The Use of a Natural Marker in Great Crested Grebes
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Population turnover in groups of wing-moulting waterbirds: the use of a natural marker in Great Crested Grebes THEUNIS PIERSMA Introduction G reb es Podiceps cristatus. F or m ore than 20 years these have congregated in large num The majority of Waterbird specics (divers, bers (up to a daily maximum of 40,000 grebes, flamingos, swans, geese, ducks, individuals) in Lake Ijsselmeer, The rails, cranes and auks) renew their flight Netherlands. It is the largest known moult feathers by a simultaneous wing-moult ing concentration of Great Crested Grebes (Stresemann and Stresemann 1966). Shed in Europe (Piersma. Vlug and Westhof ding all flight feathers at the same time 1986). During the early morning and late results in an extended period of impaired afternoon the grebes forage on the open flight capabilities. Flightlessness makes water. During mid-day and mid-night they birds vulnerable, and therefore a location assemble in one or more rafts on the shallow which provides safety from predators and water within 2 km from the shore. sufficient food is critical during the moult period. Special habitat requirements may be the reason that many waterfowl under Study area and field methods take migration (often in the opposite direction to that followed in autumn) before Fieldwork was done along the south coast of undergoing replacement of the plumage the province of Friesland (52°50'N, 5°28'E; (Salomonsen 1968). At the moulting sites inset Figure 1). where there is a 4 km long the birds often assemble in large concen and 0.5 km wide stretch of reeds (the trations (Vlug 1976; van der Wal and Mokkebank). For the first two km offshore Zomerdijk 1979; Campbell and Milne 1983; th e w ater is shallow (less than 2 m deep) is Little and Furness 1985; Storer and Jehl used by the Great Crested Grebes to roost, 1985). Protection from predators (including and is part of a protected nature reserve man) can be provided by distance from without access for the public. shore (e.g. open water for diving and sea The observations were made directly ducks) or by cover (e.g. reedbeds for sur from the shore or from an elevated hide in face feeding ducks and geese). The specific the w ater’s edge, at distances of 50—1500 m habitat requirements may mean that the from the grebes. A 15 —60 x zoom telescope birds leave as soon as the moult is com was used. A manual counter was employed pleted, leading to a large turnover at moult to sum the numerical units discerned. Due ing sites. to their foraging rhythm, the numbers of Recent studies on moulting waterbirds grebes on the roosting area peaked between have often recorded the numbers present, 12 and 16 hrs M .E .T . (Piersm a et al. 1986). but never attempted to quantify the total The daily totals therefore refer to counts number of individuals using a site through made during these afternoon periods. the period of the moult. This may by far Wing-flapping (Figure 2) takes 2 — 8 exceed the numbers present at any one seconds and it is usually easy to see whether tim e. It should, however, be possible to the grebe is in wing-moult or not. Based on quantify the turnover by sufficient regular studies of drowned grebes collected from observations. Firstly, all waterbirds have gill-ncts by local fishermen, three moult the habit of regularly flapping the wings, a phases could be discerned (Figure 3). Birds comfort movement. During wing-flapping in phase 1 have just shed their feathers with the stage of wing-moult can easily be seen. primaries less than one-third of final length. Secondly, since flight feather growth rates Phase 2 birds have flight feathers up to two- are directly proportional to the full-grown thirds of final length. In phase 3 moult wing lengths (Prévost 1983), wings-in- cannot be discerned. These birds are either moult in fact represent a natural marker non-moulting individuals or have nearly from which periodicity can be estimated. completed flight feather growth. The moult This study sets out to examine the neces phase of each grebe which was seen when sary methodology, using Great Crested wing-flapping was recorded. Given 37 Wildfowl 38 (1987): 37-45 38 Theunis Piersma sufficient samples, recording errors will number of bouts (F) was recorded within a average out. period (T), which was timed by a stopwatch The frequency of wing-flapping bouts was in minutes. The bout rate is expressed per measured by focussing on subgroups of hour, and was calculated by dividing the grebes in a roosting raft. The number of number of bouts by the number of “grebe- grebes in focus (N) was counted and the minutes”: (F/(NxT) x 60). Figure 1. Average daytime numbers of Great Crested Grebes on the moulting area near the Mokkebank, Friesland (inset), in the period 1970—1986. Up to 1984 five-year running averages are given. The two open dots indicate the seasonal averages for 1985 and 1986. Figure 2. Two examples of wing-flapping Great Crested Grebes. The bird on the left has just shed the flight feathers (moult phase 1), whereas the bird on the right has almost completed wing-moult (mou'.‘ phase 3). (Photo courtesy J. van de Kam.) Population turnover in moulting grebes 39 Figure 3. Illustration of the three different phases of wing-moult in Great Crested Grebes. 40 Theunis Piersma Results decline from halfway through August on wards, and virtually no grebes roosted near The results of the observations in 1985 and the Mokkebank in the first week of Septem 1986 on moult phase and total numbers of ber. Numbers then increased again. In 1986 Great Crested Grebes on the day-time roost no such temporary decline was apparent arc presented in Figure 4. In both years the although the numbers fluctuated strongly. numbers present increased in early August. In 1985 and 1986 peak numbers were 8,000 However, in 1985 there was a continuous and 18,000 respectively. The proportion of August September October Figure 4. Seasonal changes in the number, and moult phase composition, of Great Crested Grebes in 1985 (top) and 1986 (bottom), n indicates the number of grebes per period that were examined for moult phase during wing-flapping. Data from the entire daylight period included. Population turnover in moulting grebes 41 grebes in moult phase 1 increased from were found in the simultaneously moulting about zero, to about 50% in the second Anseriformes. Great Crested Grebes, with week of August 1985, and peaked in early an average wing length of 195 m m , w ould be September. In 1986 wing-moult may have predicted to have a grow th rate of 6 mm /day been somewhat earlier, with peak propor (when using the linear relationship calcu tions of phase 1 halfway through August. lated from the Anseriform-data), or of 4 For a straightforward estimation of popu mm/day (when using the relationship based lation turnover from total numbers present on data from all spccies). In one captive and moult phase composition, we need to Black-necked Grebe Podiceps nigricollis know the duration of each moult phase (wing length 60% of that of Great Crested (regarded as a marked “cohort”), and also, Grebes), Storer and Jehl (1985) found a to ascertain whether the frequency of wing- growth rate of 3 mm/day. Since the average flapping bouts is equal for moulting and length of the primaries is 103 mm, a growth non-moulting grebes. No direct measure duration of 17—26 days is predicted (aver ments of wing-moult duration are available age 21 days). Becausc their protein-rich diet for Great Crested Grebes but a reasonable of fish probably imposes no growth limit estimate can be obtained. Prévost (1983) ations, the growth duration is likely to be showed that, between species, flight feather closer to 17 than to 26 days. Independently, growth rate and wing length are linearly an average moult duration of 17 days was correlated, with only small differences also estimated from a sample of 74 drowned between taxonomic groups. Highest feather grebes in active wing-moult, by regressing growth rates in relation to final wing length date on percentage of primaries' final length o • • • Q -Q -- o c o oo • O. , • • — i • • O) c ° o •o* ? • o o • ^ o • • *> _ o o ? 2 o a> __ _Q : o • a • o £ o o • Z3 o C c 0 o O O o — 1----1------- 1 1 1 1 1 1 1 I 1 1 0 8 9 10 11 12 13 14 15 16 17 18 19 20 time of the day Figure 5. Daily variation in wing-flapping bouts of Great Crested Grebes. Filled dots indicate the measurements on 17th, 23rd and 28th August 1986 when daily average wing-flapping bouts varied from 2.42 to 2.62 per grebe.hr. Open dots indicate the data from 4th, 17th and 18th September when flapping rates were close to 1.7 per grebe.hr. Every dot represent sample sizes of at least 100 grebe- minutes (maximum of 1692). Only the measurements made between 8 and 17 hr M.E.T. were used in the subsequent analyses. 42 Theunis Piersma growth (see Pimm 1976 for method). Since not, the percentages of birds in active wing- the growth rate of primaries is constant moult will either be over- or under through most of the growth period, each estimated. To find out, the daily average moult phase will take about a third of the frequencies of wing-flapping bouts were total moult duration.