Growth and Morphology of Captive Female Common Eider Somater/A Moll/Ss/Ma Ducklings

GROWTH AND MORPHOLOGY OF CAPTIVE FEMALE COMMON EIDER SOMATER/A MOLL/SS/MA DUCKLINGS

P A J HAWKINS, P J BUTLER' andA J WOAKES

University of Birmingham, School of Biological Sciences, Edgbaston, Birmingham B I S 2TT, UK

'For reprint request

Five female Common Eider ducklings taken under licence from a population at the Isle of May, Firth of Forth, Scotland, were reared from eggs. Weekly measurements were made of the tibiotarsus, ulna, head, ninth primary, body mass, wing and foot area. The body components attained adult size in the following order: tibiotarsus (six weeks), ulna (seven weeks), foot area (eight weeks), body mass and head (nine weeks), wing area and wing loading (ten weeks) and ninth primary (eleven weeks). Paddle index (the ratio of foot area to body mass) remained relatively constant throughout development. Up to and including seven weeks old, body mass could be used as an estimate of age according to

the relationship A = 0.005 MB + 0.015,where A = age (weeks) and MB = body mass

2 (g) (R = 0.98). 'Feathering up' began by the second week and was completed during weeks seven to eight. There was no noticeable trough in the body mass curve when the ducks fledged at nine to 10 weeks.

Keywords: Growth, Development, Morphology, Eider Duck, Sea Ducks

The Common Eider Duck Somater;a mol/iss;ma is fledging earlier than diving species (Kear 1970, a large-bodied sea duck which is distributed Lightbody & Ankney 1984). The leg usually across most of the Arctic, Northern America and develops quickly in waterfowl, as an adaptation Northern Britain. It breeds at latitudes between for the journey from the nest to the water and 44°N and 800N (Madge & Burn 1988), and the subsequent aquatic locomotion (Lack 1968, Kear largest British breeding colonies occur at the 1970, Siegfried 1973, Brown & Fredrickson 1983, east coast of Scotland and Northumberland Leray & Yesou 1989, Bowler 1992), with flight (Owen et al. 1986). The species comprises six muscles and wings developing more slowly races (mol/issima, faeroeensis, borealis, dresseri, (Sedinger 1986, Duttmann 1993, Bishop et al. sedentar;a and v-n;grum), with differing body 1996). mass and morphometries. British and South Body growth in the Common Eider duckling Norwegian Eider Ducks are of an intermediate has been partly examined by Leray & Yesou type, falling between S. mol/issima mol/issima and (1989), who measured the body mass, culmen, S. mol/iss;ma faeroeensis (Cramp & Simmons tarsus and 'bent wing' in two captive ducklings 1977). for 13 weeks. Body mass in Eider ducklings for S. mol/issima produces a small brood of large the first 10 days post-hatch has also been ducklings (Owen & Black 1990). The precocial measured by Swennen (1989). The aims of the young leave the nest as soon as they are dry, six present study were to monitor skeletal growth to 12 hours after hatching (Mendenhall 1979) to adulthood in a group of five captive Eider and can swim, eat and dive as soon as they reach ducklings, and to describe the morphology of the water (Steen & Gabrielsen 1986). Post hatch the ducks when they attained adult size. All the growth rates of ducks are influenced by feeding hatchlings turned out to be female, so this habits, with dabblers gaining body mass and paper may not be applicable (at least

quantitatively) to male ducks, as all races of the submerging rubber bungs, mussel shells, pebbles Eider are sexually dimorphic (Milne 1976). and bricks in their pool and fastening chains and Difference in size between the sexes of another plastic cable ties to the side of their pen dimorphic species, the Tufted Duck Aythya (Hawkins 1998). fuligula, arise within the first few weeks of life At six weeks old, the birds had grown too (Kear 1970), although sexual dimorphism in large for the indoor pen and were sufficiently Lesser Scaup and Canvas back ducklings is feathered to be kept outdoors. They were 'minimal' until they are around 71 days old moved to an outdoor enclosure of 18m x 7.4 m (Lightbody & Ankney 1984). There are also x 2m high, with an area of grass, an area of likely to be quantitative differences in size gravel and a pond (8.1m x 3.8m, O.7m deep). between hand-reared and wild birds (Swennen The pen was fully enclosed by netting but was 1989, Bowler 1992, Sedinger 1992) and this not covered. In July 1994, mean temperature should be borne in mind when considering the was 18°C and mean rainfall was 56.6mm. In results presented here. August, mean temperature was 15.5°C and rainfall 48.4mm. Method Each duckling was weighed and measured on the day of hatching, then at weekly intervals Twelve Eider duck eggs were collected from a until their morphology had stabilised. colony on the Isle of May, Firth of Forth, with Measurements were taken using callipers of the the permission of Scottish Natural Heritage. ducklings' tibiotarsus, ulna, head length and ninth Eggs were taken from nests containing four or primary. The measurements were made by the more eggs, and only one egg was taken from same observer throughout the study. Wing and each clutch. Five female birds hatched (sexed foot area were measured by drawing around the by later plumage development) in the fully extended wing or foot and weighing the laboratory during May and June 1994, but the paper, having first weighed pieces of paper of remaining seven eggs unfortunately failed to known area to correlate area with mass. Paddle hatch. The ducklings were imprinted on the index and wing loading were calculated experimenter for ease of handling and according to Raikow (1973); the square root of reduction of stress. Up to eight hours a day the area of both wings (or feet) divided by the were spent with the birds during the first cube root of the body mass. The hallux was month of life, and they were taken for a daily included in the foot area (Raikow 1973). After walk for the first six weeks. Although Eider studies involving the ducks had finished, it ducklings rarely walk far in the field, walking would not have been humane to release them provided them with supplementary exercise into the wild as they had not learned to forage and enabled them to habituate to the outdoor for themselves. However, they were certified fit compound in which they would eventually live. by a veterinary surgeon and rehomed in a Initially,the ducklings were housed in an indoor private bird collection. pen of 2.1 x 3.6 m, containing a pool (2.1 x 2.2 Means are presented ± I S.E.,and differences m and 0-22 cm deep). A brooder lamp was between means were tested using Student's t- fitted, but only used by the birds on the first tests, at a 5% level of significance. Although day. Mean indoor temperature was 14.5°C and complex growth curves are often used to relative humidity 60% over the six weeks. describe increases in body components with Lighting was controlled by a time switch that age (Ricklefs 1967, 1973, 1983) these are not was set to coincide with outside daylight hours. always applicable to fledging waterfowl The ducklings were fed commercial grower's (Sedinger 1986, Bishop et al. 1996). In the chick crumbs at first, introducing SDS 'Diet A' present study, the growth of each variable with sinkers during the second week. By the third time was best quantified using linear (y = a +

b week, the ducklings were fed 'Diet A' alone and bx), power (y = a x ), logarithmic (y = a + b In grit was supplied on the bottom of their pool. x) and exponential (In y = a + bx) regressions. Their indoor environment was enriched by Only the regression that afforded the most 94 GROWTH OF CAPTIVE FEMALE COMMON EIDER DUCKLINGS accurate fit for each variable is presented. 0.0 I5, where A = age (weeks) and Me = body mass (g) (R' = 0.98). During the first eight Results weeks, head length could also be used as an estimate of age. (In A = (In H - 4.05) + 0.35; R' Figures I to 3 show the mean values of all the = 0.98, where A = age (weeks) and H = head variables measured at hatch and thereafter. The length (mm)). power regression of body mass vs. age [Me (g) 'Feathering up' began by the second week, = 178.23 A'04,where A = age in weeks] fitted with emerging contour feathers along the the data best between one and eight weeks ducklings' flanks (Figure 4). By the third week, (Table I). The increase in mass could also be contour feathers were beginning to grow on described by a linear equation between weeks the upper tail coverts, and could also be felt on one and seven (Me (g) = 197.01 A + 7.91) the abdomen. The primaries were erupting by (Table I). The slope of 197 reflects the the fourth week, and contour feathers had assimilation rate of the birds, ie they were emerged on the sides of the head, mantle and putting on 197 g in body mass per week during abdomen. Feathers were present all over the this period. All measurements were constant birds except for the rump by the fifth week, and after 14 weeks, when the ducklings were down continued to be shed throughout the morphologically identical to adult birds (t-test following week. By week seven, there was one results, in Table I). The measured variables small patch of fluff remaining near the tail, which were also checked six months later and had not was shed over the next few days, and the increased. primaries were well developed. Body components attained adult size in the following order: tibiotarsus (six weeks, Figure Discussion 2), ulna (seven weeks, Figure 2), foot area (eight weeks, Figure 3a), body mass and head Differential growth af body components length (nine weeks, Figures Ia and Ib), wing area and wing loading (10 weeks, Figures 3a In general, the legs and feet developed to adult and 3b) and ninth primary (11 weeks, Figure size before the head, bill and mass, and the 2). All the birds had begun to fly during their wings were last to develop, which is in tenth week, when they had attained 97% adult agreement with other reports on growth in mass. Anatidae (Kear 1970, 5iegfried 1973, Brown & Ulna length increased exponentially between Fredrickson 1983,Sedinger 1986, Bowler 1992, one and seven weeks, while tibiotarsus length DLittmann 1993). A previous study on two increased logarithmically from one to six weeks Eider ducklings also concluded that the ability (Table I). The limb bones thus reached adult to walk, swim and dive (for food and to escape size by seven weeks. The growth of the head predation) and the capacity to feed had priority and foot area were both described by power over wing development, so that the duckling curves from one to eight weeks. Wing area, could then satisfy the energetic demands of wing loading and the length of the ninth producing adult plumage and gaining muscle primary all increased logarithmically between mass (Leray & Yesou 1989). six and ten or eleven weeks. The paddle index The delayed development of the breast was not correlated with the age of the birds musculature in Canada Goose Branta canadensis (Figure 3b). It was highest at hatch (0.98 ± goslings reduces competition for protein with 0.03 cm'l2 tl3), lowest at I week (0.86 ± 0.0 I the other developing organs, thus allowing cm'l2 g-"'), and levelled at 0.91 ± 0.0 I cm'l2 g-II'in maximal early development of the legs and the adult birds. gastrointestinal tract for locomotion and Up to and including seven weeks, body feeding (Sedinger 1986). Following growth of mass could be used as an estimate of age the hind limbs in the Barnacle Goose Branta according to the relationship: A = 0.005 Me + leucopsis, there is only a small increase in leg muscle mass between fledging and migration, GROWTH OF CAPTIVE FEMALE COMMON EIDER DUCKLINGS 95

1600 (a)

1600

1400

1200

600

400

200

o , -+-----~~ I , o 2 6 8 10 12 14 Age (Weeks)

(b) 140

120

100

I 80 .l;: c .!!••• ..•.•., 80 :E: 40

0 0 2 4 6 B 10 12 14 Age (weeks)

Figure I. The increase in: (a) body mass; and (b) head length with age in captive female Common Eider ducklings. Vertical bars are ± I S.E. (n = 5). despite a 100% rise in body mass due to lipid Common Eider, as body mass continues to deposition and flight muscle hypertrophy. This increase after fledging, but tibiotarsus length is indicative of a shift in commitment to flight as does not (Figures I and 2). In both the the primary form of locomotion (Bishop et al. captive Eider ducklings and wild Barnacle 1996), and a similar shift is likely in juvenile Goose goslings (Bishop et al. 1996), limb bones 96 GROWTH OF CAPTIVE FEMALE COMMON EIDER DUCKLINGS

~ lj 160 ••

140 1 120 t -+- Tibiotarsus 100 I -- Nnth primBry i 80 ...... - Una , I, ~ 60

0 0 2 4 6 8 10 12 14 Age (weeks)

Figure 2. Growth of the tibiotarsus, ninth primary and ulna in captive female Common Eider ducklings. Vertical bars are ± I S.E. (n = 5).

had ceased to grow by week seven. paddle is likely to increase in area in proportion Of the variables measured in the present to the skeleton and leg musculature which has study, head length approached an asymptote to push it against the water. The adult value for latest and most steeply, so this parameter could paddle index (0.91) is far greater than the possibly be of use in estimating duckling age in paddle index of 0.79 determined for S. the field up to eight weeks (Table I). mollissima by Raikow (1973). However, it is However, this requires the assumption that difficult to interpret the significance of the data wild and hand-reared populations are presented in Raikow (1973), which was taken identical (see below). Sedinger (1986) from one duck of unknown subspecies, age and attempted to use tarsus length in Canada sex, at an unstated time of year. The six races Goose goslings, but this was unsatisfactory of Eider have differing mean body masses after 30 days, as the relationship between (Cramp & Simmons 1977), the sexes are tarsus length and age approached an dimorphic (Milne 1976) and the body mass of asymptote. It may be easiest to use body many birds can vary greatly throughout the mass for Eider ducklings, as this variable may year (Milne 1976, Mead 1983, Tuite 1984). be correlated with age up to seven weeks Information on the subspecies and sex of an and is more commonly measured than head individual, and the date when measurements length. were made, is therefore of great importance when describing avian morphology. Paddle index and wing loading The small wing area relative to body mass (ie high wing loading) found in the Eider is a Although Lack (1974) described ducklings' feet characteristic of diving ducks, which enables the as 'disproportionately large', the paddle index birds to fly at high speed but necessitates open remained relatively constant throughout waters for take-off and landing (Raikow 1973). development. This seems logical, as the growing This high wing loading is not necessarily an GROWTH OF CAPTIVE FEMALE COMMON EIDER DUCKLINGS 97

(a)

1000

900

800

700

800 ! • 500 ~ 400

300

200

100

0 0 2 4 6 8 10 12 14 Age (weeks,

(b)

2.5

0.5

o o 2 4 6 8 10 12 14 ,. (weeks'

Figure 3. Increase in (a) foot and wing area and (b) paddle index and wing loading with age in captive female Common Eider ducklings. Vertical bars are ± I S.E. (n = 5). 00 Table I. The relationships between body mass and body components and age in growing captive female Common Eider ducklings. co G)

~ I Adult size t P Relationship with age in weeks (A) R' Range of A ~ n (weeks) (one tailed) (weeks) :>- ~ ~ ~ ~ :>- Tibiotarsus 6 1.97 0.1 >P>0.05 Length (mm) = 13.65 In A +40.86 0.97 I - 6 r;; n 0 3: Ulna 7 1.76 0.1 >P>0.05 In Length (mm) = 0.26 A + 2.94 0.97 I - 7 3: 0 Z m Foot area 8 0.54 P>O.I Area (cm') = 24.11 A 0.79 0.96 I - 8 ~ '"0 c n Body mass 9 1.56 0.1 >P>0.05 Mass (g) = I78.23 A 104 0.98 I - 8 f:S z ~ Mass (g) = 197.01 A + 7.91 0.98 I - 7

Head length 9 1.61 O.I>P>0.05 Length (mm) = 57.40 A 0]5 0.98 I - 8

Wing area 10 1.38 P>O.I Area (cm') = 1119.02 InA-1695.70 0.94 6 - 10

Wing loading 10 0.55 P>O.I Loading (cm"' g .'/3)= 1.73 In A-I.42 0.90 6 - 10

Ninth primary 11 0.63 P>O.I Length (mm) = 184.69 1n A -276.43 0.94 6 - 1I

Each component attained adult size at the age shown in the second column of the table and the mean values were compared to adult values using Student's t-tests at a 5% level of significance. The dependent variables were regressed over a range of ages, and the ranges which afforded the most accurate fit (highest coefficient of determination, R') are listed in the last column. GROWTH OF CAPTIVE FEMALE COMMON EIDER DUCKLINGS 99

Figure 4. Plumage development of captive female Common Eider ducklings. (a) 0-1 week; covered with down (stippled areas), (b) 2 - 3 weeks; contour feathers emerging on flanks and tail coverts (plain areas), (c) 3 - 4 weeks; primaries erupting, (d) 4 - 5 weeks; most of body feathered apart from rump, (e) 6 - 7 weeks; covering of contour feathers almost complete and primaries well developed. adaptation to diving, nor does it reflect a predators are less able to conceal themselves, particular need for high speed flight; it may be and the ducks can escape by diving in addition engendered by the relaxation of selective to flight (Lovvorn & Jones 1994). pressure acting against high speed wings - 100 GROWTH OF CAPTIVE FEMALE COMMON EIDER DUCKLINGS

100

g ___ ~ TuIIod 60 - ! •...••• Goooe '5 -*--___ Mallard ~ 50

i 40 £

0 0 6 6 10 12 14 Ago '-0'

Figure 5. Body mass increase with age in five species of Anatidae and measured in captive populations. The three arrows indicate mass loss associated with fledging in the Tufted Duck, Barnacle Goose and White-winged Scoter.

Fledging and body mass growth period was not known, but the authors suggested that it may spread energy demands The data from the present study agree with over a long period in a niche where energy is those of Leray & Yesou (1989) regarding the not readily available and the cost of foraging is sequence of 'feathering up', and lack of a high. noticeable trough in the body mass curve at the The fledging age of 10 weeks for the Eiders point of fledging. A temporary loss of mass in the present study was at the midpoint of the during the period when flight first occurs has age range of 65 to 75 days quoted for wild been noted in captive Tufted Duck (Kear 1970), Eider ducklings by Ogilvie (1975), by which time White-winged Scoter Melanitta fusca (Brown & the skeleton of the captive ducks had attained Fredrickson 1983) and Barnacle Goose adult size. In contrast, Tufted Duck ducklings (Deaton 1998), but not in captive Mallard Anas could make short flights at seven weeks, before Platyrhynchos (Sugden et al. 1981). Body mass the wings, tarsus and head were fully grown during development in these birds is compared (Kear 1970), which could be related to a in Figure 5. The Mallard was the first to attain greater risk of predation. The trough in body adult body mass (Kear 1970, Lightbody & mass at fledging may also be an adaptation to Ankney 1984), while the White-winged Scoter enable the Tufted ducklings to fly earlier by grew very slowly. The significance of this long reducing wing loading. Many adult GROWTH OF CAPTIVE FEMALE COMMON EIDER DUCKLINGS 101

Anseriformes lose body mass during the hand-reared birds are likely to be larger than flightless period of the moult, even though food those obtained from wild birds, and variation may be abundant (Owen & Black 1990). Such a around the means is also likely to be less in mass loss, which reduces the flightless period, captive birds. This is due to a number of factors may not be necessary in the fledging Eider including uniform environmental conditions, ad duckling, as they are relatively safer at sea. This lib feeding regimes (Austin & Serie 1994), is also suggested by the fact that adult female reduced exercise and thermo-regulatory Eider ducks gain mass during moult at sea, while requirements (Bowler 1992, Sedinger 1992), males 'lose only a small amount' (Milne 1976). and, in the case of sea ducks, access to fresh water (Swennen 1989). Although the ducklings Captive and wild populations in the present study were moved outside as soon as possible and made to dive for their A possible problem with physiological and food, it is possible that they were larger than morphological studies of hand-reared birds is wild Eiders. However, measuring captive that they may not be identical to wild animals is the only method which guarantees populations. Growth rates are ultimately under that data will be obtained from the same genetic control, as they are the result of natural individuals and at the required intervals. selection, but are also strongly influenced by environmental factors (Kear 1970, Brown & We thank Scottish Natural Heritage for Fredrickson 1983, Swennen 1989, Bowler 1992, permission to col/ect the Eider Duck eggs, and Dr. Sedinger 1992). Waterfowl growth rates are Mike Harris for col/ecting them. We a/so thank often submaximal in the wild, such that most Alan Gardner for arranging to have the ducks re- species grow faster in captivity (review in homed. PAJH was funded by BBSRC. Sedinger 1992) or where supplementary food is provided (Swennen I989,Austin & Serie 1994). References Swennen (1989) reported that hand-reared Austin, JE. & Serie, JR. 1994. Variation in body Eider ducklings provided with commercial mass of wild Canvasback and Redhead chicken pellets put on more mass than those Ducklings. Condor 96:909-915 given live marine invertebrates in trays for the Bishop, CM., Butler, P.J.,El Haj,A.J., Egginton, S. & first nine days but not on the tenth and final day Loonen, M.J.J.E. 1996. The morphological on which the birds were weighed (no statistics development of the locomotor and cardiac presented). However, there may have been a muscles of the migratory Barnacle Goose behavioural explanation for this, as it took Branta leucopsis. J. Zool. 239: I-I 5 several days for the ducklings to learn how to catch the small crustaceans and polychaetes Bowler, J.M. 1992. The growth and development provided for them (Swennen 1989). of Whoop er swan cygnets. Wildfow/43:27-39 Kear (1970) found that wild, 10 week old Brown, P.W & Fredrickson, L.H. 1983. Growth Tufted Ducks were heavier than her hand- and moult progression of White-winged Scoter reared birds by 2% in males and 5% in females. ducklings. Wildfowl 34: I 15-119 When mean body mass and tibiotarsus length Cramp, S. & Simmons, K.E.L. 1977. Handbook of the measured from wild Barnacle Goose goslings Birds of Europe, the Middle East and North Africa. (Bishop et al. 1996) at one, three and five weeks, The Birds of the Western Palaearctic. Volume I: were compared with data taken from hand- Ostriches to Ducks. Oxford University Press, reared goslings (Deaton 1998), the captive birds Oxford. were significantly larger and heavier at three and Deaton, K.E. I998.Thyroid hormones and muscle five weeks old. However, Sedinger (1986) found development in the Barnacle Goose, Branta only a small number of differences (with no leucopsis. PhD thesis, School of Biological consistent pattern) in measurements taken Sciences, University of Birmingham, UK. from imprinted and wild Canada Geese. Overall, mean measurements taken from 102 GROWTH OF CAPTIVE FEMALE COMMON EIDER DUCKLINGS

Duttmann, H. 1993. Die morphologische Raikow, R.J. 1973. Locomotor mechanisms in Jugendentwicklung bei der Brandente Tadorna north American ducks. Wilson Bull. 85:295- tadorna. Die Vogelwarte 37:96-110 307 Kear, J. 1970. Studies on the development of Ricklefs, R.E. I967.A graphical method of fitting young Tufted Ducks. Wildfowl 21 : 123-132 equations to growth curves. Ecology 48:978- Hawkins, P. 1998. Environmental stimulation for 983 waterfowl: the common eider duck. Animal Ricklefs, R.E. 1973. Patterns of growth in birds. Technology 49(2): I07-115 11. Growth rate and mode of development. Lack, D. 1968. Ecological Adaptations for Breeding Ibis 110:419-45 I in Birds. Methuen and co., London. Ricklefs, R.E. 1983. Avian postnatal development. In: Farner, o.S., King, J.R. & Lack, D. 1974. Evolution Illustrated by Waterfowl. Parkes, K.C (Eds.). Avian Biology Volume 7 Harper and Row, London Academic Press, New York pp 1-83 Leray, G. & Yesou, P. 1989. Premieres donnees sur la croissance du jeune Eider a duvet Sedinger,J.S. 1986. Growth and development of Somateria mollissima. L 6iseau et Canada Goose goslings. Condor 88: 169-180 RF.O.,V.59:215-219 Sedinger, J.S. 1992. Ecology of prefledging waterfowl. In: Batt, BD.J., Afton, A.D., Lightbody, J.P. & Ankney, CD. 1984. Seasonal Anderson, M.G., Ankney, CD., Johnson, D.H., influence on the strategies of growth and development of Canvas back and Lesser Kadlec, JA & Krapu, JA (Eds.). Ecology and Scaup ducklings. Auk 10 I: 121-133 Manabement of Breeding Waterfowl. University of Minnesota Press, Minneapolis. Pp 109-127. Lovvorn, J.R. & Jones, D.R. 1994. Biomechanical conflicts between adaptations for diving and Siegfried, W.R. 1973. Post-embryonic aerial flight in estuarine birds. Estuaries development of the Ruddy duck Oxyura 17:62-67 jamaicensis and some other diving ducks. Int. Zoo Yearbook 13:77 - 87 Madge, S. & Burn, H. 1988. Wildfowl: an Illustrated Guide to the Ducks, Geese and Swans of the Steen, J.B. & Gabrielsen, G.W. 1986. World. Christopher Helm, London Thermogenesis in newly hatched Eider Somateria mollissima and Long-tailed duck Mead, C 1983. Bird Migration. Country Life CIangula hyemalis ducklings and Barnacle Books, Middlesex Goose Branta leucopsis goslings. Polar. Res. Mendenhall, Y.M. 1979. Brooding of young 4: 181-186 ducklings by female Eiders Somateria Sugden, L.G., Driver, EA & Kingsley, M.E.S. 1981. mollissima. Ornis. Scand. 10:94-99 Growth and energy consumption by captive Milne, H. 1976. Body weights and carcass mallards. Can.}. Zoo/. 59: I567-70 composition of the Common Eider. Wildfowl Swennen, C 1989. Gull predation upon Eider 27:115-122 Somateria mollissima ducklings: destruction Ogilvie, MA 1975. Ducks of Britain and Europe. or elimination of the unfit? Ardea 77:21 - 45 T & AD Poyser, Berkhamsted. Tuite, CH. 1984. Avian energetics: some pitfalls. Owen, M., Atkinson-Willes, G.L & Salmon, D.G. The relationships between body mass and 1986. Wildfowl in Great Britain (2nd Ed.) components with age in growing captive Cambridge University Press, Cambridge Pp female Common Eider ducklings. Ibis 438-440 126:250-252 Owen, M. & Black, J.M. 1990. Breeding biology. In: Owen, M. & Black, J.M. (Eds.). Waterfowl Ecology. Blackie, Glasgow. p 65