from 12.59±0.699 mL O2g−1h−1 or 0.070 W to 7.36±0.450 mL O2g−1h−1

or 0.041 W (Post-hoc Scheffé, pb0.05).A comparison of seasonal pre-acclimation Amethyst Sunbird

V̇O2 data at 25 °C also showed a decrease in V̇O2 values from winterto summer (Fig. 2b, RMANOVA, F(1, 5)=83.600, pb0.001). Fromwinterto summer, the high altitude Underberg sunbirds showed a 65.7%decrease in V̇O2 from 6.71±0.146 mL O2g−1h−1 or 0.037 to 2.30±0.143 mL O2g−1h−1 or 0.013 W (Post-hoc Scheffé, pb0.05), the inter-mediate altitude Howick sunbirds showed a 61.0% decrease in V̇O2

between seasons from 7.20±0.447 mL O2g−1h−1 or 0.040 W in winterto 2.81±0.352 mL O2g−1h−1or 0.016 W in summer (Post-hoc Scheffé,pb0.05). The low altitude Oribi Gorge again showed the least changein V̇O2 between seasons with a 48.4% decrease from 7.48±0.742 mLO2g−1h−1 or 0.042W inwinter to 3.86±0.278 mL O2g−1h−1 or 0.022Win summer (Post- hoc Scheffé, pb0.05).

3.3. Post-acclimation summer vs. winter

Post-acclimation summer and winter V̇O2 values of AmethystSunbirds were compared for each location (Fig. 3a-c). In general, theAmethyst Sunbird subpopulation from the highest altitude, Under-berg, showed higher post-acclimation V̇O2 values per temperature inwinter than in summer trials (Fig. 3a, RMANOVA, F(7, 98)=46.372,pb0.001). Post-acclimation V̇O2 values for Howick subpopulationswere generally higher in winter than in summer (Fig. 3b, RMANOVA,F(7, 546)=195.710, pb0.001, however at 25 and 28 °C summer V̇O2

values were higher than winter values, although not significantly so(Post-hoc Scheffé, pN0.05). The high altitude Underberg sunbirdsshowed a significant difference between summer and winter RMRvalues at 5 and 10 °C (Fig. 3a, Post-hoc Scheffé, pb0.05), and Howicksunbirds showed a significant seasonal change in RMR at 5 °C (Fig. 3b,Post-hoc Scheffé, pb0.05). The lowest altitude, Oribi Gorge, sunbirdsshowed no significant differences between a winter and a summerseason at any of the ambient temperatures (Fig. 3c, RMANOVA, F(7, 119)=1.369, p=0.225).

Thermal neutral zones of Amethyst Sunbird subpopulations shiftedbetween winter and summer seasons (Fig. 3a–c). The TNZ forUnderberg sunbirds in winter ranged from 10–33 °C, but in summerit was narrower, between 20–33 °C. The Howick sunbirds indicated ashift from a very narrow TNZ in winter (Ta=25–30 °C) to a broaderrange of temperatures in summer (Ta=20-33 °C). Oribi Gorge sunbirdsdisplayed a smaller shift in TNZ between thewinter (Ta=20–33 °C) andsummer season (Ta=15–33 °C) with both TNZs relatively broad.

There was a significant effect of both season and altitudinal site onpost-acclimation BMR (Fig. 4, RMANOVA, F(2, 10)=9.547, p=0.005). Under-

R

Fig. 4. Comparison of basal metabolic rates (V̇O2) of altitudinal subpopulations ofAmethyst Sunbirds for winter and summer respectively.

RET

berg sunbirds significantly decreased BMR (Post-hoc Scheffé, pb0.05) by38.8% fromwinter to summer, from 5.71±0.402 mL O2g−1h−1 or 0.032Wto 3.50±0.213 mL O2g−1h−1 or 0.020 W (both at 30 °C). Howick sunbirdsshowed a 44.8% increase in BMR from winter to summer, from 2.46±0.299 mL O2g−1h−1 or 0.014W to 3.47±0.216 mL O2g−1h−1 or 0.019W (at28 and 30 °C respectively), however this increase was not statisticallysignificant (Post-hoc Scheffé,pN0.05). Therewas very little effect of seasonon the BMR of the Oribi Gorge sunbirds (Post-hoc Scheffé, pN0.05), withonly a slight decrease of 5.8% in summer, from3.49±0.312mLO2g−1h−1 or0.019 W to 3.29±0.232 mL O2g−1h−1 or 0.018 W (at 30 °C).

3.4. Climate

Mean monthly temperatures per location were compared (Fig. 5,RMANOVA F (22, 61298)=78.503, pb0.001) and it was found that therewas a significant difference among mean temperature (°C) per monthfor all of the locations (Post-hoc Scheffé, pb0.05). The mean tem-peratures for February (hottest month) and June (coldest month) werecompared for each of the locations as representing the seasonalextremes. Underberg showed the greatest seasonal extremes intemperature with a change in mean temperature between Februaryand June of 12.2 °C. Howick (10.4 °C) and Oribi Gorge (6.9 °C) whichboth displayed less difference between seasons. Summer seasonsshowed less variability between altitudes than winter.

4. Discussion

Amethyst Sunbirds, as relatively small birds, show significantdifferences in physiological measurements between altitudes andbetween seasons, despite body mass not changing significantly(implying a negligible change in body insulation and organ mass). Ingeneral Underberg and Howick subpopulations decreased RMRbetweenwinter and summer, whereas the lowest altitude subpopula-tion, Oribi Gorge, showed very little seasonal change in RMR. This wasexpected as Oribi Gorge did not show a great change in seasonalambient temperatures, whereas Underberg and Howick weather datashowed a more marked seasonal shift in ambient temperature. TheUnderberg subpopulation showed a decrease in BMR of 38.8% fromwinter to summer. Howick sunbirds increased BMR by 44.8% insummer, whereas the Oribi Gorge subpopulation showed very littledifference between a winter and summer season (5.8%). Similarly,Cavieres and Sabat (2008) found that populations from variableenvironments exhibit more physiological flexibility than populationsfrom stable environments. Such data are still quite novel, with ourstudy being, to the best of our knowledge, only the second show this

CTE

597C. Lindsay et al. / Comparative Biochemistry and Physiology, Part A 152 (2009) 593–598

A

correlation. Clearly, more studies are needed to further examine thistrend.

Amethyst Sunbirds do not respond according to predictions for theeffect of season on the MR of small birds, thus serving to emphasizefurther the need to acknowledge the importance of altitudinal originof a studied species. Our results may also explain the conflictingresults from other studies, where some small birds have been found toincrease BMR in winter (Hart, 1962; Southwick, 1980; Cooper andSwanson, 1994; Cooper, 2002; Liknes et al., 2002; Dawson, 2003),some to decrease BMR in winter (Ambrose and Bradshaw, 1988; Hart,1962; Maddocks and Geiser, 2000; Weathers and Caccamise, 1978)and some to show no change in BMR seasonally (Hart 1962;Sharbaugh, 2001). Different subpopulations of Amethyst Sunbirdsshowed each of these seasonal responses, indicating that altitude andclimate affect studies of seasonal changes in BMR. Similarly, Black-capped Chickadees (Poecile atricapilla) studied in different areas haveshown different seasonal responses (Cooper and Swanson, 1994;Sharbaugh, 2001). In comparison, although such variations are morelikely to be prominent in sedentary birds, even long-distancemigratory birds (in particular the Knot, Calidris canutus) have beenfound to have pronounced seasonal variation in BMR (Piersma et al.,1995). Such inter-population variability within a species has notadequately been highlighted before, and clearly needs more attention.The factors that lead to different physiological responses by differentpopulations of the same species remain to be discovered, although it isclear that geographical location, in terms of altitude (this study) andlatitude (Sharbaugh, 2001) and hence variation in climate (Cavieresand Sabat, 2008) all play a role.

Climate change, or long term shifts in average weather, affect theMR of birds by requiring changes in energy expenditure (Crick, 2004).Consequently, one of the factors that could inhibit the ability to adaptto climate change, is a lack of phenotypic flexibility and thus theinability to adapt to climate change (Crick, 2004). According toBernardo et al. (2007) there is an urgent need in conservation biologyand climate change research to find criteria for assessing thesusceptibility of a species to climate change induced extinction.Many species of birds are found to exist in cold and warm climates,with metabolic parameters varying between populations of the samespecies (Furness, 2003). Surely, a promising indicator of a speciesability to survive in a changing climate, is the ability to adapt andsurvive in response to seasonal as well as altitudinal shifts intemperature thus displaying flexibility/ plasticity in metabolic para-meters and consequently the ability to survive changes in ambienttemperatures. This study of Amethyst Sunbirds showed that within aspecies, different altitudinal subpopulations and thus populationsexposed to different ranges of ambient temperatures are able to adaptand survive in changing environments. Seasonal variation in BMR andRMR in Amethyst Sunbirds and other avian species thus questions therelevance and accuracy of predictions made in avian comparativestudies that have used a mean BMR or RMR per population, and notconsidered the variation around the mean, particularly as a con-sequence of altitude or season, as an effect on these physiologicalparameters. Similarly, non-migratory species, such as AmethystSunbirds, show the ability not only to endure temperature shiftsbetween seasons, but also reduced foraging time and changes in foodavailability, and thus ecological constraints.

It is thought that changes in body insulation in small birds arelimited by their size and thus are not as marked as metabolicadjustments and only have a small role in seasonal acclimatizationand adaptation to cold (Clemens, 1988; Cooper and Swanson, 1994;Swanson and Weinacht, 1997). Studies on the Australian Silvereye(Zosterops lateralis) by Maddocks and Geiser (2000), Monk Parakeets(Myiopsitta monachus) by Weathers and Caccamise (1978) and Black-capped Chickadees by Cooper and Swanson (1994), indicated verylittle seasonal variation in body mass. Similarly, body mass inAmethyst Sunbirds did not vary seasonally or between altitudinal

RETR

subpopulations within a season. This is contrary to some other specieswhich can display a significantly higher body mass in winteracclimatized birds (Weathers and Caccamise, 1978; Blem, 1990). Asthere was no significant change in body mass between seasons withinaltitudinal subpopulations, nor within seasons between altitudinalsubpopulations, it can be assumed that changes in body fat storeswere not an influencing factor in shifts in metabolic rates and thusseasonal acclimatization.

Maddocks and Geiser (2000) found that variation within popula-tions of Silvereyes, in terms of MR, was higher in summer than inwinter. However, subpopulations of Amethyst Sunbirds consistentlyshowed greater variation around the mean in winter than in summer.Maddocks and Geiser (2000) observed a shift in TNZ from25.4–33.5 °Cin summer, to a slightly narrower TNZ range between 27.0–33.6 °C inwinter. Dawson and Carey (1976) found the zone of thermal neutralityfor the American Goldfinch (Carduelis tristis) ranged from approxi-mately 23–35 °C in both summer and winter. Monk Parakeets(Weathers and Caccamise, 1978), showed a shift in thermal neutralzones betweenwinter and summer from 24.5–38.5 °C to 28.0–40.0 °C.Amethyst Sunbird subpopulations, however, showedmixed responsesto seasonal TNZ changes. The higher altitude Underberg sunbirdsshowed a decrease in TNZ range between winter and summer,probably as a result of reduction in temperature extremes in summer.Howick and Oribi Gorge sunbirds, however, showed an increase in TNZrange fromwinter to summer. This responsewas to be expected due toexposure to a broader range of ambient temperatures in summer, andthus we expected birds to be able to cope with a broader range oftemperatures.

5. Conclusion

When considering a population to use as a representative for thespecies in physiological studies, not only does one have to takealtitudinal origin into account, but also the season to which the studypopulation is acclimatized to. It may be possible to acclimate birds tothe same temperature and altitude over a period of time, but ourresults have shown that underlying physiological differences canpersist post-acclimation. Besides this, data from birds acclimated topseudo environmental conditions may not represent the wildpopulations accurately, as significant differences were observed inmetabolic parameters between pre- and post-acclimation trials.

CTED

Acknowledgements

CL would like to thank the NRF and the Gay Langmuir bursary fundfor financial assistance. Temperature data was obtained from theSouth African Weather Service. We would also like to thank Mike andHeidi Neethling (Oribi Gorge), Bill and Pam Nicol (Howick) and Prof.and Mrs. Piper (Underberg) for allowing us to “borrow” theirAmethyst Sunbirds for the study period. Tracy Odendaal, JaclynTennent, John Lindsay, Kelly Brown, Nicolette Taylor, Ashton Mus-grave, Akeem Akilimali are thanked for their invaluable field tripassistance. Thanks also to students at UKZN for assisting with feedingand bird maintenance.

References

Ambrose, S.J., Bradshaw, S.D., 1988. Seasonal changes in standard metabolic rates in thewhite-browed Scrubwren Sericornis frontalis (Acanthizidae) from arid, semi-aridand mesic environments. J. Comp. Physiol. A. 89, 79–83.

Arens, J.R., Cooper, S.J., 2005. Seasonal and diurnal variation in metabolism andventilation in House Sparrows. Condor 107, 433–444.

Bernardo, J., Ossola, R.J., Spotila, J., Crandall, K.A., 2007. Interspecies physiologicalvariation as a tool for cross-species assessment of global warming-inducedendangerment: validation of an intrinsic determinant of macroecological andphylogeographic structure. Biol. Lett. 3, 695–698.

Blem, C.R., 1990. Avian energy storage. Curr. Ornithol. 7, 59–113.

598 C. Lindsay et al. / Comparative Biochemistry and Physiology, Part A 152 (2009) 593–598

Carey, C.C., Dawson, W.R., Maxwell, L.C., Faulkner, J.A., 1978. Seasonal acclimatization totemperature in Cardueline finches ii. changes in body composition and mass inrelation to season and acute cold stress. J. Comp. Physiol B 125, 101–113.

Cavieres, G., Sabat, P., 2008. Geographic variation in the response to thermal acclimationin rufous-collared sparrows: are physiological flexibility and environmentalheterogeneity correlated? Funct. Ecol. 22, 509–515.

Clemens, D.T., 1988. Ventilation and oxygen consumption in rosy finches and housefinches at sea level and high altitude. J. Comp. Physiol. B 158, 57–66.

Cooper, S.J., 2000. Seasonal energetics of mountain chickadees and juniper titmice.Condor 102, 635–644.

Cooper, S.J., 2002. Seasonal metabolic acclimatization in mountain chickadees andjuniper titmice. Physiol. Biochem. Zool. 75 (4), 386–395.

Cooper, S.J., Swanson, D.L., 1994. Seasonal acclimatization of thermoregulation in theblack-capped chikadee. Condor 96, 638–646.

Cossins, A., Fraser, J., Hughes, M., Gracey, A., 2006. Post-genomic approaches tounderstanding the mechanisms of environmentally induced phenotypic plasticity.J. Exp. Biol. 209, 2328–2336.

Crick, H.Q.P., 2004. The impact of climate change on birds. Ibis 146, 48–56.Dawson, W.R., 2003. Plasticity in avian responses to thermal challenges—an essay in

honor of Jacob Marder. Israel J. Zool. 49, 95–109.Dawson, W.R., Carey, C., 1976. Seasonal acclimatization to temperature in cardueline

finches I. Insulative and metabolic adjustments. J. Comp. Physiol. B 112, 317–333.Downs, C.T., Brown, M., 2002. Nocturnal heterothermy and torpor in the Malachite

Sunbird (Nectarinia famosa). Auk 119 (1), 251–260.Furness, R.W., 2003. It's in the genes. Nature 425, 779–780.Hart, J.S., 1962. Seasonal acclimatization in four species of small wild birds. Physiol. Zool.

35, 224–236.Klaasen, M., Oltrogge, M., Trost, L., 2004. Basal metabolic rate, food intake, and body

mass in cold- and warm-acclimated garden warblers. Comp. Biochem. Physiol. A.137, 639–647.

Liknes, E.T., Scott, S.M., Swanson, D.L., 2002. Seasonal acclimatization in the Americangoldfinch revisited: to what extent do metabolic rates vary seasonally? Condor 104,548–557.

Lindsay, C.V., Downs, C.T., Brown, M.A., 2009. Physiological variation in amethystsunbirds (Chalcomitra amethystina) over an altitudinal gradient. in winter. J. Exp.Biol. doi:10 1242/jeb.025262.

RETRA

Lindsay, C.V., Downs, C.T., Brown, M.A., in press. Physiological variation in AmethystSunbirds (Chalcomitra amethystina) over an altitudinal gradient in summer. J.Therm. Biol.

Lovegrove, B.G., Smith, G.A., 2003. Is ‘nocturnal hypothermia’ a valid physiologicalconcept in small birds?: a study on bronze Mannikins Spermestes cucullatus. Ibis145, 547–557.

Maddocks, T.A., Geiser, F., 2000. Seasonal variations in thermal energetics of AustralianSilvereyes (Zosterops lateralis). J. Zool. Lond. 252, 327–333.

McKechnie, A.E., 2008. Phenotypic flexibility in basal metabolic rate and the changingview of avian physiological diversity: a review. J. Comp. Physiol. B. 178, 235–247.

Piersma, T., Cadée, N., Daan, S., 1995. Seasonality in basal metabolic rate and thermalconductance in a long-distance migrant shorebird, the Knot (Calidrus canutus).J. Comp. Physiol. 165, 37–45.

Rezende, E.L., Chappell, M.A., Hammond, K.A., 2004. Cold acclimation in Peromyscus:temporal effects and individual variation in maximal metabolism and ventilatorytraits. J. Exp. Biol. 207, 295–305.

Sharbaugh, S.M., 2001. Seasonal acclimatization to extreme climatic conditions byblack-capped chickadees (Poecile atricapilla) in Interior Alaska (64°N). Physiol.Biochem. Zool. 74 (4), 568–575.

Shultz, R.E., 1997. South African Atlas of Agrohydrology and Climatology. WaterResearch Commission, Pretoria.

Smit, B., Brown, M., Downs, C.T., 2008. Thermoregulatory responses in seasonallyacclimatized captive Southern white-faced Scops-owls. J. Therm. Biol. 33, 76–86.

Southwick, E.E., 1980. Seasonal thermoregulatory adjustments in white-crownedsparrows. Auk 97 (1), 76–85.

Swanson, D.L., Weinacht, D.P., 1997. Seasonal effects of metabolism and thermoregula-tion in Northern Bobwhite. Condor 99, 478–489.

Tree, A.J., 2005. Amethyst Sunbird, Chalcomitra amethystina, In: Hockey, P.A.R., Dean,W.R.J., Ryan, P.G. (Eds.), Roberts Birds of Southern Africa, VII Ed. John VoelckerBird Book Fund, New York, pp. 981–982.

Weathers, W.W., Caccamise, D.F., 1978. Seasonal acclimatization to temperature inmonk parakeets. Oecologia 35, 173–183.

Weathers, W.W., van Riper III, C., 1982. Temperature regulation in two endangeredHawaiian honeycreepers: the Palila (Psittirostra bailleui) and the Layson finch(Psittirostra cantans). Auk 99, 667–674.

Withers, P.C., 1992. Comparative animal physiology. Saunders College Publishing, Sydney.TED

C