Blog by Simon Wotton, Senior Conservation Scientist, RSPB Centre for Conservation Science

A paper just published in British Birds provides updated estimates of population size for wintering waterbirds in Great Britain, since the previous assessment in 2011.  The paper uses recent data from the Wetland Bird Survey (WeBS), the Goose and Swan Monitoring Programme (GSMP), the Non-estuarine Waterbird Survey (NEWS III), the Winter Gull Roost Survey (WinGS), county bird reports and other sources, and new analytical approaches for some species that use smaller inland waterbodies or the non-estuarine coast.

Overwinter population estimates are presented for 98 species or populations, including seven non-native populations and, for the first time, estimates for Cattle Egret Bubulcus ibis, Glossy Ibis Plegadis falcinellus and Sandwich Tern Thalasseus sandvicensis

For most species, estimates are the mean peak wintering population during the period 2012/13 to 2016/17, although for some scarce species, mean occurrence for 2011/12 to 2014/15 was generally used, as data were extracted from county bird reports. For those species where the population is changing rapidly and there is considered to be accurate data, the estimate is based on the most recent year or two years only.

The estimated total number of wintering waterbirds in Great Britain is 12.8 million, including 4.9 million waders, 3.8 million gulls, 2.1 million ducks, 1.1 million geese, 500,000 rails, 170,000 cormorants, 70,000 swans, 60,000 herons, 30,000 divers and 30,000 grebes. For those species where most of the change in population estimate can be attributed to genuine change in population size, there has been a gain of approximately 175,000 geese and a loss of 142,000 waders since the 2011 assessment. The increased goose total is driven largely by Pink-footed Goose Anser brachyrhynchus (up by 150,000 to 510,000). The key species involved in the decrease in wader totals were Red Knot Calidris canutus (down by 60,000 to 260,000), Oystercatcher Haematopus ostralegus (down by 30,000 to 290,000), Common Redshank Tringa totanus (down by 26,000 to 94,000) and Eurasian Curlew Numenius arquata (down by 20,000 to 120,000).

Photo: In this latest assesment there is a gain of approximately 175,000 geese and this increased goose number is driven largely by Pink-footed Goose Anser brachyrhynchus (up by 150,000 to 510,000) - Photo by Andy Hay (rspb-images.com)

One group where change is particularly noticeable since the 2011 assessment is the herons. Little Egret Egretta garzetta increased from 4,500 to 11,000. Cattle Egret, a recent coloniser and not included in the 2011 assessment, has increased rapidly in recent years and the estimate of 65 individuals was based on data from 2016/17 only. The estimate for Eurasian Bittern Botaurus stellaris also increased, by 200 to 800, but the Grey Heron Ardea cinerea estimate, which is based on the Heronries Census, decreased by 16,000 to 45,000.

Photo: Herons is the one group where change is particularly noticeable since the 2011 assessment - Photo by Paul Chesterfield (rspb-images.com)

These population estimates provide crucial baseline information that underpins the implementation of international conservation obligations, including the identification of wetlands of national importance.

WeBS is run by the British Trust for Ornithology (BTO), in a partnership between the BTO, RSPB and the Joint Nature Conservation Committee (JNCC), in association with the Wildfowl & Wetlands Trust (WWT).  NEWS III and WiNGS are periodic surveys organised as part of WeBS.  GSMP is organised and funded by WWT, JNCC and Scottish Natural Heritage (SNH).

For more information contact Simon Wotton, Senior Conservation Scientist, or Chiara Ceci, Science Communications Executive, or follow us on Twitter @RSPBScience #WeBS #wetlandbirds

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