Abstract from BTO Research Report No 344:

Jacqui A Clark, Nigel A Clark, Robert A Robinson & Philip W Atkinson (2003)

Using adult/juvenile ratios of waders in winter cannon net catches in Britain & Ireland to measure recruitment.
ISBN: 1-904870-96-1

Summary

The UK is of international importance for waders. Although relatively few species of waders breed here, many more are present during passage and in the winter. For such migratory species, the UK has obligations to protect both the birds and their habitat. The UK is a signatory to a number of international conservation conventions. Of particular importance in the context of waders are the EC Birds Directive, the EU Habitats and Species Directive and the ‘Ramsar’ convention on Wetlands of International Importance. This legislation requires the UK to identify habitats and sites important for birds and designate them for protection; as part of this it is essential to monitor populations to identify, and then monitor, important sites. In addition, the UK has ratified the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA) of the ‘Bonn’ Convention on the Conservation of Migratory Species of Wild Animals. AEWA requires countries to conserve migratory waterbirds, identify sites and habitats used and ensure they are protected and managed appropriately. Again to fulfil these obligations the populations must be monitored.

Numbers of non-breeding waders present in the UK are monitored by the Wetland Bird Survey (WeBS) to assess the size of the non-breeding population, monitor trends in numbers and distribution and to assess the importance of individual sites. Monthly counts are made from September to March, although the annual index of abundance is based on the data for December to February. The annual index allows any changes in population size to be identified. Information on survival rates, productivity and dispersal allows the underlying causes of any such changes in populations to be investigated. When changes in demographic factors are identified, investigative research and subsequent conservation action can be focused on these areas. For long-lived species, changes in demography may be identified before changes in numbers (Baillie et al 1999). It is therefore important to monitor demographic factors as they may give the first warning of problems for a population that may lead to declines.

For migrant birds, the collection of demographic data may pose problems as there may not be access to information from both the breeding and wintering areas. For waders in the UK survival rates can be calculated from ringing and subsequent reports (recoveries) of ringed birds either recaught by ringers (at or away from the original site of ringing) or found dead. Ringing and recovery data can also provide information on movements between wintering sites. However, all individuals of some species breed outwith the UK and amongst other species, some individuals breed in the UK and others elsewhere. Many wader species occupy vast breeding ranges (Cramp & Simmons 1983), the breeding areas are often remote and wader chicks leave the nest shortly after hatching making the collection of extensive productivity information at breeding sites difficult.

In recent years information from various workers in the arctic describing and analysing breeding conditions has been gathered together in Arctic Birds (Soloviev & Tomkovich 1999, 2000, 2001, 2002, 2003). Arctic Birds gives reports from individual locations and summarises information on weather, rodent abundance, predators, distribution and numbers of breeding waterfowl and breeding success. However, figures for breeding success may only be available for a few sites (eg Soloviev & Tomokovich 2003). An alternative approach is to use adult/juvenile ratios on wintering grounds (ie recruitment into the wintering population) as a measure of productivity, which might provide a useful index. Indeed adult/juvenile ratios for Knot (Calidris canutus) have been used to investigate the contribution of survival and recruitment to population trends (Boyd & Piersma 2001) and data for Knot and Oystercatcher (Haematopus ostralegus) wintering on the Wash have been used successfully in a population model which suggested that recruitment rather than any long-term changes in survival had tended to drive population changes (Atkinson et al 2003).

Goose workers have used numbers of juveniles and older birds counted in autumn and winter flocks to assess productivity (eg Pink-footed Goose (Anser brachyrynchus), Fox et al 1989; Greylag Goose (Anser anser), Fox et al 1989; Barnacle Goose (Branta leucopsis) , Fox & Gitay 1991; Brent Goose (Branta bernicla), Ebbinge 1992, Ebbinge & Spaans 1995). Although they consider this may only give a crude assessment of annual productivity and be subject to bias, the information gained has been useful in giving information on changes in breeding productivity (Madsen et al 1999). Summers & Underhill (1987) also assessed productivity of Brent Geese from counts and extended this to counts and catches of wintering waders. In addition, Clausager (eg 2003) has collected adult juvenile/ratios from wings returned from quarry species. Underhill et al (1989) used information from catches of Knot in South Africa and Britain and, more recently, Minton et al (2000, 2001, 2002a, 2002b, 2003a, 2003b) have derived an index of breeding success in the previous arctic summer from the proportion of juveniles in catches in Australia in the non-breeding season.

Using data from the non-breeding grounds has the advantage that all surviving offspring are included. This is particularly useful for precocial species where it is often difficult to estimate the numbers fledged (Crick & Baillie 1996). However, there are other problems in using these data. Waders wintering in one area may come from more than one breeding area so that any index may be sampling multiple populations. Unless there are similar changes in different breeding populations this would make any changes difficult to detect.

In addition, changes in the proportion of juveniles present may occur through the non-breeding period, thus it would be important to only compare samples from different years that were caught at the same time. The time period would have to be dictated by the behaviour of the birds, possibly with a narrow time frame in passage periods but a wider timeframe in the middle of the non-breeding season when populations tend to be stable.

There may also be biases in the catching data. For example, there tends to be a higher proportion of juveniles in small catches than in large ones (Boyd & Piersma 2001, pers obs). In addition, there may be an increased proportion of juveniles in a catch if there has been much disturbance of the birds prior to the catch. Also, there may be different proportions of juveniles in different habitats with adults in the more preferred areas (eg Swennen 1984, Durrell et al 1996). Differences in the proportion of juvenile waders in catches have been found to vary with catching method, with higher proportions in mist net than cannon net catches (Pienkowski & Dick 1976, Goss-Custard et al 1981, Insley & Etheridge 1997).

Differences in the distribution of adults and juveniles could also bias the proportions found. Such biases can occur at a macro (national or whole non-breeding area), local (within an estuary) or micro (within a flock) scale. Such biases may occur due to the non-breeding distribution of the species involved, the type and quality of habitats available in an area and the distribution of birds within a flock. For example, juvenile Redshank (Tringa totanus) in Scotland have been found to occupy less favoured habitats where they are more susceptible to predation while feeding over the high tide period (Hilton et al 1999). Also, juvenile birds tend to be on the periphery of flocks (pers obs). Migratory tendency within a species may also change over time (eg Oystercatcher, Lapwing (Vanellus vanellus) – Siriwardena & Wernham 2002) and could potentially lead to changes in the proportion of juveniles present in an area over time. All possible biases need to be investigated so that their effects can be taken into account when constructing an index.

Wader catching in Britain & Ireland
Well over a million individual waders have been caught and ringed in Britain & Ireland since the beginning of the Ringing Scheme in 1909. In the early years of the Scheme, most birds caught were nestlings but with the advent of mist nets in the 1950s and rocket and then cannon nets in the 1960s the proportion of full-grown birds caught for ringing has increased. Cannon nets are used to catch a relatively large number of birds at one time. A net or nets are usually set in an area where waders are likely to congregate in a roost at high tide. Ringers catching birds in cannon nets in Britain & Ireland are required to make a return for each catch giving the numbers of each species caught. In order to gather information for a recruitment index a form dividing the catch into adults, juveniles and birds of unknown age of each species was introduced in 1990. Since then the proportion of juveniles in cannon net catches has been collected for 3,156 different samples of 124,669 waders. All cannon net catches of waders in the database were made in Britain. Note that each catch may consist of a number of samples of different species.

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