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Possible resource partitioning in pairs of Phylloscopus and Acrocephalus warblers during autumn migration through a south Wales reedswamp S. J. Ormerod
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School of Pure and Applied Biology , University of Wales, College of Cardiff , Cathays Park, Cardiff, CF1 3XF, U.K. Published online: 11 Apr 2011.
To cite this article: S. J. Ormerod (1990) Possible resource partitioning in pairs of Phylloscopus and Acrocephalus warblers during autumn migration through a south Wales reedswamp, Ringing & Migration, 11:2, 76-85, DOI: 10.1080/03078698.1990.9673965 To link to this article: http://dx.doi.org/10.1080/03078698.1990.9673965
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Possible resource partitioning in pairs of Phylloscopus and Acrocephalus warblers during autumn migration through a South Wales reedswamp Downloaded by [Cardiff University Libraries] at 06:58 13 October 2014
S. J. Ormerod Ormerod, S. J. 1990. Possible partitioning in pairs of Phylloscopus and Acrocephalus warblers during autumn migration through a South Wales reedswamp. Ring. & Migr. 11: 76-85. Though there is evidence for resource partitioning within Acrocephalus and Phylloscopus warblers on their breeding and wintering grounds, fewer data are available on partitioning during autumn migration. In several respects, this is a time when segregation may be important. Patterns of habitat distribution (between different vegetation types and different capture heights), times of passage, and daily pattern of occurrence were therefore compared between two species within each of these genera during their autumn migration through a reedswamp in South Wales in 1988 and 1989. In Reed and Sedge Warblers, passage peaked contemporaneously, diurnal patterns of occurrence were almost identical, and capture height did not differ in either reedswamp or sallow carr (though vegetation type did significantly affect capture height). However, in both years, Sedge Warblers were significantly more numerous in reedswamp than Reed Warblers. By contrast, Willow Warblers and Chiffchaffs occurred always in the same habitats, and at similar heights in the vegetation, though there was evidence of different diurnal patterns of occurrence, and strong evidence of different seasonal patterns of occurrence. One inference is that the two Phylloscopus species partitioned resources by occurring at the site at different times, while the two Acrocephalus species partitioned resources by the use of different habitats (and different foods). Other reasons for these temporal and habitat patterns (e.g. contrasting morphology in Reed and Sedge Warblers, different migratory journeys in Willow Warbler and Chiffchaff) have been suggested previously, but they are not necessarily evolutionary explanations. The segregation of resources between the species in each genus should not be overlooked as reasons for their incompletely understood migratory strategies. Though governed by some difficulties of interpretation, this area is worthy of further study using data of the type generated by ringers. Dr S. J. Ormerod, School of Pure and Applied Biology University of Wales College of Cardiff, Cathays Park, Cardiff, CF1 3XF U.K. Received 16 November 1989; revised 8 January 1990; accepted 15 February 1990.
INTRODUCTION Closely related species of animals often partition resources such as space and food in ways that reduce competition. Birds show such partitioning strongly (Lack, 1970), and many ornithologists are intuitively aware of the interspecific differences in habitat use or geographical distribution which might have arisen as a result. In migrants, such partitioning is shown not only on the breeding grounds, but also on wintering grounds which are shared with residents from the same genera (e.g. Moreau 1972).
Rather less attention has been devoted to studying competition and partitioning during the autumn migration. In many respects, this is a time when any competition might be important: populations are large following breeding, many individuals converge over a short time period on certain types of site, and in some species at some locations there may be increased energetic requirements for pre-migratory fattening (e.g. Bibby & Green, 1981). Intra-specific competition for food sometimes occurs in migrants (e.g. Bibby & Green, 1980),
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RESOURCE PARTITIONING IN WARBLERS
implying resource scarcity, though few authors have examined resource allocation between species (e.g. Barlein, 1981, 1983). Barlein's studies are particularly important in this respect, showing how warblers and other migrants had either distinct habitat preferences or peak migration periods which were consistent with resource partitioning between morphologically similar species. However, though large samples were involved, the data were from only three sites in central Europe and the patterns of habitat preferences are largely untested in other locations. The warbler genera Acrocephalus and Phylloscopus are useful for the examination of resource partitioning during migration. Species within them show evidence of resource partitioning or inter-specific territoriality both on the breeding grounds (Eddington & Eddington, 1972, Cody 1978, Catchpole 1973, Saether, 1981), and on the wintering grounds (Moreau, 1972; Aidley & Wilkinson 1986). These features combine to suggest inter-specific competition either now, or at some stage during their evolution into different species (Connell, 1980). In this study, data from a ringing programme in a South Wales reedswamp were used to investigate inter-specific partitioning between Reed Warbler A. scirpaceus and Sedge Warbler A. schoenobaenus, and between Willow Warbler P. trochilus and Chiffchaff P. collybita, during their autumn migration. Features examined included some of those initially considered by Barlein (1981, 1983) namely: comparisons of habitat use (foraging height and associations with different areas of vegetation), and times of passage. Use of the area with respect to time of day was also investigated. STUDY AREA AND METHODS All the work was undertaken in a coastal reedswamp of Phragmites australis, fringed by carr of sallows Salix spp, adjoining the upper estuary of the River Loughor at Llangennech, Dyfed, South West Wales 51°41'N, 4°04'W, c 5 m O. D.). It has a marked passage of
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Acrocephalus warblers, and is probably a fattening site for Sedge Warblers (Ormerod, 1990). Some data from this previous study are included here, but in a different context. Studies in the area were undertaken between March and October of both 1988 and 1989, although the data given here are from the migration period from mid-July to mid-October of both years. Over this time, 160 m (1988) or 216m (1989) of mist netting were deployed throughout 1-2 ha of reed and sallow carr. In 1988, nets were set so that approximately 70% of their length was in sallow or at the reed/sallow border, with the remainder in the reed alone. In 1989, nets were set in the same locations as in 1989, but an additional 56 m of netting were deployed in an area of reed to equalize the length in the two major types of habitat. At approximately weekly intervals, the nets were operated from first light for 4-5 hours on mornings with good weather (i.e. without high winds, rain or bright sunshine). Nets were patrolled every 20 minutes, and the habtitats where birds were caught were recorded as far as possible. Total catches of all species -were standardized as number per hour (i.e. catch per unit effort), which in other ecosystems gives a good approximation of abundance (Ormerod et al. 1988). In addition to the time of capture (standardized as hours from first light), the height at which birds were caught (assumed to represent foraging height) was recorded in 1989 according to the four net shelves (A = top, D = bottom). Nets in all habitats were set at the same height, with the bottom shelf about 0.3 m from the ground. Special indices are available for analysing resource partitioning between species (e.g. Saether, 1981), but all the analyses here have been made using conventional statistical methods.
RESULTS In both years combined, 319 Sedge Warblers, 410 Reed Warblers, 173 Willow Warblers and 80 Chiffchaffs were captured. A small percentage of these birds were birds recaptured after initial ringing, with retraps of Willow Warbler and Chiffchaff particularly scarce. Sedge and Reed Warbler Time of passage. Though Reed Warblers were present for around 4 weeks after Sedge Warblers had left, catches per unit effort for these species peaked almost contemperaneously in August of both years (Fig. 1; see also Ormerod, 1990). As a result, catches of the two species were correlated highly
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4-5n 1989
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30-
1-5-
I a) (D
4-5-,
1988
I
6 Sedge Warbler FIGURE 1. Catches per hour of Reed Warblers in relation to catches of Sedge Warblers during the autumn migration of the two species through a South Wales reedswamp in 1988 and 1989. Correlations in text.
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20^
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O-..
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O-
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40-n
o o 20—
1
2
3
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Hours from dawn FIGURE 2. Changes in the catch per hour of Reed Warbler (O), Sedge Warbler (•), Willow Warbler (A), and Chiffchaff (A) during the first hours after dawn during autumn migration through a South Wales reedswamp. The data are pooled from 1988 and 1989. On numerical frequencies, the patterns for Reed and Sedge Warblers did not differ significantly (xl = 0.84), but those for Willow Warbler and Chiffchaff did x] = 8.31, P< 0.05).
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significantly in both 1988 (r = 0.90, n— 14, P< 0.001), and 1989 (r=0.69, n = 1 3 , P
