UK Bird Population Trends


The British Trust for Ornithology (BTO) has been collecting data on the number of birds which visit gardens up and down the country for many years. This data is freely available through the BTO Web Site.

In order to provide a comparison between what the residents of Colwall have observed and what the bird watchers in the country as a whole have observed this page presents, in the same graphical manner as the Colwall observations, the trend figures for the BTO observations over the last decade.

As for the Colwall analysis, the analysis of the BTO data starts by examining the percentages of gardens reporting a bird during the different years, and then uses a five-year moving average calculation. A moving average is a technique used with time series data to smooth out short-term fluctuations and highlight longer-term trends or cycles. A five-year moving average uses the last five years results to calculate an average position for the current year. Applied to the data we have this results is a trend-line covering the last nine years.

The results of the population trend analysis have been grouped into five categories looking at the trend over the last few years: significant upward trend, slight upward trend, significant downward trend, slight downward trend, or marginal trend. Each bird is then assigned to one of these five categories. The assigning to a category is not an exact science but is based on the gradient of the trend line. What is shown on this page is one view of the categorisation of the trend, but others may have a different view.

For a specific bird, to see the graph of the population trend place your cursor over the name of the bird for which you wish to see the graph. This will then bring up a pop-up window showing a graph of the percentage of gardens reporting a bird per year over the last decade. The exact percentage of reported sightings per garden is given as a point on the graph. The trend (as determined through the least squares fit) is shown as a solid line. It is the gradient of this solid line which has defined the category that the bird is placed in.

Significant Upward Trend

  • Goldfinch

  • Jackdaw

  • Pigeon - Wood

Slight Upward Trend

  • Finch - Bull

  • Magpie

  • Nuthatch

  • Pigeon - Feral

  • Tit - Long - Tailed

  • Woodpecker - Great - Spotted

Significant Downward Trend

  • Dove - Collared

  • Finch - Green

  • Sparrow - House

  • Starling

  • Thrush - Song

Slight Downward Trend

  • Finch - Chaffinch

  • Thrush-Mistle

  • Wagtail - Pied

  • Wren

Marginal Trend

  • Blackbird

  • Blackcap

  • Black - Headed - Gull

  • Brambling

  • Crow

  • Dunnock

  • Fieldfare

  • Goldcrest

  • Jay

  • Owl - Tawny

  • Redwing

  • Reed - Bunting

  • Robin

  • Rook

  • Siskin

  • Sparrowhawk

  • Sparrow - Tree

  • Tit-Blue

  • Tit - Coal

  • Tit - Great

  • Tit - Marsh or Willow

  • Treecreeper


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