Mammal monitoring

To begin, select from the options above to display a graph.

Mammal recording was introduced to the Breeding Bird Survey in 1995 with a view to helping improve our knowledge of the distribution and population trends of some of our commoner mammals.

Compared with birds, the population trends of mammals are relatively poorly known. Even though mammal recording has always been a voluntary addition to the scheme, around 90% of BBS squares now hold mammal data.

Mammal trends to 2023

BBS count data are used to calculate population trends for nine relatively widespread mammal species, shown below. These trends cover the period 1995–2023.

In 2023: 

  • Mammal data were recorded on 88% of the 3,931 BBS squares surveyed, including zero counts.
  • Forty-four species of mammal were recorded in one form or another during the 2023 field season. These might be observations of live animals (counts), field signs, roadkill/dead animals, or local knowledge.

Of the nine mammals for which trends can be produced from BBS counts, six have increased significantly in the UK as a whole since 1995: 

  • Brown Hare (38%)
  • Grey Squirrel (35%)
  • Reeves’ Muntjac (284%)
  • Fallow Deer (245%)
  • Red Deer (133%) 
  • Roe Deer (129%) 

With the UK Woodland Bird Index having decreased significantly in the last five years (37%), and the known association between bird declines and deer from BTO research, the increases in the UK’s deer populations are additional cause for concern.

In the five-year period between 2017 and 2022 alone, the UK Fallow Deer population has more than doubled, and Red Deer and Muntjac populations have grown by three-quarters and two-thirds respectively.

It should be noted that trends for herding species (Red Deer and Fallow Deer) should be treated with some caution. This is because the presence or absence of a herd in a given BBS visit could heavily influence the overall trend for that species.

Three mammals have declined significantly across the UK: 

  • Rabbit (68%)
  • Mountain/Irish Hare (67%) 
  • Red Fox (49%)

In the case of Mountain Hare, 2023 represents the fifth consecutive year in which the unsmoothed index has declined.

Brown Hare, by contrast, continue to increase following a period of relative stability from 1995 to 2019. There is even a new 10-year trend available for Brown Hare in Wales, although this species does not appear to be showing the same recent increase in Wales as it is in England.

  • More information on the mammals recorded during the 2023 BBS surveys can be seen in the latest Breeding Bird Survey report (pages 28-29).

The information on species detected more often by signs of their presence than by sightings (e.g. Hedgehog, Mole and Badger) can also be used to estimate trends, although these require more careful interpretation.

Download the results from the latest mammal trends

Comparison of BBS mammal trends with the National Gamebag Census

In 2011 the JNCC funded work to compare BBS mammal trends between 1995 and 2009 with another annual scheme: the National Gamebag Census (NGC), carried out by the Game and Wildlife Conservation Trust. The NGC is a voluntary scheme that collects bag statistics from shooting estates, on average about 650 per year. The aim of the project was to produce an overview of trends in abundance and distribution. 

Of nine species tested, none differed significantly in their trends between the two schemes. For four species where BBS indicated significant increases between 1995 and 2009, the NCG trend was either not significant (Red Deer, Roe Deer and Reeves’ Muntjac) or also showed a significant increase (Grey Squirrel). Rabbit showed a significant decline in BBS whereas NGC found no significant change.

This work demonstrated the feasibility of producing joint BBS-NGC trends for assessing population change for statutory purposes where a single figure is needed. Results of the spatial mapping were also useful, in showing areas where species are most often detected and where the most marked changes had occurred. However, due to differences in sampling design and methods, the recommendation is to routinely report temporal and spatial results from the two schemes separately.

Related content