Colwall Bird Population Trends


The residents of Colwall having been busy collecting data on the birds seen in their gardens for over ten years. By collecting data over such a period of time it is possible to perform a population trend analysis to determine which birds have declined or increased over the last decade. This page presents the results of this population trend analysis.

The Colwall trends can be compared against the UK trends through the graphs shown on the "UK Trend" tab under "Population Trend". The UK figures have been derived from data provided on the BTO web site.

The trend analysis 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.

Given below are the five categories that have been used, and the bird allocation per category. The birds have been included in each category in alphabetical order and not in order of its population trend.

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

  • Buzzard

  • Dove - Stock

  • Finch - Bull

  • Finch - Gold

  • Magpie

  • Pheasant

  • Raven

  • Wren

Slight Upward Trend

  • Blackcap

  • Chiffchaff

  • Crow

  • Cuckoo

  • Finch - Chaffinch

  • Heron

  • Jackdaw

  • Jay

  • Kestrel

  • Martin - House

  • Owl - Tawny

  • Peregrine - Falcon

  • Redpoll

  • Rook

  • Siskin

  • Sparrowhawk

  • Swallow

  • Swift

  • Tit - Long-Tailed

  • Woodpecker - Great Spotted

  • Woodpecker - Green

Significant Downward Trend

  • Dove - Collared

  • Finch - Green

  • Starling

Slight Downward Trend

  • Goldcrest

  • Sparrow - House

  • Thrush - Mistle

  • Thrush - Song

Marginal Trend

  • Blackbird

  • Brambling

  • Dunnock

  • Fieldfare

  • Flycatcher - Spotted

  • Kingfisher

  • Kite - Red

  • Mallard

  • Moorhen

  • Nuthatch

  • Owl - Barn

  • Owl - Little

  • Partridge - Reg Legged

  • Pigeon - Wood

  • Pigeon - Feral

  • Redwing

  • Robin

  • Sparrow - Tree

  • Tit - Blue

  • Tit - Coal

  • Tit - Great

  • Tit - Marsh or Willow

  • Treecreeper

  • Wagtail - Grey

  • Wagtail - Pied

  • Warbler - Garden

  • Warbler - Willow

  • Woodpecker - Lesser Spotted

  • Yellowhammer


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