{"id":"water-and-wetland-birds","slug":"water-and-wetland-birds","name":"Water and Wetland Bird Index Dataset","description":"Annual abundance indices for eight groups of water and wetland birds in the UK and England, from 1975: the headline water and wetland index, four habitat groups covering fast flowing water, slow and standing water, wet meadow and reedbed, and three wintering groups from the Wetland Bird Survey. Each index is set to 100 in its first year and describes change from there — none of them counts birds.","methodology":"Collected from Defra’s official wild bird population statistics, published as a single OpenDocument spreadsheet under the Open Government Licence. The unsmoothed annual index is held for each group; the smoothed trend line published alongside is not, because it is a model with the year-to-year variation removed and it carries values for 2001 and 2020, when no survey took place. Tables are located by their published titles rather than by column position, so a table inserted upstream cannot shift a series into the wrong metric. Underlying schemes are the Breeding Bird Survey and the Common Birds Census for breeding groups and the Wetland Bird Survey for wintering groups.","domains":["ecosystem"],"metricIds":["bird-index-water-and-wetland","bird-index-fast-flowing-water","bird-index-slow-standing-water","bird-index-wet-meadow","bird-index-reedbed","bird-index-wintering-waterbirds","bird-index-wintering-wildfowl","bird-index-wintering-waders"],"subjectScope":"United Kingdom and England","cadence":"annual","version":"1.0.0","released":"2026-08-28","changelog":[{"version":"1.0.0","date":"2026-08-28","summary":"First release.","additions":["Eight water and wetland bird indices for the UK and England, 1975 onwards."]}],"limitations":["An index is not a count and cannot be turned into one. Each series is 100 in its own first year and describes proportional change from that point; no population size can be recovered from it.","Levels are not comparable between indices. Wintering waterbirds stood at 195 in 2023 and water and wetland birds at 87 in 2024, which means one group roughly doubled from its 1975 level while the other fell — not that there are more of one than the other.","Each index is a composite of several species and can sit flat while some members collapse and others increase. The headline water and wetland index covers 26 species.","No value exists for 2001 or 2020. Foot-and-mouth disease closed the countryside in the first and the pandemic cancelled fieldwork in the second, so the surveys did not happen and the gaps are left as gaps. The publisher’s smoothed series does carry numbers for both years.","The published confidence intervals are not carried, because a Hydrionis series point holds a date and a value and has nowhere to put uncertainty.","Breeding indices are calendar years; the wintering indices run July to June and are labelled by the year they begin, so aligning the two, or either to a calendar-year hydrological record, is off by up to half a year.","Coverage is uneven across the UK. These are volunteer schemes, and upland and remote areas are less well surveyed than lowland England, so the national index is weighted rather than evenly observed.","The Wetland Bird Survey’s site-level counts are not held. They are available only through a data request service and are not openly licensed, so nothing here can be resolved to a particular wetland."],"context":{"hydrionisClass":"water-dependent-system","biologicalBasis":"abundance-index","whyHeld":"Birds are the most visible measure of whether a wetland is working, and these groups are defined by the kind of water they need: fast rivers, standing water, wet grassland, reed swamp. Held so that a change in a hydrological record can be read beside what happened to the species depending on it. None of it measures water, and an index is not a bird count.","waterRelevance":["Condition of freshwater and wetland habitat, as reflected in the species depending on it","Distinct dependencies: flowing water, standing water, wet grassland and reedbed each have their own index","The UK’s internationally important wintering waterbird populations","Long-run ecological context for drought, flood and wetland loss"],"suitableUses":["Tracking the direction and pace of change in a water-dependent bird group since 1975","Comparing the shapes of two indices over the same period","Showing an index beside a hydrological record on a shared time axis, allowing for the survey-year offset","Contrasting habitat groups to see which kind of water-dependence moved"],"unsuitableUses":["Reading an index as a number of birds, or converting it to a population","Comparing the levels of two indices as though the higher one held more birds","Filling 2001 or 2020, in which no survey took place","Attributing a change in an index to drought, flood or river condition because the two moved together; land use, climate, predation and conditions on distant breeding grounds all move these numbers, and for wintering birds much of the cause lies outside the UK entirely","Locating any of it at a particular wetland, river or reservoir; these are national indices","Treating a flat composite index as evidence that its member species were stable"],"joinDimensions":[{"kind":"same-administrative-geography","on":"The United Kingdom and England as whole countries","limitation":"A national index joins to national figures and to nothing finer. It cannot be attributed to a catchment, a wetland or a site, and the underlying site counts that could be are not openly licensed."},{"kind":"time-only","on":"Annual series for the same years","limitation":"Breeding indices are calendar years and wintering indices run July to June labelled by their start year, so the two are offset from each other and from any calendar-year record by up to half a year. Bird populations also respond to conditions over several previous years, so a same-year alignment compares a slow response with a fast one."}],"relationshipConfidence":"contextual","eligibility":{"api":true,"download":true,"overlay":true,"publicPage":false}},"observationCount":768,"seriesCount":16,"subjectCount":2,"from":1975,"to":2024,"sourceIds":["uk-defra-wild-birds"],"units":["index"],"redistributable":true,"commerciallyReusable":true,"citation":"Hydrionis (2026), Water and Wetland Bird Index Dataset, version 1.0.0. Retrieved from https://hydrionis.com/data/datasets/water-and-wetland-birds/","downloads":[{"file":"/downloads/observations-ecosystem-uk-defra-wild-birds.csv","format":"CSV","label":"ecosystem observations from Defra (832 rows)"}],"apiPath":"/api/v1/datasets/water-and-wetland-birds.json","lastFetched":"2026-08-28","fields":[{"id":"bird-index-water-and-wetland","name":"Water and wetland birds abundance index","definition":"Abundance index for 26 species of water and wetland birds in the UK, relative to 1975 set to 100.","unit":"index","nativeGeography":["country"],"comparabilityNotes":"An index, not a count and not a population. Each series is set to 100 in its own first year and describes change from that point, so it says how the group moved and never how many birds there are. Levels are not comparable between indices: each is based to its own start year, so one index standing higher than another means it grew more since 1975, not that it contains more birds. Shapes may be compared; heights may not. A composite of several species. It can stay level while some members collapse and others increase, so a flat index is not evidence that nothing changed. The unsmoothed index, as published. Defra also publishes a smoothed trend line with confidence intervals; Hydrionis holds neither, because a smoothed line is a model with the year-to-year variation removed and that variation is where a drought or a hard winter shows. Two years are absent and are meant to be: no value is published for 2001, when foot-and-mouth disease closed the countryside, or for 2020, when the pandemic cancelled fieldwork. The surveys did not happen, so there is nothing to record, and the gaps must not be filled. The publisher’s smoothed series does carry values for both years, which is one reason Hydrionis holds the unsmoothed one. The published confidence interval is not carried — a series point holds a date and a value with nowhere to put uncertainty — so single years should be read as indicative and the trend as the signal. Built from volunteer schemes with fixed sites and repeat visits, weighted for coverage by the publisher. Site coverage is not uniform across the UK, and upland and remote areas are less well covered than lowland England. A calendar-year breeding-season index, so it reflects the conditions of that spring and the winter before it."},{"id":"bird-index-fast-flowing-water","name":"Birds of fast flowing water abundance index","definition":"Abundance index for 4 species of birds of fast flowing water in the UK, relative to 1975 set to 100.","unit":"index","nativeGeography":["country"],"comparabilityNotes":"An index, not a count and not a population. Each series is set to 100 in its own first year and describes change from that point, so it says how the group moved and never how many birds there are. Levels are not comparable between indices: each is based to its own start year, so one index standing higher than another means it grew more since 1975, not that it contains more birds. Shapes may be compared; heights may not. A composite of several species. It can stay level while some members collapse and others increase, so a flat index is not evidence that nothing changed. The unsmoothed index, as published. Defra also publishes a smoothed trend line with confidence intervals; Hydrionis holds neither, because a smoothed line is a model with the year-to-year variation removed and that variation is where a drought or a hard winter shows. Two years are absent and are meant to be: no value is published for 2001, when foot-and-mouth disease closed the countryside, or for 2020, when the pandemic cancelled fieldwork. The surveys did not happen, so there is nothing to record, and the gaps must not be filled. The publisher’s smoothed series does carry values for both years, which is one reason Hydrionis holds the unsmoothed one. The published confidence interval is not carried — a series point holds a date and a value with nowhere to put uncertainty — so single years should be read as indicative and the trend as the signal. Built from volunteer schemes with fixed sites and repeat visits, weighted for coverage by the publisher. Site coverage is not uniform across the UK, and upland and remote areas are less well covered than lowland England. A calendar-year breeding-season index, so it reflects the conditions of that spring and the winter before it."},{"id":"bird-index-slow-standing-water","name":"Birds of slow and standing water abundance index","definition":"Abundance index for 6 species of birds of slow and standing water in the UK, relative to 1975 set to 100.","unit":"index","nativeGeography":["country"],"comparabilityNotes":"An index, not a count and not a population. Each series is set to 100 in its own first year and describes change from that point, so it says how the group moved and never how many birds there are. Levels are not comparable between indices: each is based to its own start year, so one index standing higher than another means it grew more since 1975, not that it contains more birds. Shapes may be compared; heights may not. A composite of several species. It can stay level while some members collapse and others increase, so a flat index is not evidence that nothing changed. The unsmoothed index, as published. Defra also publishes a smoothed trend line with confidence intervals; Hydrionis holds neither, because a smoothed line is a model with the year-to-year variation removed and that variation is where a drought or a hard winter shows. Two years are absent and are meant to be: no value is published for 2001, when foot-and-mouth disease closed the countryside, or for 2020, when the pandemic cancelled fieldwork. The surveys did not happen, so there is nothing to record, and the gaps must not be filled. The publisher’s smoothed series does carry values for both years, which is one reason Hydrionis holds the unsmoothed one. The published confidence interval is not carried — a series point holds a date and a value with nowhere to put uncertainty — so single years should be read as indicative and the trend as the signal. Built from volunteer schemes with fixed sites and repeat visits, weighted for coverage by the publisher. Site coverage is not uniform across the UK, and upland and remote areas are less well covered than lowland England. A calendar-year breeding-season index, so it reflects the conditions of that spring and the winter before it."},{"id":"bird-index-wet-meadow","name":"Wet meadow birds abundance index","definition":"Abundance index for 8 species of wet meadow birds in the UK, relative to 1975 set to 100.","unit":"index","nativeGeography":["country"],"comparabilityNotes":"An index, not a count and not a population. Each series is set to 100 in its own first year and describes change from that point, so it says how the group moved and never how many birds there are. Levels are not comparable between indices: each is based to its own start year, so one index standing higher than another means it grew more since 1975, not that it contains more birds. Shapes may be compared; heights may not. A composite of several species. It can stay level while some members collapse and others increase, so a flat index is not evidence that nothing changed. The unsmoothed index, as published. Defra also publishes a smoothed trend line with confidence intervals; Hydrionis holds neither, because a smoothed line is a model with the year-to-year variation removed and that variation is where a drought or a hard winter shows. Two years are absent and are meant to be: no value is published for 2001, when foot-and-mouth disease closed the countryside, or for 2020, when the pandemic cancelled fieldwork. The surveys did not happen, so there is nothing to record, and the gaps must not be filled. The publisher’s smoothed series does carry values for both years, which is one reason Hydrionis holds the unsmoothed one. The published confidence interval is not carried — a series point holds a date and a value with nowhere to put uncertainty — so single years should be read as indicative and the trend as the signal. Built from volunteer schemes with fixed sites and repeat visits, weighted for coverage by the publisher. Site coverage is not uniform across the UK, and upland and remote areas are less well covered than lowland England. A calendar-year breeding-season index, so it reflects the conditions of that spring and the winter before it."},{"id":"bird-index-reedbed","name":"Reedbed birds abundance index","definition":"Abundance index for 4 species of reedbed birds in the UK, relative to 1975 set to 100.","unit":"index","nativeGeography":["country"],"comparabilityNotes":"An index, not a count and not a population. Each series is set to 100 in its own first year and describes change from that point, so it says how the group moved and never how many birds there are. Levels are not comparable between indices: each is based to its own start year, so one index standing higher than another means it grew more since 1975, not that it contains more birds. Shapes may be compared; heights may not. A composite of several species. It can stay level while some members collapse and others increase, so a flat index is not evidence that nothing changed. The unsmoothed index, as published. Defra also publishes a smoothed trend line with confidence intervals; Hydrionis holds neither, because a smoothed line is a model with the year-to-year variation removed and that variation is where a drought or a hard winter shows. Two years are absent and are meant to be: no value is published for 2001, when foot-and-mouth disease closed the countryside, or for 2020, when the pandemic cancelled fieldwork. The surveys did not happen, so there is nothing to record, and the gaps must not be filled. The publisher’s smoothed series does carry values for both years, which is one reason Hydrionis holds the unsmoothed one. The published confidence interval is not carried — a series point holds a date and a value with nowhere to put uncertainty — so single years should be read as indicative and the trend as the signal. Built from volunteer schemes with fixed sites and repeat visits, weighted for coverage by the publisher. Site coverage is not uniform across the UK, and upland and remote areas are less well covered than lowland England. A calendar-year breeding-season index, so it reflects the conditions of that spring and the winter before it."},{"id":"bird-index-wintering-waterbirds","name":"Wintering waterbirds abundance index","definition":"Abundance index for 46 species and races of wintering waterbirds in the UK, relative to 1975/76 set to 100.","unit":"index","nativeGeography":["country"],"comparabilityNotes":"An index, not a count and not a population. Each series is set to 100 in its own first year and describes change from that point, so it says how the group moved and never how many birds there are. Levels are not comparable between indices: each is based to its own start year, so one index standing higher than another means it grew more since 1975, not that it contains more birds. Shapes may be compared; heights may not. A composite of several species. It can stay level while some members collapse and others increase, so a flat index is not evidence that nothing changed. The unsmoothed index, as published. Defra also publishes a smoothed trend line with confidence intervals; Hydrionis holds neither, because a smoothed line is a model with the year-to-year variation removed and that variation is where a drought or a hard winter shows. Two years are absent and are meant to be: no value is published for 2001, when foot-and-mouth disease closed the countryside, or for 2020, when the pandemic cancelled fieldwork. The surveys did not happen, so there is nothing to record, and the gaps must not be filled. The publisher’s smoothed series does carry values for both years, which is one reason Hydrionis holds the unsmoothed one. The published confidence interval is not carried — a series point holds a date and a value with nowhere to put uncertainty — so single years should be read as indicative and the trend as the signal. Built from volunteer schemes with fixed sites and repeat visits, weighted for coverage by the publisher. Site coverage is not uniform across the UK, and upland and remote areas are less well covered than lowland England. Runs on a survey year from July to June, labelled by the year it begins, so the 1975 point is the winter of 1975–76 and aligning it to a calendar year is off by up to half a year."},{"id":"bird-index-wintering-wildfowl","name":"Wintering wildfowl abundance index","definition":"Abundance index for 31 species and races of wintering wildfowl in the UK, relative to 1975/76 set to 100.","unit":"index","nativeGeography":["country"],"comparabilityNotes":"An index, not a count and not a population. Each series is set to 100 in its own first year and describes change from that point, so it says how the group moved and never how many birds there are. Levels are not comparable between indices: each is based to its own start year, so one index standing higher than another means it grew more since 1975, not that it contains more birds. Shapes may be compared; heights may not. A composite of several species. It can stay level while some members collapse and others increase, so a flat index is not evidence that nothing changed. The unsmoothed index, as published. Defra also publishes a smoothed trend line with confidence intervals; Hydrionis holds neither, because a smoothed line is a model with the year-to-year variation removed and that variation is where a drought or a hard winter shows. Two years are absent and are meant to be: no value is published for 2001, when foot-and-mouth disease closed the countryside, or for 2020, when the pandemic cancelled fieldwork. The surveys did not happen, so there is nothing to record, and the gaps must not be filled. The publisher’s smoothed series does carry values for both years, which is one reason Hydrionis holds the unsmoothed one. The published confidence interval is not carried — a series point holds a date and a value with nowhere to put uncertainty — so single years should be read as indicative and the trend as the signal. Built from volunteer schemes with fixed sites and repeat visits, weighted for coverage by the publisher. Site coverage is not uniform across the UK, and upland and remote areas are less well covered than lowland England. Runs on a survey year from July to June, labelled by the year it begins, so the 1975 point is the winter of 1975–76 and aligning it to a calendar year is off by up to half a year."},{"id":"bird-index-wintering-waders","name":"Wintering wading birds abundance index","definition":"Abundance index for 15 species and races of wintering wading birds in the UK, relative to 1975/76 set to 100.","unit":"index","nativeGeography":["country"],"comparabilityNotes":"An index, not a count and not a population. Each series is set to 100 in its own first year and describes change from that point, so it says how the group moved and never how many birds there are. Levels are not comparable between indices: each is based to its own start year, so one index standing higher than another means it grew more since 1975, not that it contains more birds. Shapes may be compared; heights may not. A composite of several species. It can stay level while some members collapse and others increase, so a flat index is not evidence that nothing changed. The unsmoothed index, as published. Defra also publishes a smoothed trend line with confidence intervals; Hydrionis holds neither, because a smoothed line is a model with the year-to-year variation removed and that variation is where a drought or a hard winter shows. Two years are absent and are meant to be: no value is published for 2001, when foot-and-mouth disease closed the countryside, or for 2020, when the pandemic cancelled fieldwork. The surveys did not happen, so there is nothing to record, and the gaps must not be filled. The publisher’s smoothed series does carry values for both years, which is one reason Hydrionis holds the unsmoothed one. The published confidence interval is not carried — a series point holds a date and a value with nowhere to put uncertainty — so single years should be read as indicative and the trend as the signal. Built from volunteer schemes with fixed sites and repeat visits, weighted for coverage by the publisher. Site coverage is not uniform across the UK, and upland and remote areas are less well covered than lowland England. Runs on a survey year from July to June, labelled by the year it begins, so the 1975 point is the winter of 1975–76 and aligning it to a calendar year is off by up to half a year."}]}