Body Mass Dynamics of Wintering and Pochard A. Ferina in Switzerland

Body mass dynamics of wintering Tufted Duck Aythya fuligula and Pochard A. ferina in Switzerland

MATTHIASKESTENHOLZ

Using body mass as an indicator of body condition, data from 2973 wintering Tufted Duck and 1335 Pochard trapped at Lake Sempach or shot at Lake Constance were compared and analysed with respect to sex, age, year, seasonal trends, temperature and sites. Throughout the season, males were heavier than females in both adult and first-year age classes, and adults were heavier than first-winter birds of the same sex. Age-related differences were retained throughout winter. Only minor differences were found between years. Body mass of Pochard at Lake Sempach and of female Tufted Ducks at Lake Constance decreased with decreasing am- bient temperature. Seasonal patterns differed markedly between the two wintering sites according to feeding conditions and food availability. Diving ducks were able to keep their body mass constant until January or February at Lake Constance, while at Lake Sempach population means of body mass peaked in December when feeding conditions were favourable, and clearly dropped during mid and late winter when feeding conditions were poor. Here, body mass changes of individual ducks suggested a quick adaptation to low site-specific values. This rapid weight loss might reduce survival probability or might force ducks to change wintering sites. Body mass dynamics reflecting habitat quality provide strong evidence that diving ducks attempt to keep body weights as high as possible throughout the winter, and that energy reserves are controlled mainly by proximate factors superimposed on endogenous regulation.

Keywords: Tufted Duck, Pochard, Body Mass, Winter, Sex, Age, Habitat Quality.

Winter, with temperatures below freezing ter. Low nutrient reserves may even affect point and limited food resources, is a cru- reproductive success of female Ring- cial period in the life of diving ducks. Over- necked Ducks Aythya collaris nesting in winter survival may depend upon energy northern bog marshes characterized by reserves to cope with food shortages and low primary production (Hohman 1986). increased thermoregulatory demands dur- On the other hand, several studies with ing harsh weather conditions. Body mass is captive ducks fed ad libitum suggest an en- correlated positively with fat reserves in dogenously regulated loss of body mass Redheads Aythya americana (Bailey 1979) that may increase survival by reducing en- and Canvasbacks A. valisineria (Serie & ergetic costs (perry et al. 1986 for Aythya Sharp 1989, Hohman 1993). It can be con- valisineria, Loesch et al. 1992 for Anas sidered a good indicator of body condition. platyrhynchos) . For Canvasbacks, body mass combined Body weights of wintering diving ducks with season and sex-age classes are the have been collected from several North most important variables for the estima- American species (Bellrose & Hawkins tion of body condition (Sparling et al. 1992). 1947, Weller 1957, Ryan 1972, Haramis etal. Haramis et at. (1986) showed that birds 1986, Perry et at. 1986, Hohman et al. 1988, with a relatively high body mass and good Hohman 1993) while comparable data for body condition in early winter had a higher European diving ducks are lacking. Demen- probability of surviving the winter and the tiev & Gladkov (1952), Bauer & Glutz von following year. Data for American Black Blotzheim (1969) as well as Cramp & Sim- Ducks Anas rubripes are ambiguous: Kre- mons (1977) summarized all available data, mentz et al. (1989) did not find a positive re- combining them for various sites and lation between late-winter body mass and months. For Tufted Ducks Aythya fuligula, annual survival, but Pollock et al. (1989) Mlikovsky & Buric (1983) listed data from mentioned a higher probability of adult the literature without commenting on ducks surviving the winter if they are in them; they give mean values for each sex good condition at the beginning of the win- and month, but do not distinguish between 147 Wildfowl 45 (1994): 147-158 148 Body mass dynamics of diving ducks in winter Tufted Duck 1000

900

(/) cu(/) E 800 >, "'0 o CO 700

600 NOV NOV DEC DEC JAN JAN FEB FEB MAR MAR

Figure I. Bi-monthly mean body mass (g) of Tufted Duck trapped at Lake Sempach (:!: standard deviation). Full triangles = adult males, open triangles = first-year males, full circles = adult females, open circles = first-year females.

Table 1. Bi-monthly mean body mass (g) of Tufted Duck trapped at Lake Sempach (± standard deviation), sample size, minimum and maximum. 1-15 Nov 16-30 Nov 1-15 Dee 16-31 Dee 1-15 Jan 16-31 Jan 1-15 Feb 16-29 Feb 1-15 Mar 16-31 Mar

819±60 809±67 843±59 868±49 821±77 801±67 809±69 781±64 774±65 768±57 adM n=51 n-44 n=27 n=27 n=77 n=85 n=56 n=128 n=83 n=30 680-950 650-950 710-910 765-1015 650-975 620-955 660-995 625-975 600-915 630-920 753± 72 778± 71 810±55 821±74 791±56 760±57 770±56 731±56 729±57 731±55 juvM n=65 n=123 n=68 n=67 n= 187 n=161 n=76 n=155 n=110 n=59 560-910 570-1070 670-940 580-1005 620-925 610-915 625-900 595-895 610-850 600-850 742±62 745±54 759±40 778±28 789±56 742±59 732±74 713±62 703±73 672±39 adF n=19 n=20 n=lO n=5 n=17 n=23 n=26 n=35 n=34 n=12 630-870 625-840 690-800 730=815 680-890 625-850 600-955 595-850 585-830 605-730 692±63 708±47 732±62 752±65 740±57 690±59 709±58 660±56 680±49 674±56 juvF n=42 n=38 n=45 n=30 n= 117 n=77 n=71 n=153 n=88 n=48 545-870 620-825 570-860 635-870 610-890 580-830 580-835 490-820 575-835 560-825

Table 2. Bi-monthly mean body mass (g) of Pochard trapped at Lake Sempach (± standard deviation), sample size, minimum and maximum. 1-15 Nov 16-30 Nov 1-15 Dee 16-31 Dee 1-15 Jan 16-31 Jan 1-15 Feb 16-29 Feb 1-15 Mar 16-31 Mar

936±106 997±78 991±62 1004±76 970±76 946±70 917±69 893±63 888±60 904±28 adM n=4 n=28 n=37 n=38 n=52 n=67 n=39 n=54 n=28 n=8 830-1080 840-1250 840-1110 835-1185 720-1110 78

November December January

880±98 891±63 845± 73 adM n=13 n=23 n=91 730-1020 730-980 690-1030 830±79 769±66 795±76 juvM n=6 n=9 n=74 720-930 690-890 640-980 785±68 785±81 799±64 adF n=7 n=l7 n=54 700-910 675-925 670-970 748±114 755±59 742±70 juvF n=5 n=25 n=60 600-880 630-900 600-870

Table 4. Monthly mean body mass 00 of Pochard shot at Lake Constance (± standard deviation), sample size, minimum and maximum.

November December January

IOO7±114 I018±108 943±77 adM n=l1 n=23 n=26 800-1170 820-1230 810-1120 955±128 966±126 870±81 juvM n=4 n=12 n=5 820-1100 760-1225 790-980 938±86 950± 77 832±96 adF n=3 n=14 n=5 870-1035 800-1050 710-960 IOOO±60 972±97 687±85 juvF n=3 n=5 n=3 940-1060 830-11 00 600-770

Table 5. Body mass 00 of adult male Tufted Ducks from various wintering sites. location date mean s.d. n remarks reference

Lake Zurich January 1023 54 13 Giintert (1978) Dutch Waddenzee Jan-Feb 987 140 82 ad+juv Suter & van Eerden (1992) German Baltic coast Nov-Feb 952 79 8 Piechocki (1987) Lake Constance January 891 63 23 this study Lake Sempach January 811 73 162 this study Caspian Sea offshore January 882 ? ? Dementiev & Gladkov (1952) Caspian Sea coast January 717 ? ? Dementiev & Gladkov (1952) Camargue January 729 77 100 Bauer & Glutz (1969) adult and first-winter birds. Data for dictable (Lovvorn 1994), there is little evi- Pochards A. ferina are even scarcer than dence for genetic, time-dependent regula- for Tufted Ducks. tion of energy reserves in wintering diving This study compares body weights and ducks. their seasonal trends from two different sites. Such differences could point to different survival probabilities among habitats Material and Methods and, therefore, might also support the hy- pothesis assuing~ primarily exogenously Lake Sempach controlled body mass changes. It is as- sumed that diving ducks that face unpre- The hypertrophic Lake Sempach (47°09'Nj dictable weather or changing feeding con- 8°0TE, 504 m ASL) in the central Swiss low- ditions attempt to keep the highest lands has a surface of 14.4 km2. Zebra Mus- possible body weight throughout winter. sels Dreissena polymorpha, which consti- Because the timing of cold spells is not pre- tute the main food of diving ducks on most 150 Body mass dynamics of diving ducks in winter Pochard 1200.------

1100

U) 1000 U) CO E >, 900 "0 0 OJ 800

700

600 NOV NOV DEC DEC JAN JAN FEB FEB MAR MAR Figure 2. Bi-monthly mean body mass 00 of Pochard trapped at Lake Sempach (± standard deviation). Full triangles = adult males, open triangles = first-year males, full circles = adult females, open circles = first-year females.

Swiss waters, have not yet been recorded numbers of diving ducks decrease rapidly ([urner 1984, pers. obs.). Therefore, diving and in mid-January, counts made in 1961-89 duck populations are small and their food averaged 172±132 Tufted Ducks and 75±60 normally consists of small benthic inverte- Pochards (Kestenholz 1990). In the winters brates and submerged plants, mainly Pota- of 1987-88, 1989-90 and 1990-91, 2589 mogeton sp. In the second half of Decem- Tufted Ducks and 1221 Pochards were ber, however, numbers of diving ducks caught in a wire trap baited with corn and increase quickly up to 1500 when the birds fish offal (Hofer & Marti 1988). The ducks are feeding on the spawn of Coregonus sp. usually entered the trap during the night, fish which is temporarily available in large were held in bags for 4-6 hours to allow the quantities (Kestenholz 1990). As the avail- passage of ingesta (Haramis et at. 1982), ability of spawn ceases in early January, and released in late morning after being

Tufted Duck 1000-,------,

900

U) U) co E >, 800 'D 0 rn 700

600 DEC JAN FEB Figure 3. Monthly mean body mass 00 of Tufted Duck trapped at Lake Constance (n standard deviation), sample size, minimum and maximum. Full triangles = adult males, open triangles = first-year males, full circles = adult females, open circles = first-year females. Body mass dynamics of diving ducks in winter 151 Pochard 1200 1100 I en en 1000 ro E >, 900 f "0 tjH 0 tIt m 800 tH 700 ~ I 600 DEC JAN FEB Figure 4. Monthly mean body mass (g) of Pochard shot at Lake Constance (:!: standard deviation). Full triangles = adult males, open triangles = first-year males, full circles = adult females, open circles = first- year females.

ringed, sexed, aged and weighed to the Lake Constance nearest 5 g. The weights recorded were of unringed birds or of individuals handled Lake Constance on the Swiss-German bor- for the first time that season in order to der (7°35'Nj9°20'E, 396 m ASL) has an area avoid bias from repeated captures. of 541.2 km2. Its western part comprises two wintering sites of international impor- tance for Tufted Duck, Pochard,

Tufted Duck 1100l

1000

---. 900 -9 en ~ en ro 800 E // >, "0 I 0 ~ .D 700

600 1

500 1 NOV DEC JAN FEB MAR Figure 5. Body mass changes (g) of individual Tufted Duck wintering at Lake Sempach. 152 Body mass dynamics of diving ducks in winter

Tufted Duck

100.------,

.9 0 Q) u c - ~ ~ . •.- ~ ~ D -100 U) U) - D 0 -200 -D .

-300 -100 0 100 200 300

relative body mass (g) at capture

Figure 6. Body mass difference 00 of individual Tufted Duck wintering at Lake Sempach in relation to relative body mass at capture rmC 00. Relative body mass is referring to corresponding population means of sex and age.

Tufted Duck 100

.•

.9 0 Q) u c ~Q) ~ . '6 -100 . U) U) - D 0 -200 ..a

-300+------,,------.------1~ o 10 20 30 40 time-lapse between capture and recapture (days)

Figure 7. Body mass difference 00 of individual Tufted Duck wintering at Lake Sempach in relation to time-lapse (days) between capture and recapture. Body mass dynamics of diving ducks in winter 153

Tufted Duck 10 ~ CI:1 -0 -9-- >. o CI:1 . - - -0•.. - - Q) • a.. - Q) .- Q) c -10 CI:1 .c u (/) (/) CI:1 E -20 >. -0 o .D

-30 o 10 20 30 40 time-Iapse(days) between capture and recapture

Figure 8. Body mass change per day (gjday) of Individual Tufted Duck wintering at Lake Sempach in relation to time-lapse (days) between capture and recapture.

Red-crested Pochard Netta rufina and Gad- of various combinations of day 1 to 7; best wall Anas strepera (Marti & Schifferli 1987). results are given). At Lake Sempach, 29 Numbers of diving ducks peak in November Tufted Ducks were retrapped up to three (Schuster et ai. 1983) and decrease to times within 40 days. Since mean body 43,000 Tufted Ducks and 19,000 Pochards mass changed markedly in the course of in mid-January (averages of 1976-87, Suter the winter, relative body weights for cap- 1991). They feed almost exclusively on ture rmc and recapture rmR were calcu- zebra mussels which have mean densities lated as the differences between body mass of 4500 g freshweight j m2 (Suter 1982). and the corresponding means (Figure 1) of Body masses of 384 Tufted Ducks and 114 sex, age and season. Statistical tests were Pochards were collected from hunters' carried out using program SPSSjPC+ V2.0 bags in 1982-83, 1983-84 and 1984-85 (Meile (SPSS Inc., Chicago, 1988). 1991). Birds were sexed and aged according to plumage characteristics and these facts Results were confirmed by cloacal examination if necessary (Bauer & Glutz von Blotzheim Body mass 1969). In this paper, first-year or young refers to birds hatched in the preceding At Lake Sempach, the body weight of breeding season, adult refers to at least one Tufted Duck (Figure 1) varied by sex year old birds. (three-way ANCOVA Fl,2576=672, P

Human activities, e.g. water sports and fish- food availability results in emigration of ing, might also influence body conditions most of the diving ducks. Trends of re- but are not discussed in this paper. Distur- trapped Tufted Ducks indicate that body bances by hunting may also affect mass weights quickly adjust to low lake-specific negatively. However, in spite of important values. By contrast, favourable feeding hunting activity at Lake Constance, masses sites promote accumulation of fat reserves. in December were the same as at Lake Sem- At Lake Constance, body weights of Tufted pach. Ducks feeding on Dreissena polymorpha mussels are constant and significantly Effects of sex and age higher than at Lake Sempach except for De- cember. These clear differences in body For both species, males were heavier than mass and, especially, in seasonal body females and adult ducks heavier than juve- mass trends fit well with feeding conditions niles. This corresponds to most other stud- while climate is the same for both sites. ies (Ryan 1972, Sparling et al. 1992) but Tufted Ducks from the German Baltic coast Hohman (1993) reports that in wintering (piechocki 1987) are slightly lighter in De- Canvasbacks females are heavier than cember and slightly heavier in January males although males are larger, and Gau- than at Lake Constance. Their main food thier et al. (1992) did not find body mass consists of Mytilus edulis and Cardium sp. differences between sexes of wintering mussels. The area of Lake Ijsselmeer and American Black Ducks which show no clear the westernmost part of the Dutch Wad- sexual dimorphism in plumage coloration. denzee provide even better feeding condi- At both sites, age differences were retained tions with important mussel concentra- until late winter. Thus, first-year birds from tions, mainly Mytilus edulis. This major Switzerland will not attain adult weights be- wintering site holds up to 160,000 Tufted fore spring or summer, probably causing Ducks and 50,000 Pochards. Diving ducks reduced breeding success because of de- freshly drowned in gill nets in February layed spring migration, reduced nutritional 1980-89 show even higher winter masses reserves or failed pair formation. Physical (e.g. 987±140 g for male Tufted Ducks, Suter condition as well as reproductive success & van Eerden 1992). Highest winter masses (clutch size and one-day-old duckling of Tufted Ducks (e.g. 1023±54 g for adult weight) of female Pochards and Tufted males) recorded from Switzerland are Ducks nesting at the Engure Marsh, Latvia, those of 41 birds drowned at Lake Zurich in were clearly age-dependent (Blums et al. January 1974 (Giintert 1978) where feeding 1985). On the other hand, Ring-necked conditions were excellent due to large num- Ducks wintering in Florida adjust to adult bers of Zebra Mussels. Water in the birds' body mass during winter (Hohman et al. lungs might partly explain these extremely 1988). high values, but Giintert (1978) also re- ported massive fat deposits. Thus, food Effects of habitat quality on body mass and availability plays a decisive role in regulat- seasonal trends ing body weight; the type of food is much less important (Hohman 1993). Ducks win- Average body mass and seasonal pattern tering at the most southern sites with a vary greatly among sites (Table 5) suggest- mild winter climate show a constant in- ing a major influence of habitat quality even crease in body mass during winter. In the if this comparison is very tenuous because Camargue, southern France, mean body of the different sampling techniques. Food weights of Tufted Ducks and Pochards in- availability, winter climate and distur- crease until February (Bauer & Glutz 1969), bances (predation, hunting) are among the Ring- necked Ducks wintering in southern main factors determining the quality of a Florida put on fat reserves throughout the wintering site. At Lake Sempach, in Decem- wintering period (Hohman et al. 1988) and ber, large quantities of fish spawn offer Redheads from two sites in Louisiana also temporary feeding conditions (Kestenholz gained body mass during winter (T. Michot, 1990) comparable to Lake Constance, and in lift.). In northern latitudes, however, body mass is the same at both sites. But body mass of wintering diving ducks nor- this food resource is available for only a mally peak in mid winter and decrease sub- few weeks. As a consequence, body mass sequently to reach weight minima in late drops thereafter. The rapid decrease in winter (e.g. Ryan 1972 for Aythya ameri- 156 Body mass dynamics of diving ducks in winter cana). Therefore, starvation is most likely ded in experiments with captive Mallards to occur at the end of the wintering season. fed ad libitum (Loesch et al. 1992.) or in The body mass decrease from mid to late wild Mallards at profitable feeding sites winter was suggested to be an endoge- (Whyte & Bolen 1984). But Lovvorn (1994) nously regulated adaptive reaction to re- observed that Canvasbacks, Redheads and duce energy costs for maintenance, flight Lesser Scaup Aythya affinis reduced or costs and predation risk (perry et al. 1986, even stopped feeding during very cold Gauthier etal. 1992, Loesch et al. 1992). Pro- weather. Thus, harsh weather or poor feed- vided that such endogenous patterns are ing conditions may trigger emigration. At involved, it seems that they are often su- Lake Constance, a significant relation be- perimposed the influence of environmental tween body mass and temperature was factors and therefore are not always per- found only in female Tufted Ducks, which ceptible. The area of the Caspian Sea pro- were the smallest birds considered. At vides another example of environmentally Lake Sempach, an influence of temperature influenced body weights where flock size was observed for Pochards but not for seems to be an indicator of habitat quality. Tufted Ducks. Here, the situation is confus- Big flocks of Tufted Ducks foraging offshore ing because most ducks are forced to leave had higher individual body mass than small the lake as soon as feeding conditions de- groups at the coast (Table 5, Dementiev & teriorate. In the worst case, cold spells co- Gladkov 1952). And for Canvasbacks win- inciding with depleted food resources may tering at different sites in the USA, body cause severe mortality. Such a mass star- mass pattern varying widely among lati- vation of diving ducks was recorded at tudes and years indicate that Canvasbacks Lake Constance in late February and March maintain greater reserves if proximate con- 1986 (Suter & van Eerden 1992). ditions allow (Lovvorn 1994). Body conditions reflecting habitat qual- Diving ducks facing insufficient habitat ity seem to be related to fitness (Haramis et quality have to change behaviour in order al. 1986). Future research, therefore, to avoid loss of body condition. Low tem- should focus on annual survival in relation peratures could be compensated with in- to different wintering locations. This creased food intake as long as excellent should help to estimate the amount and feeding conditions prevail. For instance, a type of habitat needed to support winter- temperature influence has not been recor- ing populations of diving ducks.

I am most grateful to l. Hofer for providing the data from Lake Sempach and to P. Meile for col- lecting the ducks from Lake Constance. I wish to thank 1. lenni and B. Naef-Daenzer for statistical advice and B. Bruderer, F Liechti, C. Marti, 1. Schifferli, s.c. Steams and W Suter for critically reviewing and improving the manuscript.

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Matthias Kestenholz, Swiss Ornithological Institute, CH-6204 Sempach, Switzerland.