differences in population structure between two ...

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DIFFERENCES IN POPULATION STRUCTURE BETWEEN TWO OYSTERCATCHER HAEMATOPUS OSTRALEGUS ROOSTS ON THE BURRY INLET, SOUTH WALES SARAH E.A. LE V. DIT DURELL!, STEVE J. ORMEROD2 & PETER J. DARE3 Durell S.E.A. Le V. dit, S.I. Onnerod & P.I. Dare 1996. Differences in population structure between two Oystercatcher Haematopus ostralegus roosts on the Burry Inlet, South Wales. Ardea 84A: 383-388. Comparison is made between Oystercatchers caught by cannon netting at two roost sites on the Burry Inlet in the winter months of 1990 and 1991. Proportionally more adult birds were caught at a large roost at Whiteford Point, and more juveniles at a smaller roost at Llanelli North Dock. Adults caught at Whiteford Point were also less likely to be suffering from deformities and more likely to be d' d' . These findings are contrasted with previous work on the Wadden Sea, where Oystercatchers caught at large roosts containing more adult birds and fewer deformed birds were more likely to be QQ. Mechanisms for assortive roosting are discussed. It is suggested that the population structures of the different Oystercatcher roosts on the Burry reflect the feeding preferences of the birds present, but that feeding preferences may also be influenced by a bird's social status, or dominance. Key words: Oystercatcher - Haematopus ostralegus - Burry Inlet - sex ratio llnstitute of Terrestrial Ecology, Furzebrook Research Station, Wareham, Dorset BH20 5AS, U.K.; zSchool of Pure and Applied Biology, University of Wales College of Cardiff, Cardiff CFI 3TU, U.K.; 3Ministry of Agriculture, Fisheries and Food, Fisheries Laboratory, Pakefield Road, Lowestoft, Suffolk NR33 OHT, U.K.

INTRODUCTION Differences are known to occur in the distribution of roosting birds both within and between roost sites. An individual's position within a roost can be related to its status in relation to the other birds present (Pienkowski & Dick 1976, Swingland 1977, Furness & Galbraith 1980, Weatherhead & Hoysack 1984, Still et ai. 1987). Differences between roost sites can also be explained in these terms: interactions within a roost site may cause birds of lower status to roost elsewhere. Alternatively, the population structure of a particular roost may merely reflect differential utilisation of nearby feeding grounds; all birds from a particular feeding area roosting in the same place. In a study of Oystercatcher Haematopus ostraiegus roost sites on the Wadden Sea, Swennen

(1984) concluded that birds did distribute themselves between roosts according to their social rank, but that this distribution was determined on the feeding grounds. In other words, birds of higher dominance excluded birds of lower dominance from the preferred feeding areas and the composition of a particular roost site reflected the 'quality' of the nearby intertidal flats. In this paper we describe differences in the age structure, sex ratio and the proportion of deformed birds found at two Oystercatcher roost sites on the Burry Inlet, S. Wales. Our findings support those of Swennen but with one exception: the larger roost, with the high proportion of adults and low proportion of deformed birds, had a high proportion of male, rather than female birds. We discuss this contrast between the Burry and the Wadden Sea in terms of sex differences in diet.

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ARDEA 84A, 1996

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Llanelli .~

i.

Whiteford Point

..e/ o _

o Fig. 1.

•

2km

Intertidal flats saltmarsh mussel beds

main roost sttes

Map of the Burry Inlet showing the main Oystercatcher roost sites.

The study area The Burry Inlet (Fig. 1) is one of the major British estuaries for overwintering Oystercatchers. With intertidal mud- and sandflats totalling 4800. hectares, the Burry supports peak numbers of around 19 000 Oystercatchers (Prys-Jones et al. 1989). A detailed description of the Oystercatchers' movements within the estuary is given in Prys-Jones et al. (1989). At low water, around 13 000 birds feed within the estuary, mostly on Cockles CerastoderYfUl edule. A further 4-6000 birds feed just outside the estuary on mussel Mytilus edulis beds NW. of Whiteford Burrows. At high tide, most of the oystercatchers roost in the extensive salt marshes along the southern shore of the estuary and on shingle spits near Whiteford. Small numbers also roost on the north shore, near Llanelli.

METHODS Cannon-net catches were made at two roost sites on the Burry: Whiteford Point and Llanelli North

Dock. Whiteford Point is a relatively large roost site, with average numbers of around 2500 birds, whilst Llanelli North Dock is a smaller roost, with average numbers of 500-1000 birds. The majority of birds roosting at Whiteford Point feed on the mussel scar nearby, whereas those at Llanelli feed on the intertidal flats within the estuary. Two catches of over 150 Oystercatchers were made at Whiteford Point in October and November 1991. Two catches of over 100 birds were made at Llanelli in January and February 1990, and two catches of over 50 birds in October 1990 and November 1991. Nets were set in different parts of the roosts at each catch. Birds were aged as juveniles (in their first year), irnmatures (second to approximately fourth year) and adult (Prater et al. 1977). Birds were weighed to 1 g, using a Pesola balance, and their wing length (maximum flattened) and bill length (to feathers) measured to 1 mm, using a stopped rule and vernier calipers. Any deformities or injuries were also noted. Sex ratios for adults at each roost were calculated in two different ways, both using bill length.

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Durell et af.: POPULATION STRUCI1JRE ON TWO ROOSTS

385

length frequency data from the birds caught in 1990 and 1991. Method 2 predicted the proportion of d d from the mean bill length of the 80 whole sample, using a regression equation calcu•• lated from corpses obtained and dissected from • I I/J 60 both the Burry and Morecambe Bay in the 1960s 75 mm: 10%. These proportions were then applied to bill

RESULTS In total, 342 Oystercatchers were caught at Whiteford Point and 349 at Llanelli North Dock. As the population structure of overwintering Oystercatchers is extremely stable over the winter, and from one winter to the next (Goss-Custard et al. 1982), data were combined for all catches at each roost. Proportionally more adults were caught at

Table 1. Characteristics of Oystercatchers caught at a large roost at Whiteford Point (WP) and a small roost at Llanelli North Dock (LND) on the Burry Inlet. n: numbers caught, SE: standard error, % occ. = % occurrence, % def. = % deformities. Roost Age

WP

n

Adults Immatures Juveniles

260 71

LND Adults Immatures Juveniles

% occ. % def. %

(J (J

1 0 0

67 54

11

76 21 3

181 74 94

52 21 27

11

45 33

0 2

Bill length

Wing length

Weight

mean

SE

mean

SE

mean

SE

72.4 71.5 74.8

0.3 0.6 5.5

549 546 560

2.4 4.6 8.5

265 260 261

0.5 0.7

75.3 75.4 75.2

0.4 0.6 0.7

589 552 526

4.2 4.5 4.5

267 263 259

0.7 0.7 0.5

1.3

ARDEA 84A, 1996

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Whiteford Point and more juveniles at Llanelli North Dock (x2 on numerical frequencies = 102.31, df = 2, P < 0.001) (Table 1). Of the adults caught at Llanelli North Dock, 11 % had missing or deformed legs, a significantly greater incidence than the 1% of those caught at Whiteford Point (x2 = 22.54, df = 1, P < 0.001). Few immatures or juveniles suffered from deformities at either roost. Sex ratios also differed between roost sites, with more (J (J being caught at Whiteford Point. In adults, method 1 and method 2 gave similar results, with the proportion of (J (J at Whiteford calculated to be 66.9% and 67.9% respectively, and at Llanelli North Dock 45.6% and 44.6% (x2, 1 df= 19.96,p < 0.001). With immatures the proportion of (J (J at Whiteford Point was calculated to be 54.2% and at Llanelli 32.9% (x2, 1 df = 6.94, P < 0.01).

DISCUSSION There were marked differences in the population structure between the two oystercatcher roosts. Compared with Llanelli North Dock, more adults, more (J (J and fewer deformed birds were caught at Whiteford Point. Swennen (1984) also found that larger roosts were characterized by a high percentage of adults and a low percentage of birds with anatomical defects. In contrast, however, he found that it was