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R i ng i ng & M i g r a t i o n ( 1 9 9 5 ) 1 6 , 9 1 - 9 9

Biometrics of Sanderlings Calidris alba from the Taimyr
MIKHAIL YU. SOLOVIEV Department of Vertebrate Zoology & General Ecology, Biological Faculty, Moscow State University, 119899 Moscow, Russia PAVEL S. TOMKOVICH Zoological Museum of Moscow State University, Herzen Str. 6,103009 M osc ow , R ussi a
Biometric data were collected for Sanderlings breeding at northern Taimyr in 1982-92. They allowed the calculation of discriminant functions, using wing, bill and (or) total head lengths, for sexing Sanderlings, belonging to Siberian population. Geographic comparisons revealed that birds from Taimyr have shorter bills than birds on South African wintering grounds, wh i c h l e a v e s t h e p r o b l e m o f o r i g i n o f l o n g - b i l l e d S o u t h - A f r i c a n S a n d e r l i n g s u n s o l v e d . Sanderlings had minimal body mass in pre-nesting and brood-rearing periods, and noticeably larger mass during incubation. Females were consistently heavier than males (regardless of

size]. The mean body mass fell in late seasons due to a probable decrease in food availability. he bre ed in g ra n ge of t he Sa nd erli n g hig h Ar ctic wa der po p ulatio ns. I n additi o n, n Calidris alba includes several regions of the mefasurementls i of 23 oStan-derlie gs Twere tianken i2n t oca nm high Arctic. The large population of the Taimyr di9f8er.enn lgeneirtaesdofanwrehewesttairned afior yarlmo1t98he 1 4I l at r ob st Peninsula, north-central Siberia, is believed to whole breeding season, from 8 June to 5 August. migr ate m ainly to Africa n win ter quarters T h e b ir d s w er e ca u g h t w it h t h e "l u c h o k " - tr a p through Europe, the Black Sea and Caspian (Priklonsky 1960) for ringing and individual colourtr s, t b s Sea areas (Cramp & Simmons 1983, Summers mareki,ng uornnghetihenesrte-bweieh inrootdm,eori,n in ea dfiew etal. 1987, Lebedeva 1985, Tomkovich etaL cas itsatsd anid on dependerd td rrigtoriies of ufnpeairng ha b f e ed 1994). However, there are some indications ma le s . T hirt y-e i ght bir ds w ere c oll e ct e d a nd th at A fr ic a r e ce iv e s Sa n d erl i n gs fr o m t he measured on the day of capture. Wing length was Greenland population as well, also migrating measured with a stopped ruler to the nearest 0.5 hr p s n ac go e (1 ) through Europe (Green & Greenwood 1978, mnmlaitnetneee wonigtioors micoirdiunm tchSvdnssoning984):, uf t d i nm or ("w 1" G u d m u n d s s o n & L i n d s t r o m 1 9 9 2 ) . T h e flattened but not straightened wing ("wing 2"), and c o m p o s i t i o n o f t h e w i n t e r p o p u l a t i o n o f maximum length - flattened and straightened wing Sanderlings in South Africa is not clear at ("wing 3"). Bill length from bill tip to the featherpresent, an d we ha ve poor k n owle dge of line on the forehead, and tarsus length were taken al i e t . 199 19 biometric data on adult birds from the Taimyr wittahl cenlgphrs ftobitlhleanearheesad0.(1 mmn I1n980) 0-nd 9i2 to l t o nd Gree an breeding population (Summers et al. 1987). 1991 and 1992 length of middle toe without claw T h e m e a n b i l l l e n g t h o f S o u t h A f r i c a n w er e a l s o m e a s u r e d . T h e b ir d s w er e w ei g h e d t o Sanderlings is larger than those of birds from the nearest 0.5 g with a Pesola spring balance in the other areas, resulting in the opinion that these latter years or, more precisely, with a beam balance n he or y . e bo y m a long-billed Sanderlings are from Siberia (Prater iayitng ffemmlers weearresexThuded drom assaldaits. of eggl ae cl f an ys et al. 1977, Su m m ers et al. 1987). In the Birds were sexed by gonadal examination on current paper, biometric and body mass data dissection. Males song during the period of territorial are presented for Sanderlings of the Northern defence and mating behaviour were used for sex e f i of v y l ma i Taimyr, and methods of sex identification from idnnttihicratsoning icndtieriidunallwacobour-dth rked cboradsa. oe e o e of l measurements are discussed. Data on the mass A erpendiculxar to trhie body axss), rmaeasured withca (p i changes in Sanderlings during breeding season ruler to the nearest 0.5 mm. The measurement of help us to understand adaptations to extreme t he e xt e r na l pa r t of t he c l oa c a l pr ot ube r a nc e environment of the high Arctic, and fill a gap c or r e s p on ds t o t he ma xi m a l w i d t h of c l oa c a at d sn a e t at . in the kno wle dge abo ut annual chan ges of unfeMhereof psatrttiisntiictal uprcontdrurcets dwsereec(aFirged1)o.ut ost a oce ri body mass in this species. using Systat 3.0 and the discriminant analysis was done in SAS. The latter was used to work out the MATERIAL AND METHODS best sexing criterion for Sanderlings on the basis of Most of the biometric data (from 82 adult birds) was the following measurements: maximum wing, bill, collected in 1990-1992 in the Knipovich Bay area, total head, tarsus and middle toe lengths. Average north-central Taimyr (76° 05N, 98° 32E) during pr ob ab ilitie s of er r o n eo u s cla ss if icati o n wer e s t udi e s of br e e di n g bi ol o gy a n d s oc i a l s t r uc t ur e of estimated following the formulae of Deev (1977) (for

T


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M. Yu. Soloviev and P. S. Tomkovich in Sanderlings during breeding season. In order to control for the effect of body size on birds' mass we included the first principal component of wing, bill a nd t a r s u s l e ngt h s i n t he mo de l a s c o va r i a t e . Sexual dimorphism It is expected that the cloaca size of females, which have recently finished egg-laying, is different from that of males and this character can be used for sexing birds. In our study, the breadth of unfeathered part of cloaca did not exceed 7.5 mm for 20 Sanderlings, definitely sexed as males, while in definite females it could reach 14 mm (n=14). On this basis, we i n c l u d e d a l l b i r d s w i t h c l o ac a b r e a d t h exceeding 8 mm into the training sample as " females" in addition to birds, sexed by gonadal examination. The analysis of normal probability plots (Afifi & Azen 1979) for the data demonstrated that distribution of characters in males, females and the total available sample was close to normal, permitting use of parametric methods (Table 1). The measurements of wing length, taken b y t h r e e d i f f e r e n t m e t h o d s a r e s t ro n g l y intercorrelated (r > 0.9, P < 0.0001 for all three pairs). As maximum wing length is now the standard measurement and provides the most precise measurement, the following r e g r e s s io n eq u a t io n s f o r co n v e r s io n o f measurements taken by different methods are given: Wing Wing Wing Wing 3 3 1 2 = = = = 31.973 31.945 -13.385 -13.299 + + + + Wing Wing Wing Wing 1 2 3 3 * * * * 0.789 0.784 1.055 1.060
RESULTS

Figure 1. The cloaca breadth measuring technique. a comparative review of methods see Ahrens & Lauter (1981)). The possibility of unequal covariance matrices was considered and appropriate tests (Davis 1986) m a de . T he r e j e c t i o n of t h e n ul l h y p o t he s i s i n a l l cases (P> 0.1) permitted to use linear discriminant analysis. Equal prior probabilities of belonging to any sex were used in the estimates. However, the resulting functions can be easily employed with any prior probabilities (Ahrens & LДuter 1981, Afifi & A z e n 19 7 9) . Analysis of variance (ANOVA) was chosen a principal approach for studying body mass changes

Th e s q u a r e d m u l t i p l e c o r r e l a t i o n c o ef f ic i en t ( r 2 ) e x c e ed s 0 . 8 3 0 f o r a l l regressions. M e a n v a l u e s o f e a c h s t ru c t u r e w e r e significantly different between males and females (P= 0.039 for middle toe length, and P< 0.001 for other structures, Student's f-test). To compare our data with those given by Wood (1987) we calculated a length of head itself, by subtracting bill lengths from the total head length. Mean values of this structure for males and females were 25.2 (SD = 0.8, n = 18)


Table 1. Measurements (in mm) and body mass (in g) of Sanderlings Calidris alba from northern Taimyr.

Structure n Min. wing chord Flattened wing Max. wing length Bill length Total head length Tarsus length Middle toe length Body mass 31 31 31 31 18 31 16 33 m e an 119.6 120.3 126.0 24.1 49. 4 24. 7 18.0 54.3

Male SD
2 .9

Female range 114.5-128.0 115.0122.022.848.023.216.442.7129.0 134.0 25. 8 51.2 26.1 18.8 59.7 n 36 36 36 36 29 37 19 34 mean 123.5 124.3 129.5 25. 6 51.1 25. 9 18.4 62.3 SD 2 .6 2 .7 2 .4 1 .0 1 .2 1 .0 0 .6 6 .3 range 118.0-130.0 119.0123.523.949.323.617.548.6130.5 135.0 27.4 53.1 27.8 19.3 70.5 n
104 104 104

The whole sample mean SD range 114.5-130 115.0122.022.446.923.216.342.7130.5 135.0 27. 4 55.5 27.8 19.3 70.5

121.2 3.3 122.0 127.6 24. 9 50.5 25. 4 18.2 57.9 3 .3 2 .9 1 .2 1 .5 1 .0 0 .6 6 .6

2 .8 2 .3 0 .8 0 .0 0 .7 0 .5 4 .0

105 82

104 59 103


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M. Yu. Soloviev and P. S. Tomkovich with the probability exceeding 0.9, for 25 out of 38 formerly unsexed birds in our sample. Along with the birds of a priori known sex, they formed a total sample of 46 males and 46 females. So, the sex ratio in the sample is 1:1. This extended sample was used to revise the data on the changes in cloaca size. The breadth of cloaca in males of the extended sample did not exceed 7.5 mm (Fig. 2), as in the initial one, thus confirming that birds with cloaca breadth of 8.0 mm and more were females. It was important that in the period between 24 June and 12 July, corresponding to the three-week period from the start of incubation of the earliest egg-clutches at the northern Taimyr (pers. obs.), no females were recorded with cloaca breadth less than 7.0 mm, while among males such birds were common. Mean cloaca breadth of females in the period was significantly larger (P< 0.05), than in average for the breeding season, being 9.6 mm and 8.0 mm, respectively. Thus, a Sanderling with cloaca breadth of 6.5 mm or less (allowing for any error in measurement) in the period of first three weeks after start of egg-laying in the region can be reliably classified as male. Body mass changes The first principal component of wing length, bill length and tarsus length accounted 66.3% of variance in these structures. It was used as a covariate in body mass analysis and the effects associated with the covariate are hence forth referred to as the "size" effects. In the study area, incubating Sanderlings were found from 19 June to 3 August, birds with unfledged chicks from 10 July to at least 7 August, and roving birds, yet not breeding, or which have already finished it, from 7 June to late August (Tomkovich et al. 1994). The first variant of ANACOVA included three factors with fixed effects: ( 1 ) sex; (2) breeding state - birds in pre-nesting period (before 21 June), incubating birds and those with broods; (3) year - early spring with prolonged snow melting in the year 1990, and later spring with quick snow melting in the year 1991. The absence of brood-rearing Sanderlings in 1992 did not permit to include data, obtained in this year in the analysis. The

and 25.5 (SD = 0.6, n = 29), respectively. Significant differences were found neither between sexes (P= 0.245), nor between our data and those of A.G. Wood (P> 0.1, for males and females) taken for Sanderlings, wintering in the Teesmouth, NE England. Th e c o e f f i c i e n t o f v a r i a t i o n w a s significantly higher in females than in males only for the body mass (P < 0.05, Student ttest), indicating similar variability in other structures between sexes. The stepwise discriminant analysis applied to a sample of 47 birds showed that wing length and bill length is a minimal necessary combination of structures for sex identification, because the omission of any of them significantly reduces sexing efficiency (P < 0.05). The use of total head length together with bill and wing lengths does not improve sexing significantly in comparison with the application of two latter structures only, but the average probability of error in both males and females is slightly less for the case with three variables (0.146 and 0.142 against 0.147 and 0.147), and, as a result, more birds from the training sample were sexed correctly with the probability exceeding 0.9 (0.619 against 0.522). Therefore, measuring of wing, bill and total head lengths is recommended for highest possible probability of correct sexing. Two linear discriminant functions are given here to meet both situations described:
Z = wing * 0.932 + bill * 0.645 + total head * 0.625 Z = wing * 0.577 + bill * 1.731

A bird was sexed as male if the calculated va lu e wa s l es s th an th e co n t ro id v alu e Z0= 166.059 for a function of three variables, a n d Z0 = 1 1 6 . 8 0 2 f o r a f u n c t i o n o f t w o variables. In the opposite situation a bird was classified as female. The probability that a bird with calculated Z value was female was determined from the formula of Afifl & Azen (1979): P= 1/(1 + EXP(-Z + Z0)). where EXP is the exponential function, and Z0 - centroid value for a given type of the discriminant function. The probability for a bird being male was 1 - P. The procedure described permitted sexing


Sanderling biometrics

95

Figure 2. Changes of Sanderling cloaca size during breeding season, m - males, f - females.

palrwise Interactions of factors were included in t h e m o d el . To check the influence of date on the body mass we included it in the first variant of model in two covariate forms - linear and quadratic, which account for a simple type of nonlinear relationship. The effect of year and its interactions with two other factors appeared to be insignificant (P > 0.2 5), an d their furt her pre sen ce in the model was ascertained unnecessary. T h e r ef o r e , t h e s a mp l e o f b o d y ma s s m e a s u re m e n t s w a s i n cr e a se d , t a ki n g y e a rs , ot h er th a n 1 99 0 a n d 1 99 1. Th e i nf lu e nc e o f

date was also insignificant (P> 0.5) and related covariates were not included in further models. The estimates were repeated for a model with one covariate (" size" ), main effects of two factors (sex and breeding state), and effect of th e ir i n te r a ct i o n. The influence of " size" on body mass of Sanderlings was insignificant (P> 0.9), while the influence of the main effects of sex and breeding state appeared to be highly significant (P= 0.0008 and P= 0.0001 respectively). The effect of interaction of sex and breeding state had no significant influence (P= 0.361). The esti mat e o f lin ear co ntra cts per m i tted t o fin d


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M. Yu. Soloviev and P. S. Tomkovich

out that significant influence of the breeding state was d ue to di ffer en ces b e tw een me an body masses of incubating birds in comparison w it h t h o s e t a ke n i n pr e - ne s ti n g a n d br o o d rearing periods (P< 0.0001 for both contrasts). There was no difference in body mass between birds in pre-nesting and brood rearing periods (P = 0.637). Mean values of body mass of Sa n derl in gs in diff ere nt c o m b inat io ns o f breeding state and sex are presented in Table 2, as well as means, adjusted to the differences in size (see W.G. Cochran (1957) for statistical details). They correspond to those values of body mass, which Sanderlings of a given sex and breeding state would have had, if being of an avera ge size. A djuste d me a ns of bod y masses for males and females were 55.1 and 59.9 g respectively. Twelve pre-nesting and incubating Sa nd erlin gs w ere m ea sur ed in the ver y late seaso ns of 1983 and 1992, and a separate variant of ANACOVA with factors of sex, breedin g state (with two levels) and year (normal and late) was fulfilled. It confirmed the conclusions of the basic variant about lesser mass of males (P< 0.001) and pre-nesting birds (P< 0.001), but also show that Sanderlings in late seasons had lesser mass than in normal (adjusted means 56.4 and 58.8 g, respectively, P < 0. 0 4 4).
DISCUSSION

Ringing recoveries of Sanderlings from South Africa at the Taimyr make it most interesting to compare measurements of birds from these p o p u l at i o n s. A c c o r di n g t o S u m m e r s e t a l . (1987) birds wintering in South Africa have mean bill length 24.9 mm (SD = 1.1, n = 53) in males and 26.2 mm (SD = 1.0, n = 20) in femal es. These are 0.8 and 0.6 m m, or 3.3 and 2.3%, respectively, longer than in Taimyr trapped or freshly dea d San de rlings. The difference between populations is significant (/>< 0.001 and P< 0.05 in males and females correspondingly, Student' s t-test). U nf ort un ate ly b ill le ngt h is s ubj ec te d to seasonal variation (Summers etal. 1987), and therefore comparisons of measurements taken in different seasons should be interpreted with ca ut i on . I n th e S o ut h A fr ic a n sa m p le, o n ly

Geographic comparisons

1 4 % of s e x e d bi r d s w e r e m e a s u r e d i n N o v e m be r to J a n ua ry , w h e n t h e w in te ri n g bird s ha ve their ma xi mu m bill le ngt hs. M ost of the Sa nd erlin gs fro m the bre edi ng rang e were measured from mid June to late July, i.e. after at least one month and a half since the pe ri o d of m i ni m u m b ill le n gt h i n w in te r qua rter s ( M arc h and A pril), wh en plu m ag e probably is the freshest after the pre-breeding moult. Thus, the recorded differences can not be interpreted as the result of comparison of breeding birds in almost fresh plumage (and short bill) with wintering Sanderlings in the heavily worn one (long bill). The sample sizes of sexed Sanderlings from South Africa and Taimyr were relatively small. Therefore, additional comparisons of unsexed adult birds from Taimyr and the Langebaan Lagoon, South Africa, captured in different months, were made. Taking into account the sex rati o of on e fe mal e to 2.6 m ales in th ese wintering grounds, we created a sample of the same sex ratio from the Taimyr birds, randomly selecting females for it. The mean bill length o f S a n d er li n g s i n t hi s s a m p l e w a s 2 4. 5 m m (SD = 1.0, n = 43), which is less than in birds fr o m So u t h Af ri c a i n a n y mo n t h f r o m November to May (P< 0.05), and even slightly le s s [ P < 0. 1 ), t h a n i n O ct o b e r, w h e n m e a n bill length was a minimum (24.9, SD = 1.5, n = 177) and birds were probably in fresh winter plumage. These results demonstrate that Sanderlings from Taimyr have shorter bills than birds on South African wintering grounds. Therefore, large bill size of South African Sanderlings can not be ex plai ne d by pre se nc e o f birds fro m Siberian population. Perhaps, then, Greenland birds also winter in South Africa (Gudmundsson & Lindstrom 1992). Unfortunately, the samples of sexed Sanderlings from Greenland population are so small (review is given in Summers etal. 1987) that conclusions on similarity or difference in bird sizes are not possible. For migrants in Iceland, which pass from Europe and Africa to Greenland (Gudmundsson & Lindstrom 1992), the mean bill length in late May and early June was 24.6 m m (SD = 1.6, n = 224), 0.3 m m less than in pooled Taimyr sample (P < 0.1). This small difference can be explained either


Sanderling biometrics Table 2. Mass (in g) of Sanderlings in different breeding state.
Breeding state of birds

97

S ex

n

mean
52.8 57.3 58.0 65.3 54.3 57.3

SD

A dj u st e d m e a n s by se x both se x e s 55.1 61.6 55.7

Pre-nesting Incubating Brood-rearing

M F
M

8 10 13 17 11 6

3 .5 4 .5

52. 9 57.2 58.1 65.2 54.3 57.2

3 .0
4 .3 4 .2 3 .0

1
F

:

M

by lesser wear of feathers on forehead of birds d u r i n g s p r i n g mi g r a t i o n t h a n i n t h o s e measured a month later, or by some prevalence of males in the Icelandic sample of unknown sex ratio. In any case an origin of the longbilled Sanderlings from South Africa remains unclear. Comparison of wing length provides little help to solve the problem. In the artificial sample of Taimyr birds with sex ratio of 2.6 males to 1 female, the mean wing length was 126.7 mm (SD = 2.7, n = 43), close to wing measurements from South Africa in October and Nove mber (127.2 and 126.9 mm respectively) and does not differ significantly (P > 0.05) from the April sample with mean value of 127.7 mm. Another opportunity is a possible migration o f S an d er l i n gs, b r eed i n g i n no r t h - east er n Canadian Arctic to winter grounds of the Old World (Myers et al. 1990). In this case some long-billed Sanderlings from the New World should come in a non-stop flight over Iceland to Eu rop e an d th en to South Africa. Th ere are no good samples of breeding birds taken in the Canadian Arctic for comparison, as there a r e fr o m A me r i c a n wi n t e rin g g r o u n d s. H o we v e r , t h e b i l l l e n g t h s o f s e x e d b i r d s wintering in California cannot be compared with other data because these measurements were taken to the proximal end of the nares (Maron & Myers 1984), but the larger tarsus length in comparison with Taimyr birds (P<

0.0001 and P< 0.001 in males and females respectively) gives support for the idea about t h e l a r g e r si z e o f C a n a d i an b r e ed in g Sanderlings in comparison with Taimyr birds. This idea is also partly supported by significantly (though based on very small museum samples) increase in bill length in Sanderlings wintering in Caribbean and South America (mean values: 25.6, SD = 0.8 mm, n = 9 and 26.4, SD = 0.8 mm, n = 11 in males and females respectively) (Cramp & Simmons 1983) over those from Taimyr (P< 0.0001 and P< 0.05) for males and females respectively). Thus, we can not claim that biometric data from Taimyr breeding Sanderlings substantially clarified the problem of origin and proportions of birds from different geographic breeding popu l ati ons wi th in win t er ing gr ound s. Nevertheless, some new priorities for further studies of the problem became clear, other than ringing in the breeding range. Primarily, the lack of good samples of sexed birds from Nearctic and New Siberian Islands'breeding populations is obvious. Samples of unsexed birds are practically useless for the outlined tasks because of the unknown sex ratios. Also data collection is extremely desirable for those features in which seasonal variability caused by moult and feather wearing off is absent, e.g. tarsus length and total head length. It is vitally important that measurement techniques are standardized internationally, because we still c a n n o t e x cl u d e a p o ss i b i l i t y t h at t h e f o u n d


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M. Yu. Soloviev and P. S. Tomkovich Acantlis hornemanni and A. flammea as a response to the shortening of feeding day and decrease of temperature (Brooks 1968). After o n s et o f in cu b a t io n , Sa n d e r lin g s lo s e a possibility to feed for rather long continuous periods, when they have to stay on the nest which probably has the same effect as a break of feeding for the night. With the onset of brood-rearing an opportunity to replenish e n e r g y l o s s in a n y m o m e n t o c cu r s an d compensatory fatness disappears. At any stage of breeding season females are heavier than males. The larger body mass of females is known also for Sanderlings w i n t e r i n g i n En g l a n d ( W o o d 1 9 8 7 ) , i n particular in the pre-migration period. The difference is perhaps due to the female' need s to have additional energy reserves for egg laying soon after arrival to breeding grounds. The females at the northern Taimyr were on a ve r a ge 4 . 3 g h e a vi e r t h a n ma l e s i n p r e b r e ed in g p e r io d an d th e d if f e r e n c e w as retained later as well. Th e l o w e r m a s s o f p r e - n e s t i n g a n d incubating Sanderlings in the late seasons, when abundance and availability of food probably decreases, is not surprising. However, it is worth mentioning that year conditions thus affect the bird breeding exclusively in high Arctic and having, pr esuma bl y, cert ain adaptations to this kind of environmental changes. This study was carried out in the field camp of the International Arctic Expedition of the Institute of Evolutionary Morphology and Animal Ecology, Russian Academy of Sciences, and it was possible due to financial and logistic s u p p o rt o f t h e e xp e d i t i o n o r g a n i z e r s . Therefore we are extremely obliged to the head of the expedition, Academic Professor E.E. Syroechkovski and to the expedition staff, especially to N.V. Vronsky, E.E. Syroechkovski Jr., M.M. Zabelin, M.G. Sinitsyn, A. Abolits. We also greatly appreciated the pleasant company and valuable help searching for nests and bird catching of the expedition participants in different years: O. Hilden, P. Yesou, G. Th. deRoos, E.E. Syroechkovsky Jr., E.G. Lappo, A.V. Rybkin, V.O. Yakovlev. Dr. R.W. Summers,
A C K N OW L E D GE M E N T S

difference between South African and Siberian Sanderlings is due to some variability in measuring between observers. Summers etal. (1987) have shown that mean lean body mass of Sanderlings in South Africa varies in different periods in range of 50.3 to 54.1 g. Mean adjusted masses of Sanderlings in pre-breeding and brood rearing periods are 55.1 and 55.7 g respectively, indicating low fat reserves in these periods. For the prebreeding period this can be explained by consequences of long migration and by high c o s t o f m al e t e r r i to r y d e f en s e an d e g g formation in females. It is more difficult to explain body mass decrease in adults with chicks, noting coincidence of hatching time in arctic waders, including Taimyr breeders, with period of high abundance of insects, especially flying Diptera (e.g. Holmes 1966a, Myers & Pitelka 1979, Lantsov & Chernov 1987, Underbill et al 1993). The small mass in the brood-rearing period is likely to conflict with the necessity of fat accumulation before migration. It is known, however, that in the north many long-distant migrants begin to move southward with a small amount of fat (Blyumental 1971), including such waders as Dunlin Calidris alpina in Sweden (Mascher 1966) and Alaska (Holmes 1966b), Pacific Golden Plover Pluvialis fulva and Western Sandpiper Calidris mauri in Alaska (Johnston 1967). Odum (1960) and Evans (1966) supposed that long-distance migrants accumulate a maximum amount of fat at the moment of transgressing a substantial ecological barrier. Sanderlings have none of these, at least at the beginning of their flyway, and probably avoid accumulation of large amount of fat during brood-rearing, because the lesser mass provides better aerodynamic quality to be used in predator avoidance and chick care. Large mean mass (61.6 g) of incubating Sanderlings can be interpreted as an energetic insurance for periods of negative energy budget. Compensatory fattening of such kind is known for waders wintering in Europe, as an insurance for bad weather periods (Dugan et al 1981) and in arctic and subarctic Redpols

Body mass changes


Sanderling biometrics Prof. L.G. Underhill and an anonymous reviewer provided valuable comments on the manuscript.
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