271

Body Mass Changes in Waders (Charadrii) in a High Arctic Area at Northern Taimyr, Siberia

Mikhail Y. Soloviev and Pavel $. Tomkovich

SOLOWEV, M. Y. & R S. ToivnvowcH (1997): Body mass changes in waders (Charadrii) in a high arctic area at nor thern Taimyr, Siberia. J. Orn. 139: 271-281. - Body mass data were collected for 5 species of tundra waders during 6 breeding seasons to study variation in nutrient reserves relative to high arctic climatic conditions and the stage of reproduction. Structural size was accounted for by the 1st Principal Component of external linear measure- ments: wing length, bill length and tarsus length. Sandpipers Calidris spp. were on average lighter in phenologically late seasons, while mass of Turnstones (Arenaria interpres) was similar in all years. Mass of waders was lower during brood-rearing than during incubation. Body mass during pre-nesting was most variable between years, reflecting the unpredictable weather conditions in this period. Monogamous species (Knots Calidris canl~tUs and Turnstones) began to breed earlier, and were on average, as hea W during the pre-nesting period as during incubation. In contrast, species with a proven or suspected rapid double- clutch breeding system (Little Stints C. rninuta and Sanderlings C. alba) began to breed later and had during the laying period masses close to those of adults attending broods, and lower than during incubation. Body mass of the two latter species and Curlew Sandpipers (Calidris ferruginea) varied during incubation, reaching peak at 7 to 13 days after clutch completion.

(MYS) Department of Vertebrate Zoology and General Ecology, Biological Faculty, Moscow State University, 119899 Moscow, Russia; (PST) Zoological Museum of Moscow State University; Bol. Nikitskaya Str. 6, 103009 Moscow, Russia.

Introduction

Waders (Charadrii) go through significant changes in body mass in the course of the annual cycle Variation in mass has been most thoroughly studied ha wintering areas and on migration routes. These studies have been instrumental in understanding wader adaptations to long-distant flights and survival in winter (e,g. J o n s o n 1985, SUMMERS et al. 1987, ENS et al. 1990, CASTRO et al. 1992, Su~t~vmRs et al. 1992, LIND- STROM & PmRSMA 1993). During reproduction, waders engage in a number of energy demanding activities like territory defense, egg-laying, incubation rand attending young that should be reflected in changes of body mass. Energetics of r~producfion must be especially limiting in the harsh climatic conditions of the Arctic, which is used for breeding by many wader species. However, previously studies of wader mass dynamics at their breeding grounds in the High Arctic were focused mainly on certain periods within a breeding season (~ g., MORRISON & DAVIDSON 1990, PIERSMA & MORVdSON 1994), and usually did not provide material for inter-year comparison. The most recent studies at the Taimyr peninsula revealed a peak of body mass during incubation compared to brood-rearing in Sandertings (Calidris alba; SoLovmv & TOMKOVICH

272 Journal fiir Ornithologie 138, 1997

1995), Little Stints (Calidris rninuta) and Curlew Sandpipers (Calidris ferruginea; SCHEKKERMA?q & VAN ROOMEN 1995). However, for wader species other than the Sanderling, neither patterns of body mass dynamics in the pre-nesting period, nor the influence of a season's phenology on body mass changes are known. Since three previously considered species belong to uniparental incubators, no data is available on regularities of mass changes in waders with both parents participating in incubation. Addressing these problems, we undertake comparative analysis of patterns of mass dynamics in Little Stints, Curlew Sandpipers, Knots (Calidris canutus) and Turnstones (Arenaria interpres) inhabiting different localities of northern and northwestern Taimyr, Siberia, in an attempt to understand adaptations to the environment of the High Arctic. When studying variation of body mass within breeding stages we also re-analyzed the Sanderling data, because this problem was not accounted for in our previous study of this species.

Materials and methods

Data were collected within the arctic tundra subzone of the Taimyr peninsula (CHn<NOV 1985) in four sites, located at 76°05'N; 98 °32'E (1990--92), 73 °33'N; 80°35'E (1982--83), 75 °16'N; 89 °30'E (1983) and 73 °37'N; 82 °20'E (1984). The sites were situated within 20 km of the Kara Sea, and were characterized by hilly or low mountain landscape (heights up to 200 m asI.) with gentle slopes. Large rivers in different localities were either absent, or had no developed flood-plains belonging to mountain-type streams; lakes were also scarc< The watershed ridges were dominated by nanopolygond spotty (medallion) tundra (CH~RNOV 1985), which was replaced by moss tundra and marshes of sedge-grass Carex stans at the middle and lower parts of the slopes, respectively.

The climate was cold and humid with regular fogs brought by prevailing northern and northwestern winds. The average July air temperature was approximately 4 °C. Years 1982, 1984, 1990 and 1991 were characterized by average dates of snow-mek in June, while 1983 and 1992 were extremely late and cold. The areas and breeding conditions of birds in different seasons have been described in detail elsewhere (ToMKOVICH & VRONSKY 1988 a,b, TOMKOVICH & VRONSKY 1994, TOMKOVICH et al. 1994).

The study period usually covered June and July, with only a few birds measured in August. Birds were caught for ringing and individual color-marking on their nests and near broods with the "luchok'-trap (PRIKLONSKY 1960), a variation of the bow net (BuB 1991). A number of mostly pre-nesting birds were also collected.

Body mass measurements were taken from 133 adult Curlew Sandpipers, 231 Little Stints, 67 Knots, 84 Sanderlings and 31 Turnstones. They were weighed to the nearest 0.1 g (Little Stints) or 0.5 g (other species) with Pesola spring balances in 1990-92 and with beam balances befor~ Body mass data from egg-laying females were excluded from the analysis as they contained developing eggs. Birds were not weighed repeatedly in order to avoid stress from re- captur~

We also measured flattened and straightened wings (SvENSSON 1984) with a stopped ruler to the nearest 0.5 mm, bill length from bill tip to the feather-line on the forehead and tarsus length with calipers (4-0.1 mm). Birds were sexed in the field with different techniques: gonadal examination at dissection for collected specimens, details of mating and territorial

M. Y. SoLovmv & R S. TOMKOVICH: Mass changes in waders 273

behavior of marked birds, breadth of cloaca, measured with a ruler to the nearest 0.5 ram. As waders are sexually dimorphic with respect to size, discriminant analysis of linear measure- ments increased the sample sizes of Sanderlings and Little Stints, sexed with probability exceeding 0.9. All the sexing procedures are described in detail in SoLovmv & TOMKOVlCH (t995).

Analysis of variance (ANOVA; AFIn & AZ~N 1979) was used for statistical analysis of body mass. To control for the effect of nutrient-reserve independent structural size (PmRsMa 1984) on mass, we included the first principal component of wing length, bill length and tarsus length (PC1) in the model as a covariate, which transformed ANOVA into analysis of covariance (ANACOVA).

The basic ANACOVA included three factors with fixed effects: (1) sex (male and female); (2) breeding state (birds in the pre-nesting period, incubating birds, and birds attending broods); (3) year ('normal' and 'late'). Pairwise interactions of factors were included in the model. If the influence of the main effect of sex and its interactions was found insignificant we excluded it from further reestimates, which permitted us to enlarge the sample size by using unsexed birds. Since male Curlew Sandpipers do not participate in incubation and attending broods (ToMKOVlCH 1988), and leave breeding grounds in early July, regardless of the current stage of incubation by females, estimates were made separately for females and pre-nesting males, for this species. To check whether the date rather than the breeding state was responsible for body mass change during the season, we constructed models without the factor 'breeding state', but instead including the Julian date as the covariate (in the form of a linear or quadratic function).

Dates of clutch completion were determined for 432 nests of five species in order to check whether temporal differences between species in reproductive strategies are connected some- how with the pattern of mass dynamics during breeding season, and to study variation of mass within breeding states. These dates were derived from direct observations of laying birds, or back-calculated from the hatching dates on the basis of incubation periods determined by TOMKOVICH et al. (1994). Since data on the dates of clutch completion were not available for pre-nesting birds and also entirely missing or extremely scarce in different species of waders in the late years 1983 and 1992, we estimated separate ANACOVA (when the factor 'sex' parti- cipated) or multiple regression models with PC1 of linear measurements and the time after clutch completion in the form of linear and quadratic functions as independent variables separ- ately for periods of incubation and brood-rearing in normal seasons.

Analysis of studentized residuals allowed us to detect one to two outliers in every species' sample which were excluded from further calculations. Statistical e~imates and graphs were made using SYSTAT 5.0 (WiLKiNSON 1990).

The studies of PST in 1982--84 were financed by the Zoological Museum of the Moscow State University, while the data of 1990-92 were collected by both the authors in the field camp of the International Arctic Expedition of the Institute of Ecology and Evolution, Russian Academy of Sciences, thanks to the financial and logistic support of the expedition organizers. Therefore we express our gratitude to the head of the expedition, Acad. E. E. SYROEC~KOVSX~, and to the expedition staff, especially to N. V. VRONSKY, E. E. SYROECHKOVSKI Jr., M. M. ZAB~LIN, M. G. SINITSYN, A. ABo~Ivs. The study- of MYS was also supported by grant ~ 96-04-49290 of the Russian Fund for Fundamental Studies awarded to G. N. S~KiN. We greatly appreciated the pleasant company and the help in the search for nests and bird catching of the expedition participants in different years: O. HaLngN, E. G. Lappo, G. Th. n~ Roos, A. V. R~xIN, E. E. SYROeCHKOVSS¢I Jr., N. V. VRONSKY, V. O. YAKOVLeV, R Y~sou. We are thankful to I. BVRKJ~DaL, H. HOT~:ZR, J. S. SEDINOER and tk SUMMERS for valuable comments on the manuscript, and to J. TUR~BULL for correcting the English in the final draft.

274 Journal fiir Ornithologie 138, 1997

Resu l t s

The data on body mass of waders in different seasons and breeding states are sum- marized in Table 1. Inclusion of the date in statistical models instead of the factor of breeding state resulted in a decrease of multiple square correlation coefficients, in- dicating a reduction of models' fitness, and the Julian date was not considered in the further re-estimates. The measure of structural size, the first principal component of wing, bill and tarsus length was responsible for 47 to 66 % of their total variance in different species. In the ANACOVA PC1 score was significantly related to body mass of Curlew Sandpipers (P<0.004), Knots (P=0.042), Little Stints (P<0,003), but not to that of Turnstones (P>0.05) and Sanderlings (P>0.9). Body mass of Sanderlings and Little Stints was influenced by sex (P<0.005), while in Knots and Turnstones its influence on body mass after controlling for PC1 score was not significant (P>0.25). Interaction of sex with other factors was not significant for all species (P>0.25). On this basis, both the main effect of sex and its interactions were excluded from further re-estimates for Knots and Turnstones, while models for Sanderlings and Little Stints retained the main effect of sex.

The breeding state had a significant effect on body mass in sandpipers (P<0.001). The high mass of incubating birds, followed by the significant decrease of mass in

Tab. 1. Mean body mass of waders at Northern Taimyr. Average±SD; sample size. n: normal; 1: late.

Species Sex Season Breeding state

pre-nesting incubating brood-rearing

M n

Sanderling 1 F n

1 M n

Little Stint 1 F n

1 M n

Knot 1 F n

1 Curlew M n Sandpiper 1

F n 1

M n Turnstone 1

F n 1

53.2__+3.5; 7 58.3-+-2.8; 12 53.4___4.4; 13 50.8+2.2; 5 53.5; 1 59.7+2.9; 7 64.7±5.1; 16 57.3__+3.0; 6 53.0+__2.8; 5 63.0; 1 26.4--I-1.3; 3 29.1-+-2.3; 36 25.4-+-1.0; 4 25.4+2.2; 5 28.6+2.0; 15 26.2±1.5; 7 30.1-t-2.6; 7 31.4±2.2; 58 27.7--1-1.8; 9 30.5; 1 30.7+1.8; 27 29.0±0.9; 13

133.3+6.5; 8 137.6+10.5; 15 126.3-+-5.8; 17 113.4+3.0; 3 107.7; 1 157.0; 1 138.4__.6.4; 9

59.3-+-4.3; 25 57.5-1-3.4; 13 66.6; 1 65.5___4.4; 63 58.5-+-2.7; 13 67.5; 1 62.1+-5.8; 13 59.1-+-11.8; 2

102.6+11.8; 3 101.8-+-3.8; 6 105.5; 1 107.1; 1 105.4__+9.0; 5

105.1-+-5.9; 7 106.1±5.7; 6

M. Y. SOLOVIEV & I). S. TOMKOVICH: Mass changes in waders 275

brood-rearing waders, was the most pronounced change in mass and was common to all species (Table 2; P<0.001 for the linear contrasts of two respective levels of the factor of breeding state in Calidris species). Body mass of incubating Curlew Sand- pipers in 'normal' seasons was 15.0 % greater than the adjusted mean body mass during brood-rearing. Comparative values were 14.2 % in the Little Stint, 12.3 % in the San- derling, 8.5 % in the Knot, and 9.6 % in the Turnstone. The small sample of Turn- stones with broods did not permit us to show a significant influence of the factor in general (P=0.153), though a linear contrast of incubating and brood-rearing birds, with pre-nesting left out, illustrated presence of a similar tendency (P=0.059).

Little Stints and Sanderlings were significantly lighter (P<0.001) in the pre-nesting than during incubation, having average body mass close to that of birds attending broods (P>0.25). In contrast, pre-nesting Knots and Turnstones were as hea W as incubating birds (P>0.4), the former species significantly differing (P<0.005) from lighter brood-rearing conspecifics (Table 2).

Tab. 2. Adjusted for differences in body size mean masses of waders in 'normal' seasons. Average 4- SD; sample size.

Species Breeding state

pre-nesting incubating brood-rearing

Sanderling 56.4___4.6; 14 61.9-}-5.1; 27 55.1___3.7; 17 Little Stint 28.9+2.6; 9 30.5___2.3; 89 26.7+1.9; 13 Knot 134.34-10.0; 10 136.74-7.9; 25 126.04-7.2; 28 Curlew Sandpiper 59.74-3.8; 24, ce 65.84-4.3; 62, 9 57.24-3.0; 13, 9 Turnstone 103.54-11.8; 3 103.54-5.1; 13 94.44-8.9; 3

Waders were significantly lighter in late years than in normal years (P<0.04), with the exception of Turnstones (P=0.432) and Little Stints (P=0.08). In the latter species, however, there was an interaction between season and breeding state (P=0.011). It took effect in a greater difference between the mass of pr~nesting birds in 'normal' and 'late' years than it was predicted by the sole influence of the main effect of year, while the difference between the mass of waders in other states was, in contrast, diminished and even inverted, though not significantly, for brood-rearing Little Stints. This interaction was not significant related to body mass in species other than the Little Stint (P>0.6). However, in sandpipers the absolutely lightest birds were pre-nesting waders in 'late' seasons.

Median dates of clutch completion were significantly later in 'late' seasons than in 'normal' in all five species (P<0.035, Mann-Whitney test=MW, with the difference ranging from 5.5 to 14 days for a species). However, in species with considerable samples of nests in 'late' seasons (Curlew Sandpiper, Little Stint and Turnstone), the

276 Journal f/ir Ornithologie 138, 1997

between-year difference in median dates was smaller (5.5 to 6.5 days), but more signifi- cant (P<0.004, MW). Curlew Sandpipers, Turnstones and Knots were the first to complete clutches in 'normal' seasons and did not differ from each other in laying dates (P>0.136, for pairwise MW; Figure). These species were followed by Little Stints, having significantly later median dates (P<0.005, MW), while Sanderlings were the latest breeders, compared to four other species (P<0.005, MW).

The species breeding order in 'late' seasons was similar to that in 'normal' ones, though smaller samples for late seasons failed to confirm in pairwise comparisons significant differences in laying dates between the Litte Stint and either Curlew Sand- pipers or Knots (P>0.6, MW). Also, the significance level of differences between Sanderlings and four other wader species decreased, ranging from 0.011 to 0.094 (MW).

Curlew Sandpipers had larger variance of dates of clutch completion in 'normal' seasons than Little Stints (P=0.004, F-test for comparison of variances), Knots (P=0.019) and Sanderlings (P=0.093). Failure to detect a difference with Turnstones (P=0.190) is likely to be due to the small sample of the latter species. We found no difference among other species in variability of laying dates (P>0.15). This particular feature of Curlew Sandpipers is explained by the bimodal type of their arrival to the breeding grounds, when a portion of birds are the first to appear already in pairs, while an additional party of females arrives later and consequently begins to breed later (Toiv~ovIcu et al. 1994).

July 20

10

June 30 *

10 t n=15

Turnstone

n=52 n=68 n=133 n=60

Knot Curlew Little Sanderling Sandpiper Stint

The dates of clutch completion in five wader species in 'normal' years at Northern Taimyr. Box plots show nonparametric statistics: median, quartiles, and outside values of the data (asterisks).

M. Y. SoLoxaEv & P. S. TOMKOVICH: Mass changes in waders 277

The date relative to time of clutch completion contributed significantly both as linear and quadratic functions to variation of mass during incubation in 'normal' seasons in Little Stints (P<0.05). Sanderlings (P<0.06) and Curlew Sandpipers (P<0.01), but not in Knots (P>0.6). Coefficients of equation obtained for the three former species correspond to an inverted U-shape relationship between time after clutch completion and mass. According to these curves the peak of body mass is reached on the 12.6, 10.1 and 7.7 day of incubation in Little Stints, Sanderlings and Curlew Sandpipers, respectively. The samples of incubating Turnstones and brood-rea- ring waders with the exception of the Knot and the Sanderling were too small (less than 15 observations) to justify estimates of curvilinear relationships on their basis. The multiple regression estimate for brood-rearing Knots and Sanderlings failed to show significant influence of time after hatching on body mass in any species (p>0.3).

Discussion

By simultaneously including in statistical models the PC1 score of linear measure- ments and sex of birds, we expected to gain satisfactory control for structural size and interpret possible differences between sexes in mass as mostly nutrient-reserve depen- dent. The results obtained for Sanderlings were discouraging, because for this species the statistical model reported absence of variation in mass, associated with linear dimensions. Instead, a strong influence of the factor of sex, which we expected to be size-independent, was revealed. However, sexual dimorphism in size is known for this species (Socovmv & TOMKOVICH 1995), which means that size-dependent variation of body mass almost completely concentrated in the effect of sex and cannot be separated from size-independent differences between sexes. This precludes any conclusions about sexual difference in mass of Little Stints, which was also significant, for we do not know whether it really means that females of a given size are heavier than males, or that a fraction of size-related variation was attributed to sex. Ambiguity, associated with interpretation of the sources of mass variation, explained by the factor of sex, made us avoid separation of data in table 2 by sexes. Still, we suppose that either the sex, the PC1 score, or both in different species controlled for most of variation in mass due to structural size, leaving us with mostly reserve-dependent effects.

All the waders studied at Northern Taimyr had a common pattern of a decrease in body mass during chick-rearing, compared to incubation. Being found previously in Sanderlings (SOLOVmV & TOMKOVlCH 1995) and in Little Stints and Curlew Sand- pipers (ScHEKI~KUaN & VAN ROOMEN 1995) the high mass of incubating birds was interpreted in both studies as an energetic insurance necessary for birds, spending much energy (field metabolic rate of incubating Turnstones equaled four times their basal metabolic rate, according to PIERSMA & MOKRISON (1994) and having constrained feeding time This adaptive explanation for the accumulation of energy stores by incubating birds is similar to that for the peak body mass of waders, wintering in temperate areas like England and the Dutch Deka (JOHNSON 1985, SCtmKKEKMANN et al. 1992). There, the extra mass was proposed to serve as an energetic insurance for

278 Journal fiir Ornithologie 138, 1997

periods of negative energy budget (DUGAN et al. 1981). The field metabolic rate of Knots during the temperate winter period is about 4 times the basal metabolic rate at this time of the year (POOT & PIERSMA 1994), being identical with measured field metabolic rates of a similarly sized shorebird species, the Turnstone during the incuba- tion period in the Canadian High Arctic (PIEr, SMA & MORRISON 1994).

A possible advantage of lower mass during brood-rearing as adaptation to cheaper flight when performing anti-predator behavior near chicks was mentioned both in studies of SOLOVIEV & TOMKOVICH (1995) and SCt-mKKEI~'aAN & VAN ROOMEN (1995).

Confirming the peak of body mass in incubating Little Stints and Curlew Sand- pipers, we also found it for Knots and Turnstones. The extra mass relative to brood- rearing of biparental incubators the Knot and the Turnstone (8.5 and 9.6 %, respec- tively) was smaller than that of spedes with a single incubating bird (12.3 to 15.0 %), which obviously have to spend more time on the nest. This provides an additional indirect support to the hypothesis about the compensatory nature of stores deposited by incubating birds.

SCHEKKERMAN &; VAN ROOMEN (1995) found that the mass of Curlew Sandpiper females decreased as hatching approached. They lacked, however, weight data for the first half of the incubation period, which, probably, precluded revealing the curvilinear nature of mass variation within the incubation state, found for Little Stints, Curlew Sandpipers and Sanderlings in our study. Given the need to accumulate compensatory stores for incubation, a bird can hardly be expected to have a full stock at the very moment of completing the energy demanding process of egg-laying. The gradual increase of body mass during the first one or two weeks of incubation, probably, corresponds to the steady transition from one physiological condition to another. As the season progresses, environmental conditions (air temperature and food supply) improve, diminishing the need in energetic insurance, at one hand, while approach of hatching demands to reduce body mass in advance, at the other hand, that can explain a decrease of mass in the second half of incubation. The failure to reveal similar mass dynamics during incubation in Knots is, probably, connected to the fact that peak of mass is less pronounced in this species than in uniparental incubators.

The pronounced influence of phenologically unfavorable seasons on wader mass during the pre-nesting period seems natural, because birds have to spend much energy either on territory defense (males), or egg production (females) (t~CKLeFS 1974, ASHKENAZIE & SAFRIEL 1979, GAV~tOV 1988). The first shorebirds of different species appeared from 6 to 10 June (median 8 June) in 'normal' years and from 10 to 19 June (median 12 June) in 'late' ones (based on the data from TOMKOVICH & VRONSKY 1988a,b, TOMKOVICH & VRONSKY 1994, TOMKOVICH et al. 1994), while the snow melting developed in 1992 with an approximate delay of 10 days, compared to 1991 (our data). This indicates lesser variation in arrival dates than in snow conditions, which may prevent feeding and breeding of birds at arrival in late seasons and make them wait for the appearance of snow-free patches, meanwhile depleting energy stores left after the flight from the last staging area. Poor feeding conditions at the beginning of reproduction may still affect the body mass during incubation, but by brood-rearing

M. Y. SOLOVIEV & P. S. TOMKOVICH: Mass changes in waders 279

a new generation of surface-active insects appears (MYERS & PITELKA 1979, LANTSOV & CHERNOV 1987), and food-shortage can no longer be connected with weather conditions at the start of the season.

The critical role of the pre-nesting period with its instability finds indirect evidence in the dynamics of numbers of the islandica subspecies of the Knot in Britain in January, that tended to be lower after cool Junes in the breeding range, and to be higher after warmer ones (BoYD 1992).

The inter-species differences in patterns of body mass change, when swkching from pre-nesting activities to incubation, correspond approximately to differences in breeding schedules. Two early nesting species (Knots and Turnstones) incubate with a body mass similar to that obtained during a phenologically normal pre-nesting period, while two late breeders (the Little Stint and the Sanderling) have low pre-nesting masses, which increase after the start of incubation. Assuming shorebirds arrive at arctic breeding areas with substantial nutrient reserves (MoP.RISON & DAVIDSON 1990), we hypothesize that Lktle Stints and Sanderlings use their stores for some energy demanding activities longer than Knots and Turnstones, thus reaching a lower body mass. Variation among species in mass dynamics may result from interspecific differen- ces in breeding strategies. Knots and Turnstones are monogamous (CRAMP & SIMMONS 1983), for Little Stints the double-clutch system (rapid multiclutch system after LENINGTON (1984)) was proved (ToMKOVICtt et al. 1994), while for Sanderlings it was strongly suspected (PateemLEE & PAYNE 1973, personal observations), but not con- firmed directly. A species, in which a female lays two clutches and a male mates with two females in succession (HILDgN 1975), should have later average nesting dates and deplete more stores during longer pre-nesting periods, that indeed occurs in the case of Little Stints and Sanderlings.

Thus, waders breeding at Northern Taimyr have similar dynamics of body mass in the middle and end of the summer, while patterns of transition from pre~nesting to incubating masses differ among species in accordance with differences in breeding schedules and associated reproductive strategies.

The above attempt to interpret the observed patterns of mass dynamics is based on the general biological information about species and does not involve data on energy budgets, measurements of field metabolic rates and other information, apparently necessary to solve the problem. Also some gaps in body mass data for certain species in some periods (e g., pre-nesting Curlew Sandpiper females, pre-nesting and young- attending Turnstones) should be filled to make the picture complete. However, even this limited approach permitted us to suggest explanations, worth being checked by more focused studies in future.

Zusammenfassung Von fiinf Tundra-Watvogdarten wurden in sechs Brutperioden Daten zur K6rpermasse gewonnen, um die Dynamik der Energiereserven in Abh~ingigkeit yon den klimatischen Bedingungen der Hocharktis und yore Fortpflanzungsstadium zu analysieren. Der Einflu~

280 Journal fiir Ornithologie 138, 1997

der K6rpermaf~e auf die Masse wurde bei der statistischen Auswertung durch die Einbezie- hung der 1. Hauptkomponente von Fliigel-, Schnabel- und Tarsusl~nge beriicksichtigt. Strand- l~iufer Calidris spec. erwiesen sich in sp~ten Jahren als leichter, w'~hrend in der K6rpermasse yon Steinw~ilzern (Arenaria interpres) keine Unterschiede festgestelk werden konnten. Die K6r- permasse aller untersuchter Arten war w~hrend der Betreuung der J.ungv6gel geringer als w~h- rend Briitezeit. Die Masse in der Zeit vor der Brut variierte in Ubereinstimmung mit den wechselnden Wetterbedingungen in den einzelnen Jahren am st~irksten. Monogame Arten (Knut Calidris canutus und Steinw~lzer) begannen friiher mit der Fortpflanzung; ihre K6rper- masse in der Phase yon der Brut und w'~hrend der Briitezeit war im Mittel etwa gleich. Im Gegensatz dazu begannen Arten, bei denen ein ,Doppelbrutsystem" nachgewiesen wurde oder vermutet wird (Zwergstrandl~ufer C. rninuta und Sanderling C. alba), sp~ter mit dem Nestbau, wobei die K6rpermasse auf ein Niveau absinkt, das dem w~ihrend der Betreuung der Jungv/Sgel nahekommt und geringer ist, als das w'~ihrend der Briitezeit. Bei den beiden letzten Arten sowie beim Sichelstrandl~iufer (Calidrisferrugunea) ver'~inderte sich die K6rpermasse w'~ihrend des Briitens und erreichte ein Maximum zwischen dem 7. und dem 13. Tag nach Beendigung der Eiablage.

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