Weight studies in wood sandpipers (Tringa glareola), migrating by way of SW Scania in late summer and spring. with notes on related species

In the Öresund area, SW Scania, Sweden, the supply of food for waders staging in late summer and autumn is highly variable, due to stochastic change of water levels. When wind flats are bare and brackish marshes lining the beaches begin to dry up, there is a surplus of food, that will allow excessive rates of fattening in all-round species like Dunlin, Redshank and Greenshank. The Wood Sandpiper is not as all-round as these species and has a more narrow habitat preference, still at least 20% of the birds reach weights in excess of 70 g in the same area. The calculated average weight gain was 1.15 g/day. If the extra weight is all fat, Wood Sandpipers should be able to fly from S.Sweden to new staging areas (the Camargue, Gulf of Venice) in one flight. Spring birds weighed on average 5 g more than autumn adults, part of the extra weight may be invested into breeding

INTRODUCTION AND MATERIAL

In Sweden the mass ringing of waders is history; in the late nineties the ringing volume is only 10 - 20 % of what it used to be in the sixties. As a consequence the existing knowledge about wader migration is more or less biased, since much material was collected during a period of heavy hunting pressure in Western Europe (particularly France and Italy) and little field activity in Eastern Europe. Today the balance has changed, there are few (reported) hunting recoveries from France and Italy, while nations like the Czech Republic, Hungary, Slovenia and Croatia have established ringing activities. In addition the winter distributions of many species are changing, due to habitat destruction and climatic change. The possibility of such biases must be taken into account in the case of the Wood Sandpiper.
The first Swedish paper (Myhrberg (1961)) dealing with this species was written almost precipitously, before the great bulk of recoveries came in between 1960 and 1979. Still, the general picture given here (and confirmed by Roos 1984) is correct; the majority of Swedish Wood Sandpipers migrates S-SSW to West Africa. But there was some uncertainty as to the origin of birds flying SSE from Swedish ringing sites in autumn and only cautious assumptions about a loop in spring. Four decades later numerous recoveries of Wood Sandpipers ringed in all parts of the annual distribution show that many (if not all) birds fly on a more easterly course in spring (probably as a result of flying NE, against the prevalent wind, over the Sahara), passing over Italy, Greece and Turkey.
The general development of Swedish Wood Sandpiper ringing is shown in Table I:

Table I. Ringing totals and recovery rates of Wood Sandpipers in Sweden 1960-1995. From the Annual reports of the Swedish Museum of Natural History Bird Ringing Centre.

Period	Ringing total	Recoveries reported	Recoveries in %
1960-69	11223	281	2.5
1970-79	5970	191	3.2
1980-89	3706	36	1.0
1990-95	1310	12	0.9

With declining activities in Western Europe - included here a general decline for field ornithology in favour of a commercially oriented "bird watching", that will eventually come to haunt Central Europe as well - recoveries of ringed waders from Scandinavia are very scant today. As a result the remaining ringing groups and stations spend more interest and more time in collecting biometry and studying behaviour and adaptations during migration. This development is evident at all ringing sites e.g. in the Baltic area, but there is one problem, concerning the sharing of information. In the basic study of movements (speed, direction, time-tables), recoveries served as a general informant about the activities of neighbouring schemes, but when recoveries get scarce we suddenly know very little about the running affairs of our closest neighbours, and the lack of information is reciprocal. This paper is an attempt to repair the Swedish deficit; within the framework of the project "Tringa glareola 2000" it introduces recent work done by the leading Swedish ringing group on waders.

Fig. 1. Sites where waders have been caught in SW Scania between 1983 and 1998.

In the present paper biometry from Wood Sandpipers migrating in the Öresund (The Sound) area from Falsterbo Peninsula (55.24 N, 12.50 E) in the south to Löddesnäs (55.44 N, 13.00 E) and Örtofta (55.46 N, 13.16 E) in the north is presented and discussed in the particular Öresund context of wind flats and more or less brackish marshland. The average salinity of sea water varies from 1.0 % at Falsterbo to 1.5 % at Löddesnäs. The ringing sites are shown in Fig. 1. Material concerning other species has been drawn in for comparison, and since little has been written about the feeding ecology of waders in this particular area, the text will zigzag between necessary background information and data related to the main object of study: Wood Sandpipers. The work at Löddesnäs, Lund and Örtofta (area A; at the inland sites the birds were caught at sedimentation ponds for sewage) was performed by Ulf Lundwall from 1983 onwards, with particularly valuable spring catches between 1992 and 1994; at the SW corner (area B; Skanör, Ljunghusen, Foteviken, Klagshamn - no spring catches here) by Per Nothagen, Peter Olsson and Christer Persson from 1989 onwards. Wing lengths were measured to the nearest mm according to the "maximum" method (e.g. Evans 1986), bill lengths to the nearest tenth of a mm according to three different methods (from feathers, from nostril, with head included), weights to the nearest gram with 300 gram Pesola balances. The birds were caught with wader-nets and attracted by means of tape recorders playing endless tapes, most catches derive from late evening and night hours.

RESULTS

1. GENERAL BIOMETRY

The overall biometry of Wood Sandpipers from SW Scania is summarized in Tables II - IV. Note different methods for measuring bill length at different sites!

Table II. Measurements of Wood Sandpipers ringed at Löddesnäs in spring, 1992-1994.

Period	n	Item	Span	Mean±1 s.e.	s.d.
22.4-17.5	43	Weight(g)	56-86	71.1±1.0	6.5
		Bill+head(mm)	52.6-58.5	55.9±0.2	1.3
		Nalospi(mm)	21.3-25.8	23.4±0.2	1.0
		Wing(mm)	122-136	128.8±0.4	2.8

Table III. Measurements of Wood Sandpipers ringed at Löddesnäs, Lund and Örtofta (area A), 1983-95. Two 2y+ before 16.7 in the "July" material. "Aug I" = 1-15.8, "Aug II" = 16-31.8.

Period	n	Item	Span	Mean±1 s.e.	s.d.
July;2y+	35	Weight(g)	53-84	66.3±1.3	7.9
		Bill+head(mm)	52.2-58.3	56.1±0.3	1.5
		Nalospi(mm)	21.3-25.5	23.3±0.2	1.2
		Wing(mm)	122-131	125.9±0.4	2.5
July;1y	101	Weight (g)	51-75	61.6±0.6	5.9
		Bill+head(mm)	50.6-58.4	54.8±0.2	1.5
		Nalospi(mm)	19.4-25.5	22.4±0.1	1.2
		Wing(mm)	119-137	127.3±0.3	3.4
Aug I	128 (1y)	Weight (g)	48-91	64.4±0.9	9.7
		Bill+head(mm)	51.2-58.1	54.8±0.2	1.5
		Nalospi(mm)	20.0-25.8	22.5±0.1	1.2
		Wing(mm)	121-136	127.3±0.3	2.9
Aug II	29(1y)	Weight (g)	53-81	64.0±1.5	8.1
		Bill+head(mm)	51.4-56.3	54.1±0.4	1.5
		Nalospi(mm)	20.5-24.0	22.4±0.2	1.1
		Wing(mm)	123-135	127.7±0.6	3.0

Table IV. Measurements of Wood Sandpipers ringed between Skanör and Foteviken (area B), 1989-95. "Aug I" etc as in Table III

Period	n	Item	Span	Mean±1 s.e.	s.d.
July;2y+	5	Weight(g)	56-85	66.5±5.6	12.6
		Bill from f.(mm)	27.6-29.1	28.2±0.3	0.6
		Wing(mm)	122-129	125.5±1.3	2.8
July;1y	24	Weight (g)	51.5-79	62.3±1.5	7.4
		Bill from f.(mm)	26.0-32.9	28.1±0.4	1.8
		Wing(mm)	123-140	128.8±0.9	4.2
Aug I	36 (1y)	Weight (g)	49-77.5	60.7±1.2	7.1
		Bill from f.(mm)	25.2-32.0	27.9±0.2	1.4
		Wing(mm)	121-137	128.5±0.6	3.8
Aug II	25 (1y)	Weight (g)	48-92	66.0±2.3	11.0
		Bill from f.(mm)	24.5-29.5	27.2±0.3	1.4
		Wing(mm)	123-136	128.7±0.7	3.7
Sept I	4(1y)	Weight (g)	56-71	62.0±3.2	6.4
		Bill from f.(mm)	25.1-27.4	26.3±0.5	1.0
		Wing(mm)	125-132	128.5±1.4	2.9

Wings are moderately worn in July adults, spring weights ca 7 - 8 gms higher than the mean weight in late summer juveniles. There may be a tendency for shorter bill from feathers among juveniles from July to September, but the "nalospi" measure remains stable, so the reason may simply be feather growth. One item is of particular interest: the spread of weights in August; the maximum standard deviations of mean weights in the two main areas being 15 and 17 % respectively, peak weights 91 gms (Table III) and 92 gms (table IV).) Fat loads well below this magnitude will allow birds to fly to e.g. the Camargue or Northern Italy in one flight - but is this a regular, recurring pattern? In order to investigate the general prospects for fattening in our area, data from nine wader species are presented together in Table V.

Table V. Comparative data for Northern Ringed Plover (49 2y+, 19 1y , Aug-Sept 1998, Ljunghusen), Curlew Sandpiper (1y, Aug-Oct, sea shore, Falsterbo Peninsula), Little Stint (1y, Aug-Oct; sea shore, Falsterbo Peninsula), Wood Sandpiper (5 2y+, 90 1y, late July-early Sept; brackish marshland, Skanör, Foteviken), Greenshank (2 2y+, 102 1y, Aug-Oct ; brackish marshland, mainly Foteviken), Dunlin (1y, c. 2/3 staging centralis and 1/3 wintering alpina during a period of fattening in early November, Falsterbo Peninsula + Foteviken, all kinds of habitat), Redshank (1y, summer; no robusta involved), Reeve (10 2y+, 13 1y; brackish marshland, but many birds feeding part of the day on farmland). All ratios between mean values.

species	n	s.d. in % of mean weight	bill : wing	bill : (weight)1/3
tundrae	68	5.5	0.10	3.6
ferrug.	25	12.5	0.28	9.7
ochropus	28	14.0	0.23	7.8
glareola	95	14.2	0.22	7.0
nebularia	104	14.5	0.28	9.7
alpina	146	15.4	0.27	8.3
minuta	44	15.5	0.18	6.2
totanus	28	16.6	0.25	8.1
pugnax	23	18.2	0.19	6.4

Some of the percentages s.d./mean based on few measurements will jump a few % up or down when material from a particularly poor or good year is added - but the relation between species remains roughly unaltered. Northern Ringed Plovers are content to subsist for a day or two (they probably can do no better in this area), and Curlew Sandpipers seldom stay longer than 24 hours; the cause for high or low weights in these species should be sought elsewhere. The larvae of Coelopa pilipes in banks of decaying seaweed along the shores are the basis for astonishing fat levels in the Little Stint, but this habitat is only second choice for the Dunlin (wind flats are first). Three Tringa species behave almost as one single species mastering three slightly differing niches, while the two 100-g-species Ruff and Redshank seem to be the most effective feeders in the overall area. Summing this up: the Öresund area, in spite of low salinity and absence of tides, will allow small calidrids as well as all occurring Tringa to reach substantial average fat levels before their continued migration journey. And in all species staying in the area for some time, the fattest birds are 60 - 75 % (4 s.d.) heavier than lean birds. The reasons for this apparent abundance of food will be outlined in the next section.

In the case of Wood Sandpipers, there are 18 (17 1y, 1 2y+) recaptures of ringed Wood Sandpipers in area A in late summer, these are shown in Fig. 2. The average duration between two catches was 5 (5.2±1.1) days, the weight increase during this period 6.4±1.6 g, s.d. 6.8 g. The average weight increase was 1.15 g/day, but as can be seen from Fig. 2 there are individual differences; the highest weight gain (20 g) was achieved in 10 days. Two birds in spring were losing weight: from 79 g on 3. May to 74 g on 5 May and from 64 g on 5 May to 59 g on 7 May. A general regression of the flight muscles and a return to normal fat levels may be involved here - on the other hand many individuals are still 1000 - 1500 km from their breeding-grounds.

Fig. 2. Weight gain in 18 Wood Sandpipers recaptured 1 - 15 days after ringing at Löddesnäs and Örtofta, 1983-92.

2. NICHE WIDTH AND CAPACITY FOR FATTENING

The extension of the feeding-grounds available to migrating Charadriidae and Scolopacidae in the Öresund area is largely determined by the capricious water level of this "river" connecting the Baltic with the Kattegat. Öresund represents the shortest way for water streaming from the Baltic to Kattegat or vice versa, and water levels rising with strong easterly or westerly winds will get particularly accentuated here; the amplitude of the sea level amounts to 0.5 - 0.6 m when there is a wind shift and/or a change in air pressure (and in extreme cases it will reach 1.5 m). Water levels will be influenced not only by wind direction, but by air pressure and water currents as well; the shore proper being brimmed by "wind flats" (they might as well be termed "pressure flats" or "current flats") reclaimed from the sea when water levels fall, flooded again when they rise. In addition the marshland (particularly at Skanör and Foteviken) behind the sea-shore gets flooded (or drained) through numerous inlets. At the other end of the scale, in summer during highs with prolonged periods of drought, the same marshlands will be completely dried up, and small fish and insects caught up in the remaining pools will be at the mercy of foraging waders. The bare wind flats and the dried-up marshland are the main "surplus" resources of the area, and the prospects for fattening are completely dependent on this unpredictable, strongly stochastic phenomenon. (Regularity is not far away; e.g the meteorologists speek of an "Eastern high" in the Baltic, in many springs creating low water levels and wind flats for staging waders). When water levels remain high throughout a season (as in 1998) many wader species simply overfly the area, and there is notable fattening mainly in small sandpipers and plovers feeding on Coelopa in banks of seaweed. Under these conditions there is always a premium for a generalist or even opportunist feeding pattern, while more rigid and specialized species (e.g. the Snipe Gallinago gallinago) are forced to fly on to more favourable feeding-grounds.Niche width, here approximated as {habitat choice + food recognition + feeding pattern} determines if the bird is going to overfly Öresund or use it as a staging area for the continued migration journey. Table VI is a sketchy attempt to characterize the differences in habitat choice of the nine species listed in Table V, these data will be connected with feeding patterns in the discussion.

Table VI. Habitat choice of migrating waders in SW Sweden, July-November. RP=Ringed Plover, GS=Green Sandpiper, CS=Curlew Sandpiper, LS= Little Stint, WS=Wood Sandpiper, GR=Greenshank, DL=Dunlin, RS=Redshank, RE=Reeve. Preference for one type of habitat is denoted "main", neutral presence "yes", presence under certain conditions "(yes)", absence "-"

habitat	RP	GS	CS	LS	WS	GR	DL	RS	RE
open shore,wind flat	main	-	main	main	(yes)	yes	main	yes	(yes)
shore lined by Scirpus	-	(yes)	(yes)	-	yes	yes	yes	yes	(yes)
brackish marsh	-	yes	yes	yes	yes	yes	yes	yes	yes
flooded meadow	(yes)	yes	-	(yes)	main	yes	yes	yes	yes
ditches	-	yes	-	-	yes	yes	-	-	-
farmland	-	-	-	-	-	-	-	-	yes

It should be added that migrating Ringed Plovers may visit farmland in spring, but never do so in autumn. Dunlins and Curlew Sandpipers feed at sites with poor overview (sea shore lined by Scirpus) mainly during night hours. Green Sandpipers have a "snipey" behaviour and avoid bare and unprotected areas even in marshland, while Wood Sandpipers will enter sheltered mudflats (Foteviken), particularly at night. In rough outline the nine species may be characterized in the following way:

1. One narrow specialist: Charadrius hiaticula tundrae.
2. Three "moderate" specialists:Tringa ochropus, Calidris ferruginea and Calidris minuta.
3. One "in-between" species, a moderate generalist: Tringa glareola.
4. Three generalists with opportunist tendencies: Calidris alpina, Tringa totanus and Tringa nebularia.
5. One extreme opportunist: Philomacus pugnax.

3. FLIGHT RANGE ESTIMATES

Thus, part of the fattening of Wood Sandpipers in the Öresund area in late summer depends on a particular local condition: the alternating inundation and drainage of wind flats as well as meadows and marshland adjacent to the sea. If the birds are quick to exploit the end phase of drought (say: when inland water levels fall below 5 cm) the weight gain will be quite extraordinary. Such conditions occur in at least one out of two summers (with a couple of dry or wet and windy summers tending to follow in succession). In area A (Fig. 1) 22 % of 293 (1y+2y+) Wood Sandpipers had weights 70-91 gms (mean value 76.2±0.7, s.d. 6.0 g, regression: WEIGHT = 0.68 WING - 23.14; t=4.39, p<0.001, r=0.249) when caught, and 20 % out of 90 (1y+2y+) in area B weighed 70-92 gms (mean value 76.9±1.3, s.d. 5.9 g; regression n.s.). The mean value of all 1y autumn birds (areas A + B; n=350) was 63.3±0.5 g, s.d. 8.5 g, of all 2y+ autumn birds (n=41) 66.2±1.3 g, s.d. 8.3 g. The measurements of spring birds are listed in Table II. Using the mean values of the different categories ("fat birds" = 76 g) and the formulae given by Davidson (1983, 1984) the still-air flight range of different categories of Wood Sandpipers can be estimated. Data are given in Table VII.

Table VII. Still-air flight range estimates in four categories of Wood Sandpipers, SW Scania 1983-1998. Value chosen for flight speed: 60 km/h.

category	mean weight (g)	est. lean weight (g)	est. flight range (km)
fat birds, 1y+2y+	76	50	2.500
all 1y	63	51	1.250
all 2y+, autumn	66	47	1.975
2y+, spring	71	47	2.425

DISCUSSION

Feeding conditions for migrating waders in the wind flats of the south Baltic area have been discussed by Kube (1994), many of his conclusions apply to the Öresund area as well (both areas characterized by low salinity and stochastic change of water levels). There are four major prey species in the purely marine habitat, Nereis diversicolor, Hydrobia spp., Mytilus edulis and Corophium volutator; the average size of all prey decreasing and the reproductive season getting deterred into summer months with decreasing salinity. Kube observed little competition between species in his area; intraspecific interference in species with an optical searching technique seemed to be of greater importance. Furthermore, referring to Zwarts (1978) and Zwarts et al. (1990), he suggests that the average prey size of the south Baltic makes fattening difficult in species weighing more than c.200 g, e.g Curlew (Numenius arquata), Whimbrel (Numenius phaeopus) and Oystercatcher (Haematopus ostralegus). (The Whimbrel should not be included here, since part of its fattening in Scandinavia and Germany is based on the berry Empetrum nigrum. More than 200 adult Whimbrels stage on Falsterbo peninsula from c.15 July in most years; their departure weight lies in the vicinity of 1/2 kilo!)
In SW Scania it is obvious, that the wind flats can never be treated in isolation from the seaweed banks, lining the shores, and the brackish marshland behind the coastline; foraging waders cross the borderlines and use all habitats available in this "Rastplatzmosaik" (Kube 1994) during their stay in the area. To begin with Arenicola marina is of greater importance at South Swedish flats - at least where the local sources of pollution have been stemmed. The marshland adds important new prey species to the overall food supply, particularly small fish (flatfish Pleuronectes spp., sticklebacks Gasterosteus spp. etc.), left behind when the water level sinks (but many other genera (gastropods) may be involved as well). When fish is added to the diet, species weighing c.100 g (Redshank, Reeve) may add 50 g to their original weight, while species starting from 160 - 180 g (Greenshank, Ruff (not included in the table)) will end up as high as 250 - 260 g. This is a case of truly gargantuan feeding, characteristic of three species with strong opportunist tendencies. According to Deen Petersen (1981) Dunlin and Bar-tailed Godwit at Tipperne both increase their food intake in spring by 60 % in order to increase their weight by 0.5 - 1.1 g/day; in the Foteviken case both intake and increase rate must be of quite another order of magnitude, the increase rate very swift, but the fish diet should contain much more fat than the spring prey at Tipperne. Other species - Green Sandpiper, Spotted Redshank, Wood Sandpiper - occur at the outskirts of the same situation, and they all seem to perform almost as well as the more generalist species, although their shift between adjacent habitats is not equally light-footed. (According to Moreau (1972) all Tringa species except the Common Sandpiper Tringa hypoleucos also stick to the same type of slowly drying habitat in Africa). The obvious reason for the overall good performance is that the different species take different size fractions from the supply available, and even a medium-sized species like the Green Sandpiper may take great amounts of the smallest fish. The question is if Wood Sandpipers do so as well; we are not convinced that they do. The matter is hard to settle under poor light conditions; stomach contents should be investigated.
Turning again to the Wood Sandpiper, lean 1y birds arriving in S. Sweden in July and August typically weigh 49 - 50 g, confirming the value calculated from the formula of Davidson (1983). Within 1 - 2 weeks, dependent on conditions, these birds have added 10 - 15 gms to their initial weight. The substantial weight spread (48 - 92 g) of course partly reflects the fact, that birds are caught at different stages in the process of fattening. But a majority of the Wood Sandpipers does not reach weights exceeding 70 g, most birds depart with fat loads of 10 - 15 g. The reason for this is obvious from what has been said above: the extreme fat-loads are collected under conditions of accentuated competition, and many Wood Sandpipers simply get marginalized, or even excluded from the suddenly occurring resource. This exclusion is obvious already in pure flocks of Dunlin or Greenshank, where dominant birds simply run into inferior individuals, colliding with them (or feigning collision) and thus forcing them away from the most awarding spots, night and day. (Such behaviour probably "centrifugates" light-weight alpina from heavy-weight centralis in mixed flocks, or - more often observed by us - adults from 1y birds). Green Sandpipers obviously defend some sort of feeding territory, while the spacing process seems to be of a more calm nature among Wood Sandpipers.
In this context the bill length or the bill/body ratio is of interest, mean values for nine species are presented in Table V as a preliminary basis for discussion. The niche width, or simply the utilization of the resources available, will be greatly enhanced if the bird is capable of both probing deep and taking food items from the shore-line or water surface. In our area this capacity seems to be most rewarding in species like Dunlin and Redshank, while fattening seems to be very difficult for the short-billed, optically searching Ringed Plover. Still, bill length per se is not conclusive; different species take different size fractions of prey at different depths, and a less than optimal bill:body ratio (Little Stint, Reeve) obviously may be outbalanced by behaviour (feeding pattern).
When it comes to niche width, the Wood Sandpiper has an in-between position: it is not as "handy" or as competitive as Greenshank or Dunlin in aberrant situations, and avoids too purely marine habitats. Its preferred habitat is the flooded meadow, where competition is reduced by a seemingly automatic spacing. The same paddyfield type of habitat is met with in other parts of the annual distribution, and Wood Sandpipers seemingly apply the same rate of spacing whether in France, Greece, Tunisia or S Sweden (own observations). In other species such a generality may not be valid; there is for example a size reduction and a strong decrease in the amount of food available from Kattegat to the south Baltic shore, the number of marine animal species reduced by a factor ten, and this increasing scarcity of course leads to a change in behaviour and niche width in many species. Judging from table V, 100-g-species with generalist feeding behaviour have a slight advantage in the particular Öresund mosaic of habitat, but smaller (Dunlin, small Tringa) and larger (large Tringa) perform almost as well, and the general weight status of most species allows direct flight to the Channel area or the Mediterranean. Others, like the Snipe, poor surface feeders with short legs and long bills, overfly the area when inland water levels are too high (or too low), while Northern Ringed Plovers always fly more or less directly to the rich tide-flats of W Europe, where they should be far more efficient feeders than e.g. Greenshanks.
Turning for the last time to Wood Sandpipers, distances from S. Scania to the main target areas, Golfo di Venezia and the Camargue, are 1.100 and 1.450 kms respectively, and since 1 g of body weight is equivalent to 100 km flight distance in still air (Table VII) in this species, the birds should be able to fly non-stop to the Mediterranean under favourable conditions. The departure from Scania often takes place in calm evenings; 10 - 20 birds gaining height and calling intensely before crossing the shoreline. In Edelstam (1972) this evening departure even seems to be accentuated towards the end of the migration period. Still, numerous recoveries show that many Scandinavian Wood Sandpipers do not reach the Mediterranean in one flight; after flying 600 to 800 kms due south they are grounded in eastern Germany or the Czech Republic. A 2y+ bird from Foteviken was in the Camargue after 17 days, two 1y birds from Löddesnäs and Foteviken (weighing 82 and 79 g at ringing) in the Czech Republic after 10 and 11 days. Weather could play a role here; when confronted with e.g. thunderstorms the birds may choose to go down and replenish fat reserves rather than risking to deplete them. Another possibility for Wood Sandpipers is to migrate only during dark hours; if this is the case they will have to fly by the wind in order to reach Italy or France in one flight. There is another possibilty as well: for the fattest birds with fat-loads exceeding 30 gms to fly non-stop to North Africa. If they do not at least attempt this journey, the excessive rate of fattening seems rather pointless - in strong contrast to the functional, instrumental spring fattening in species like Dunlin, Knot and Sanderling. The same consideration applies to the fat levels in spring; Wood Sandpipers should easily be capable of reaching the northern limits of their breeding range in Norway, N. Sweden or Finland in one flight. Still, the fat reserves in spring seem unnecessarily large for the remaining flight distance, and part of the energy stored could be invested into breeding, the food supply of the breeding habitat being very poor when the first birds arrive in spring.

LITERATURE

Davidson, N.C. (1983): Formulae for estimating the lean weight and fat reserves of live shorebirds. Ring. & Migr. 4: 159-166. - Davidson, N.C. (1984): How valid are flight range estimates for waders? Ring. & Migr. 5: 49-64. - Deen Petersen, B. (1981): Vadefuglenes fouragering og predation på bundfaunaen på Tipperne. DOFT 75: 7-22. - Edelstam, C. (1972): The Visible Migration of Birds at Ottenby, Sweden. Vår Fågelvärld, suppl. 7. - Evans, P.R. (1986): Correct measurement of the wing-length in waders. WSGB 48: 11 - Kube, J. (1994): Aspekte der Nahrungsökologie ziehender Limikolen an der südlichen Ostseeküste. Corax 15: 57-72. - Moreau, R.E. (1972): The Palaearctic-African Bird Migration Systems. Academic Press. London. - Myhrberg, H. (1961): Grönbenans (Tringa glareola) sträck genom Europa. Vår Fågelvärld 20: 115-145. - Roos, G. (1984): Flyttning, övervintring och livslängd hos fåglar ringmärkta vid Falsterbo (1947-1980). Anser, suppl. 13. - Zwarts, L. (1978): Intra- and interspecific competition for space in estuarine bird species in a one-prey situation. Proc. XVII Internat. Orn. Congr. (Berlin): 1045-1050 - Zwarts, L., A.-M. Blomert, B.J. Ens, R. Hupkes & T.M. van Spanje (1990): Why do waders reach high feeding densities on the intertidal flats of the Banc d'Arguin, Mauritania? Ardea 78: 39-52.

This paper was written by Christer Persson, first version published on the web October 1st, 1998, corrected 28.5.99. The material was collected by Ulf Lundwall, Per Nothagen, Peter Olsson and Christer Persson.

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