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Migration patterns and moult of Common Terns Sterna hirundo and Sandwich Terns Sterna sandvicensis using Teesmouth in late summer Robin M. Ward
a
a
Dept. of Biological Sciences, University of Durham, South Road, Durham, DH1 3LE, U.K. E-mail:
[email protected] Available online: 11 Apr 2011
To cite this article: Robin M. Ward (2000): Migration patterns and moult of Common Terns Sterna hirundo and Sandwich Terns Sterna sandvicensis using Teesmouth in late summer, Ringing & Migration, 20:1, 19-28 To link to this article: http://dx.doi.org/10.1080/03078698.2000.9674223
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Ringing & Migration (2000) 20,19-28
Migration patterns and moult of Common Terns Sterna hirundo and Sandwich Terns Sterna sandvicensis using Teesmouth in late summer
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Robin M. Ward*, Dept. of Biological Sciences, University of Durham, South Road, Durham DH1 3LE, U.K Count, recovery and biometric data were analysed to assess the conservation status and use of Teesmouth by Common Terns Sterna hirundo and Sandwich Terns Sterna sandvicensis in late summer. The recognition of the international importance of Teesmouth to migratory Sandwich Terns was endorsed. Peak late summer counts for 1990-1997 averaged 1,835, over 1% of the international population. Analysis of information from both Common and Sandwich Terns ringed or recovered in late summer at Teesmouth identified the existence of a trans-Pennine migratory pathway of Common Terns between Teesmouth and Merseyside. Trans-Pennine migration of Sandwich Terns is also suspected from visual observations of departures from Teesmouth. 55% of adult Common Terns using Teesmouth have yet to initiate primary moult. This, and the recovery patterns, are consistent with the involvement of a substantial Baltic contingent of non-moulting adults amongst the late summer Teesmouth population. Of those adults in active wing moult, primary moult had progressed less far than for birds in the Wadden Sea at the same time of year. This suggests a cline through northern European breeding populations in the timing of moult. Visible migration, retrap data, arrival dates of different populations and the progression of primary moult, all identify rapid turnover amongst the late summer Common Tern population at Teesmouth, suggesting that over 1 % of the UK breeding population, with their young, migrate through Teesmouth
C
onservation of a migratory species requires knowledge of key staging sites used on migration, together with identification of any discrete populations that may depend upon a site's existence. Teesmouth, north-east England (54°37'N 01°10'E), is the first Special Protection Area to be designated in recognition of its importance as a staging site to migratory tern populations (SPA citation 1993), in this case the Sandwich Tern Sterna sandvicensis. This assessment has been based primarily upon count data, such as those collated by Armstrong (1997), Bell (1997) and Cranswick et al. (1997). The origin and migration patterns of the late summer tern populations at Teesmouth have yet to be identified for either Sandwich Tern or Common Tern Sterna hirundo, the other species to frequent *E-mail: R.M.
[email protected] © 2000 British Trust for Ornithology
Teesmouth on passage in substantial numbers (Bell 1997). Furthermore, the site may be much more important than is apparent from count data alone, as turnover rates at staging areas, may be high (Evans 1984, Smit & Piersma 1989). The present paper uses ringing recoveries and biometrics to describe migration patterns of the late summer populations of Common and Sandwich Terns at Teesmouth. In the absence of dye-mark data to provide a better measure of turnover rates (Evans 1984), primary moult data are also explored to provide information on Common Tern movements in terms of numbers and discrete breeding populations. As a consequence, the conservation status and use of Teesmouth by the two tern species in late summer can be re-assessed.
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Robin M Ward
maximum chord (Baker 1993), bill length measured from bill tip to feathering, and primary Count data has been examined from three moult score calculated as recommended by Ginn sources: & Melville (1983). The terminology used 1 Wetland Bird Survey (WeBS) data (1990-97), regarding feather generations follows that of 2 Weekly low water counts across Teesmouth's Baker (1993). Any observed differences in principal inter-tidal areas undertaken by the biometrics or moult strategy that may be author for the Tees Barrage Monitoring attributed to discrete populations, were assessed Programme (1990-97) (Evans et al. 1996). with both parametric (Z-test, t-test & One-way These surveys cover different parts of the Anova) and non-parametric tests (Chi-square) estuary on two successive days. To minimise (Fowler & Cohen 1986). any day-to-day changes in bird distribution, counts from the two tidal periods are treated RESULTS separately. 3 Opportunistic counts by the author. Peak Numbers at Teesmouth The mean peak count (July - October) is calculated from these data to evaluate the site's Late summer populations of Common and importance to each species on an international Sandwich Terns peak in mid-August at level, using the recognised 1% criterion Teesmouth (Fig 1). Peak numbers of the latter species continue to qualify Teesmouth as (Cranswick et al. 1997). The migration patterns of terns utilising internationally important (Table 1). No Teesmouth were identified from all relevant population estimate is currently available of the ringing recoveries regardless of recovery autumn Common Tern population in Britain and circumstances. These data were extracted from Ireland to enable calculation of a 1% threshold the British Trust for Ornithology's national for national importance (Cranswick et al. 1997). recovery database. All biometrics and moult Although the maximum numbers of Common data used had been collected at Teesmouth Terns recorded at Teesmouth exceeded 1% of opportunistically since 1985, by the Tees Ringing the estimated breeding population in Britain and Group and the University of Durham, from birds Ireland (assuming 14,700 pairs and two young caught by mist netting and cannon netting per pair), ringing recoveries show that the respectively. No account has been taken of any population is swollen by a significant proportion variability that may exist amongst the 10 or more of migrants (see later). measurers involved. Wing length was taken as
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MATERIALS & METHODS
4O0O
3OOO
Common Tern Sandwich Torn
2OOO
1OOO
J1
J2
J3
J4
A1
A2
A3
A4
S1
Weeks during July -September
Figurel. Peak counts of Common and Sandwich Terns at Teesmouth, 1990-1997 © 2000 British Trust for Ornithology, Ringing & Migration, 20, 19-28
S2
S3
S4
Migration patterns & moult of Terns
21
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Table 1. The conservation status of Teesmouth for Common and Sandwich Terns
Mean of late summer peak counts ± one standard error 1990-97 Breeding pairs in Britain and Ireland, 1985-87 (Lloyd et al. 1991) 1% threshold for international importance (Cranswick et al. 1997)
Common Tern
Sandwich Tern
630 ± 22.5
1835 ± 32.1
14,700
6,000
18,400
1,500
Table 2. Origins of Sandwich Terns ringed as pulli and recovered in late summer at Teesmouth Natal colony
Belgium Netherlands Great Britain: Essex Norfolk Northumberland Lothian Fife Grampian Orkney Denmark Ireland (Ulster)
Age (Euring code) at recovery 5 7 9
2 1 2 21 1 2 -
11
12 +
40
Breeding origins Sandwich Tern
The natal sites of the majority of ringed Sandwich Terns recovered at Teesmouth in late summer are the two nearest breeding colonies, Coquet Island and the Fame Islands in Northumberland (Table 2). Birds have also been recovered in late summer at Teesmouth from colonies around the North Sea (Belgium, Netherlands, Denmark, and the entire British East Coast) and from Northern Ireland. For the North Sea colonies within year recoveries relate only to juveniles ie birds ringed and recovered within the same summer, as expected because the vast majority of terns ringed in colonies are pulli (eg Appleton et al. 1997). The recoveries of birds ringed in Ireland
may relate to immatures prospecting (three were recovered as second-summers) or adults breeding away from their natal colony (Cramp 1985). No other recoveries, regardless of date or age, link Teesmouth with western Britain or Ireland. The northwards movement of pulli from Essex and Norfolk probably form part of a general dispersal after fledging, as has been found also for birds from the Northumberland colonies (Langham 1971). Common Tern
The natal sites of Common Terns recovered at Teesmouth in late summer extend much further east than those of Sandwich Terns. Most originated from colonies around the North Sea, from Baltic Sea countries and from north-west
© 2000 British Trust for Ornithology, Ringing & Migration, 20, 19-28
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Robin M Ward
Table 3. Origins of Common Terns ringed as pulli and recovered in late summer at Teesmouth
Natal Colon}/
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7
Lithuania Finland Sweden Norway Great Britain: Orkney Highland Grampian Northumberland Tyne & Wear Durham Lancashire Clwyd West Midlands Lincolnshire Norfolk Essex Netherlands
2 6 2 9
1 1 1 16 3 4 -
1 5 2
9
13
-
2
-
1
-
-
-
1
1 -
1 2 1 -
-
-
-
-
-
-
1
-
-
-
-
-
1
-
-
-
-
-
1
-
-
1
-
-
1 _
_
_
Britain (Table 3). A colour-ringed Common Tern seen at Teesmouth in August was a one-year old from an inland breeding colony in the West Midlands. Although it is not possible to test them statistically, the data available so far show that most Norwegian juveniles are recovered at Teesmouth in August, with those from further east in the Baltic being recovered in September (Table 4). Furthermore, the late summer recoveries at Teesmouth of juveniles from southern North Sea colonies are mostly in September but those from north-east British colonies chiefly in August. These suggested differences in the date periods of arrival are likely to apply equally to adults, given they accompany their young in late summer (Cramp 1985). Migration routes & wintering areas Sandwich Tern
By October juvenile Sandwich Terns ringed at Teesmouth have reached the Gulf of Guinea (1 recovery) and Senegal (2). Other recoveries of
Age (Euring Code) at recovery 15 17 31 33
birds as juveniles are of four in March in Senegal and the Ivory Coast, and one in Spain in February. All remaining recoveries of birds ringed in late summer at Teesmouth relate to adults (third calendar year or older). Wintering areas of these birds extend along the west African coast (Senegal to Ghana, 5), Congo (1), Angola (1) and South Africa (1), though a January recovery also exists from the Netherlands. Recoveries of adults en route between Africa and the North Sea colonies are restricted to France (1) and Portugal (1) in April and September respectively. Common Tern
Recoveries during migration of Common Tern pulli and juveniles ringed at Teesmouth in late summer are limited to France (1), Portugal (1) and Morocco (2), the last two in September. Winter recoveries (October onwards) of juveniles and of immatures (up to third year of life), come from North-west Africa (9 recoveries; Senegal, Ghana, Gabon, Liberia, Guinea Bissau, Togo).
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Migration patterns & moult of Terns
23
Table 4. The percentage of the total number of juveniles recovered at Teesmouth originating from a given region for half-month periods in late summer
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Early August Baltic Sea (east of Norway) Norway North-east Scotland North-east England Eastern England The Netherlands
10 12.5 0 17 0 0
Late August 30 75 100 48 33 0
Two of five winter recoveries relating to adult birds, all of them along the Atlantic coast of Africa, are from Namibia and South Africa. No known Teesmouth-bred birds however have been found along the Southern African coast. An unusual winter recovery relates to an adult found during February in Hampshire. All Common Tern recoveries relating to nonbreeders within European waters other than those already mentioned, are from Seaforth Docks, Liverpool Bay, NWEngland (34 in total). Three of these are of third calendar year birds at Seaforth during early summer (mid May & June) whilst the remainder are of third calendar year or older birds during late summer (July September). Twenty-five of the recoveries are of birds ringed at Teesmouth as pulli; the other nine were ringed as juveniles, second calendar years or old during the late summer. Biometrics Wing length
Early September 60 0 0 30 50 100
Late September 0 12.5 0 4 17 0
Total no of recoveries
10 8 3 23 6 2
Common Tern
Wing lengths of adults in late summer were much more variable than those of Sandwich Terns; the mean was 272.1mm (se = 0.39, range 243-295mm, n = 424). On dividing the late summer period into half months, an increase in mean wing length between mid July- mid Sep of 270.2 -273.0mm was found to be significant only at the 10% level (One-way ANOVA: F = 2.21, P = 0.09, n = 421). Mean wing length of juveniles, many with primaries still in active growth was 259.4mm (se = 0.44, range 215-277mm, n = 414). Bill length Adequate data on bill lengths were available only for Common Terns in August. No significant difference was found in bill lengths of adults between early and late August birds (Z-test: Z = 0.34, P = 0.37, n = 165). Use of the probability paper technique suggested a unimodal distribution of these data.
Sandwich Tern
Body mass
Wing lengths of adults in late summer were unimodal with a mean of 306.5mm (se = 0.47, range 293 - 326mm, n = 195); mean wing lengths of juveniles was 278.3mm (se = 1.68, range 227309mm, n = 102) though primaries of most individuals were still in active growth. Adult Sandwich Terns showed no significant variation in wing length with time (One-way ANOVA: F = 1.32, P = 0.27, n = 191).
Sandwich Tern
Sandwich Tern body masses were grouped by half-months for all years combined. The difference in mean body mass of adults and juveniles between combined catch data for late July and early August (Table 5) was not significant at the 5% level (Z-test: Z = 1.31, P = 0.19, n = 189 for adults; Z = 0.26, P = 0.79, n = 132 for juveniles), but the body mass of both age classes was significantly more variable in the later catches during August (F-test: F = 2.85, P < 0.001, n = 37
© 2000 British Trust for Ornithology, Ringing & Migration, 20, 19-28
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Robin M Ward
Table 5. Body masses of adult and juvenile Sandwich Terns at Teesmouth
Date Period
Adults se
N
241
1.3
157
221
1.8
107
244
2.2
32
220
3.2
28
246
9.5
5
228
9.1
16
Mean (grams)
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Late July Early August Late August
for adults; F = 4.77, P < 0.001, n = 44 for juveniles). Common Tern
When data for all years were combined, significant variations were found in mean body mass grouped by half monthly periods from mid - July — mid - September in both juvenile and adult Common Terns (One-way ANOVA: F = 5.52, P < 0.001 for juveniles; F-Test: F = 3.68, P < 0.001, n = 194 for adults). However the mean half-monthly body masses fluctuated through time with no directionality. Sufficient data for a within year analysis exists only for 1995 (Table 6). A significant difference was found between the mean body masses of adults of some catches (One-way ANOVA: F = 2.45, P = 0.04, n = 147). Juvenile Common Terns also varied significantly in mean body mass amongst the six 1995 catches (One-way ANOVA: F = 7.54, P < 0.001, n = 120). Moult of primary feathers Sandwich Tern
Adequate moult data exist for only three catches at Teesmouth, 23 July 1994, 28 July 1996 and 1 August 1993. The percentage of adults that had begun to moult their primaries by these dates were 87.5% (n = 8), 91.7% (n = 25) and 87.5% (n = 32) respectively. Moult scores ranged from 0 to 19. All birds exhibited sequential replacement of primaries starting from the innermost, with between none and three primaries in simultaneous growth. Common Tern
The observed difference in the proportion of Common Terns in primary moult amongst half monthly periods (early Aug - early Sep) was not
Mean (grams)
Juveniles se
N
statistically significant (x22 = 5.46, P > 0.05); the overall proportion was 44.7% (Table 7). Amongst those birds that had began primary moult, the variation in mean moult score of the three samples was statistically significant (F2128 = 9.76, P = 0.0001, n = 131), the mean score rising from 3.2 to 8.1. Primary moult scores from 0 to 24 were recorded between July and early September, though the scores of only 1% of birds (3 of 293) exceeded 14. The primaries were replaced from the innermost, with no more than three in simultaneous growth. A significant difference was found in body mass between moulting and non-moulting adults (Z-test: Z = 2.04, P < 0.05, n = 263); the mean for moulting birds was 137.2 (se = 1.40, n = 121) and those birds not moulting 133.0 (se = 1.34, n = 142). The number of first generation primaries retained from the previous series of moults in adult Common Terns (Baker 1993) showed a significant relationship with wing length (Product Moment Correlation Coefficient: r = 0.25, t = 3.81, P < 0.05, n = 215) but not with date (r = -0.11, n = 244, f = 1.747, P > 0.05). The mean number of retained first generation primaries in Common Terns during the months July — September was five. DISCUSSION Population size Teesmouth's SPA citation (1993) identifying the site's international importance to migratory Sandwich Terns, is endorsed by my low water counts. The use of peak counts alone does not identify Teesmouth to be of international importance for migrant Common Terns. Although the maximum numbers of Common Terns recorded exceeded 1% of the estimated UK breeding population, ringing recoveries show that the passage population includes a
© 2000 British Trust for Ornithology, Ringing & Migration, 20, 19-28
Migration patterns & moult of Terns
25
Table 6. Body masses of adult and juvenile Common Terns from catches at Teesmouth in 1995
Adults Mean (grams)
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Date
1 August 4 August 20 August 23 August 2 September 3 September
Juveniles se
N
Mean (grams)
N
142
2.9
7
128
1.8
136
1.9
20
130
1.8
5 14
131
1.2
46
122
1.2
49
130
1.8
30
118
3.0
11
131
2.2
21
124
2.9
13
131
2.4
23
132
1.5
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Table 7. Proportion of adult Common Terns in primary moult at Teesmouth
Early Aug No primary moult In active primary moult % in active moult Total no. of birds
Late Aug
Early Total no. of birds Sept
67
70
25
162
37
70
24
131
36
50
49
45
104
140
49
293
significant proportion of continental breeders (see later). Origins & movements The origins and movements of Teesmouth's Sandwich and Common Tern populations, both breeders and migrants, conform to the patterns expected from previous reviews of recoveries of ringed birds (eg Langham 1971, Cramp 1985). Since these analyses, however, the ringing of pulli and fledged birds at Teesmouth, in conjunction with the reading of rings in the field at Seaforth Docks, Merseyside has identified an autumn migratory flyway between the two sites. This was confirmed when an adult Common Tern was observed at Seaforth in August 1995, seven days after it was dye-marked at Teesmouth; only 24 birds were marked. Flocks of Common and Sandwich Terns are regularly observed to depart southwestwards high from Teesmouth around dusk on a presumed trans-Pennine migratory path; smaller numbers are observed arriving from the southwest in spring (RMW pers. obs., E.Wood pers comm.). Liverpool Bay lies due southwest from Teesmouth. Recoveries of
Common Terns in Liverpool Bay suggest that both Baltic and North Sea breeders are involved in the trans-Pennine movement (Fearon & Pennington 1984, White 1991). Radar studies identified a narrow direct migratory route from Teesmouth south-west to the Ribble estuary during late summer that was attributed to shorebirds (Evans 1968) but probably is also the flyway used by terns. Certainly the four criteria used by Evans to attribute the radar echoes to shorebirds apply equally well to terns whose use of the Tees estuary in the 1960's would appear not to have differed significantly from the present (Stead 1969). Overland migration routes of Common Terns elsewhere in Europe (eg Alerstam 1985) are known to far exceed the distance and altitude barriers presented by what is amongst the shortest possible overland crossings of northern Britain. The preponderance of second and third calendar year Common Terns from Teesmouth at Seaforth Docks is almost certainly a consequence of 1000+ pulli being ringed at Teesmouth between 1993 - 95 but comparatively
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Robin M Ward
few before that period. The possibility exists that a proportion of these immatures remain west of the Pennines as first time breeders, when aged 2 years or usually greater, since Common Terns often settle in a colony other than their natal one (Cramp 1985). Whereas for Common Terns the opportunity exists for reading rings in the field at Seaforth Docks, this is not so for the post-breeding concentrations of Sandwich Terns in Liverpool Bay (J.D. Fletcher pers. comm.). Thus the lack of recoveries of Sandwich Terns along the west coast of Britain does not cast doubts on transPennine migration for this species, which visual observations at Teesmouth indicate. Moult & biometrics The extent, progression and patterns of primary moult observed in Sandwich Terns at Teesmouth in late summer are much as expected for a North Sea location (Cramp 1985, Olsen & Larsson 1995). This applies equally to those biometrics examined. There was no evidence of any temporal differences that could be attributed to identifiable cohorts eg geographical populations. In Common Terns, during August and early September, primary moult data separate adults at Teesmouth into two distinct groups, their respective proportions not changing with time. The larger group of birds, 55.2%, have yet to initiate primary moult at a time of year when the majority of breeding adults in at least one Dutch colony are in primary moult (Walters 1979). Whereas in the Baltic Sea only 10% of birds have initiated primary moult by late July/ early August (Olsen & Larsson 1995) no published data yet exists for moult strategies of breeding adults from northern North Sea colonies. The progressive increase in the mean moult score but the lack of change in the percentage of moulting birds, is compatible with the existence of two discrete cohorts at Teesmouth. Furthermore, the recovery pattern is consistent with the involvement of some South African winterers, these adults initiating primary moult only upon arrival from the Baltic (L.G.Underhill pers. comm.). Given that primary moult is rarely initiated after August in European Waters (Cramp 1985), it is likely that Teesmouth's late summer population includes a substantial Baltic contingent of non-moulting adults.
Certainly the seasonal pattern of Common Tern migration through Teesmouth (Fig 1) matches that of Baltic breeders observed in southern Sweden (Alerstam 1985). The progress of active moult amongst Common Terns at Teesmouth in late summer conforms with that described in Liverpool Bay (Bainbridge 1977), as would be expected if they derive from the same population. However during August only 2 of 108 Teesmouth moulting birds had reached a moult limit comparable with the majority in the Wadden Sea i.e. between primaries 4-5 (Olsen & Larsson 1995), whilst 78% had reached only 2-3 or 1-2. It is unlikely that differences in age and body condition can explain the observed differences in moult between the two areas. The observed difference in primary moult between Common Terns predominately of northern North Sea origin and those in the Wadden Sea is likely to be real. Perhaps, therefore, the northern North Sea populations adopt a timing of moult that intergrades with that of Dutch (Walters 1979) and (eastern) Baltic breeders (Olsen & Larsson 1995, L.G.Underhill pers. comm.). The advantages, in terms of energetic costs, of adopting a moult strategy that alters with latitude, have been reviewed for many other migratory species (Ginn & Melville 1983) and could provide an explanation for observed Common Tern moult patterns. The presence over several weeks of birds initiating moult, although the proportion of the population in moult remains unchanged, suggests a turnover of moulting Common Tern adults at Teesmouth. Furthermore this can be taken to imply a turnover also of non-moulting adults, possibly the later arrival of Baltic migrants, as suggested from the temporal variation observed in recovery patterns. The fact that body mass is significantly different between catches is consistent with the arrival of different waves of migrants that vary in body condition. No temporal variation was identified within the other biometrics taken but geographical variation in size within the European subspecies S.h.hirundo is small (Cramp 1985, Olsen & Larsson 1995). Counts show considerable fluctuations in numbers of both Sandwich and Common Terns at Teesmouth within the few weeks of each late summer period, one feature indicative of high
© 2000 British Trust for Ornithology, Ringing & Migration, 20, 19-28
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Migration patterns & moult of Terns turnover (Evans 1984). For example, from a peak of 1,470 Common Terns at Teesmouth in mid August 1995, numbers declined to less than 100 before rising to a second peak of 750+ in early September (pers. obs.). Rapid turnover was also indicated in 1995 by the absence of recaptures in later nights of any of the 552 Common Terns mist-netted on 5 nights. During each of the evenings, up to several hundreds of Common Terns were observed climbing high at dusk and departing from the estuary on a southwesterly bearing. Assuming that most of the moulting adults were from British colonies and that each pair was accompanied by two juveniles, within the second half of late summer 1995 over 950 Common Terns of UK origin passed through Teesmouth. This figure equates to over 1% of the UK breeding population (Lloyd et al. 1991) with their young, thus providing another example of a site's importance being far greater than is apparent from peak count data alone (Smit & Piersma 1989). Given the disparity between Teesmouth and the Wadden Sea in the moult patterns observed during late summer, most Baltic Common Terns migrating east-west over southern Sweden (Alerstam 1985) probably go via trans-Pennine routes rather than the southern North Sea. Common Terns which begin primary moult whilst in European waters, according to several authors (eg Cramp 1985, Ginn & Melville 1983), suspend moult before they begin long distance migration. Only one bird was encountered at Teesmouth by early September (n = 293) that had probably suspended primary moult; the moult limit was between primaries 1-2 with two third generation primaries in positions 2 and 3. Over 19% of adults examined (n = 246) exhibited between one and four comparatively fresh inner primaries but these were assigned to the third wave of primary moult that some birds undertake prior to breeding (Ginn & Melville 1983). That eight of these birds were actively moulting out these innermost primaries confirms that they were not first generation primaries in suspended moult.
27
CONCLUSION Integration of count and ringing data for Teesmouth endorses the international designation for Sandwich Terns and suggests that over 1% of the U.K. breeding population of Common Terns migrate through Teesmouth in late summer. An equally large proportion of the U.K. breeding population is likely to form the substantial post-breeding concentration of Common Terns in Liverpool Bay to which a trans-Pennine flyway from Teesmouth has now been identified. These Common Terns are drawn from breeding populations that differ in terms of the time of onset of post-nuptial primary moult from that documented for Wadden Sea birds. Whether most of the adults yet to initiate primary moult are eastern Baltic Sea breeders warrants further investigation. Elsewhere, the identification of the origin of passage migrants, using primary moult timing and patterns should be possible if a gradation in the onset of postnuptial primary moult across western Europe can be confirmed and detailed. Taking this further could allow quantification of the importance of those populations utilising particular migratory staging sites in north-west Europe by integrating moult and count data. ACKNOWLEDGEMENTS
I wish to thank the many individuals whom have participated in the tern catches taken by Tees Ringing Group and Durham University at Teesmouth. Eric Wood and Geoff Myers have in particular provided much help and constructive discussion throughout the study. WeBS has been a success at Teesmouth because of a team of dedicated volunteers co-ordinated by Martin Blick and Mike Leakey. Interpretation of these Teesmouth data have very much benefited from the reading of metal rings in the field by Steve White at Seaforth. I am very grateful to Prof. Peter Evans for both his encouragement and constructive comments on drafts of this paper. Part of the research on which this paper is based was funded by the Teeside Development Corporation under the Monitoring Programme of the effects of the Tees Barrage.
© 2000 British Trust for Ornithology, Ringing & Migration, 20, 19-28
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Robin M Ward
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