Interpreting changing wader counts

Blog mixed flockWhen you count the number of Redshank on your local estuary and discover that there are fewer now than there were last year –  or five, or twenty years ago – what are the implications? Is this part of a national or international trend or has something changed within the estuary itself?

The first question to ask is, ‘what is happening elsewhere?’ and then to wonder about the ways in which numbers of birds in different sites might relate to each other. What actually happens to local counts when national counts go down – or go up, for that matter?

The UK Wetland Bird Survey (WeBS) data provide comprehensive and long-term monitoring of estuarine wader populations around our coastline. Thanks to volunteers who collect monthly counts each year, these data present an excellent opportunity to explore how bird distributions can change over time. A new paper by Verónica Méndez and colleagues from the University of East Anglia and British Trust for Ornithology uses counts of 19 species (18 waders plus Shelduck, an honorary wader) over a 26-year period to ask what happens to distribution and local abundance across our estuaries when overall population sizes go up or down.

Consequences of population change for local abundance and site occupancy of wintering waterbirds Verónica Méndez, Jennifer A. Gill, José A. Alves, Niall H. K. Burton & Richard G. Davies Diversity and Distributions. 2017;1–12. DOI: 10.1111/ddi.12653

The counters

Blog Counter 2A WeBS count can be a tough assignment for a volunteer birdwatcher. Being allocated to a stretch of an estuarine coastline and asked to visit it, whatever the weather, on a given weekend of every month, in every winter, is not the same as an invitation to go birdwatching in September to look for a Curlew Sandpiper.

The counts used in this paper are from the months of November through to February, when waders are largely settled for the winter and the weather can be less than clement. Many of the data-points for individual sites have been collected by the same person in all of the years analysed in the paper and every contribution is important.

WeBS70logo6a_smallWeBS is the successor of other, similar count schemes which are celebrating 70 years of continuous monitoring this year. It is organised by BTO, in partnership with RSPB and JNCC and in association with WWT. There are over 3000 registered WeBS volunteers, collecting data about the waterbirds that can be found on estuaries, in wetlands, on inland lakes, along river valleys and in local parks and villages.

Understanding change

Many populations of migratory birds are changing in number quite rapidly at present, but are these changes more likely to result in changes in occupancy (eg colonisation of or extinction from some sites) or changes in abundance within sites? Put simply, if extra waders arrive in the autumn, how do they distribute themselves across available sites? If fewer arrive, where will the gaps be found?

DN and BTG graphs

The contrasting fortunes of Dunlin and Black-tailed Godwit over the period covered in the new paper

Over the period studied in the paper (between 1980-85 and 2002-07), populations of five species declined, with the greatest losses occurring in Purple Sandpiper and Shelduck (both declined by about 25%), while there were increases for four, the biggest of which were Avocet (+1690%), Golden Plover (+554%) and Black-tailed Godwit (+418%). No birdwatcher will be shocked by the figures for Avocet or Black-tailed Godwit, wintering numbers of which have shot up, but Golden Plover is more of a surprise and may be linked to a move from inland fields to estuaries.

When the size of a wintering population of waders declined, Verónica Méndez and her colleagues found that the main consequence was a reduction in numbers across all sites. Birds were likely to stay on a site even if local numbers were dwindling. This suggests high levels of site-fidelity by individual waders, which is something we know from ringing and tracking studies. If you’re still alive then just do the same again – there could be a better site with a higher number of conspecifics somewhere else but it would be risky to try to find that out.

 

Blog Avocets

Thirty years ago, few people can have expected that they would ever see such a large flock of Avocets on the Humber

If the size of a wintering population increased, generally numbers went up within all occupied sites. The exceptions tended to occur in species for which original numbers were very small, such as for Greenshank, or species for which the change in numbers has been rapid – as seen in Avocet and Black-tailed Godwit.

If adults don’t change their wintering sites then increases are presumably being driven by juveniles. Their settlement decisions may be influenced by the distribution of adults of the same species, resulting in increased local abundance, rather than colonisation of new sites. For the two rapidly-expanding species, Black-tailed Godwit and Avocet, there was colonisation of 25 and 15 new sites, respectively, between 1980-85 and 2002-07, with some indication of more similar increases in sites that were closer together, which may reflect local movements among groups of nearby sites.

Designating sites

Blog Blackwit

Juvenile waders may well settle in areas where there are already populations of wintering adults

One of the key conservation measures for waders across Europe is the Special Protection Area (SPA) network, a collection of sites that are designated because they hold internationally or nationally important numbers of species, measured as a percentage of the population. Designated sites need to maintain numbers of all the species that hit this threshold percentage. However, if a national or European population gets larger (for example because of high breeding success) but the number on a particular site does not grow (or grows more slowly), then the species might drop below the threshold for protection, even if the site is unchanged. Theoretically this could affect a site’s protected status for that species, although is unlikely to be a problem, as most sites are designated for many species.

This new paper shows that gains and losses tend to be fairly constant across all sites, making it unlikely that a site designation would be affected by national or European-scale changes. In only one species (Ringed Plover), have numbers declined so much in some sites that the total number of sites exceeding the threshold for that species has decreased.

Keep counting!

Blog Counter 1Habitat availability and site fidelity, along with species longevity, may explain the strong tendency for local population abundance to change much more than site occupancy, in our wintering waders. Given the statutory importance of maintaining waterbird populations in designated protected areas, it is important to continue local and national surveys that can identify changes in local abundance and relate these to large-scale processes.

Returning to the earlier question – When you count the number of Redshank on your local estuary and discover that there are fewer now than there were last year –  or five, or twenty years ago – what are the implications?

This new paper shows that it is unlikely that a local drop in a species’ numbers is caused by a redistribution of birds. Factors that might lie behind a local decline need to be investigated locally, if the trend is not replicated elsewhere. The authors could only reach this conclusion because they had 26 years of WeBS data from a range of sites at their disposal. Future generations of WeBS counters will hopefully continue to monitor the conditions of our estuaries, working together throughout the country to interpret local counts within a national framework.

Paper

Consequences of population change for local abundance and site occupancy of wintering waterbirds Verónica Méndez, Jennifer A. Gill, José A. Alves, Niall H. K. Burton & Richard G. Davies Diversity and Distributions. 2017;1–12. DOI: 10.1111/ddi.12653

Blog RINGOS


 GFA in Iceland

WaderTales blogs are written by Graham Appleton, to celebrate waders and wader research.  Many of the articles are based on previously published papers, with the aim of making wader science available to a broader audience.

@grahamfappleton

 

 

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