The expense of comprehensively assessing biodiversity is enormous. One estimate is that an all-taxa inventory of just one hectare of tropical forest might take 50–500 scientist years to accomplish (Lawton et al. 1998). This has led to much interest in finding proxy taxa that can act as indicators for biodiversity as a whole.

There is no perfect indicator taxon, but some are much better than others. The kind of indicator taxon that works best depends on whether the purpose is to track environmental changes, or clarify biodiversity patterns; on the scale involved; and on the kind of habitat being looked at.

Ten reasons why birds are good indicators for biodiversity

REASON 1 Bird taxonomy is well known and relatively stable: e.g., the number of recognised bird species has grown by just 5–8% per decade in recent years compared to 15–24% for mammals and amphibians.

REASON 2 Bird distribution, ecology and life history are well understood: e.g., over 16,000 scientific papers on bird biology are published per year.

REASON 3 Birds are generally easy to identify, survey and monitor, and there are valuable historical data sets for a wide range of species: e.g., birds comprise over 50% of the populations included in global wildlife trend indicators.

REASON 4 Birds are diverse, found in nearly all habitats and occur across the world: e.g., there are over 10,000 bird species globally with, on average, over 400 species occurring per country.

REASON 5 Bird habitat requirements are typically fairly specialised: e.g., more than half of all bird species predominantly occur in one or two habitat types.

REASON 6 Birds usually occupy high trophic levels in food webs and are relatively sensitive to environmental change: e.g., trends in farmland birds in the UK correlate with trends in land-use intensity and climate.

REASON 7 Bird population trends often mirror those of other species: e.g., mammals, reptiles, amphibians, plants and invertebrates have shown trends similar to farmland birds in the UK since the 1940s.

REASON 8 Bird distribution generally reflects that of many other wildlife groups: e.g., the network of key sites for bird conservation (IBAs) covers 80% of the area of those identified for other wildlife groups.

REASON 9 Birds are economically important: e.g., pest control by birds in Canada’s boreal forests is estimated to be worth Can$5.4 billion per year.

REASON 10 Birds are flagships for nature—they are popular, engage the public and resonate with decision-makers: e.g., 20% of people in the USA and 30% in the UK watch or feed birds regularly.

Birds score very highly on many of the broad criteria defined for selecting indicator taxa (Pearson 1995). Their most significant advantage is that we have, relatively speaking, so much information about them, and their biology and life-histories are so well understood. Birds are also taxonomically well-known and relatively stable, and their populations are readily surveyed. Birds are widespread, occurring almost everywhere in the world. Bird families and genera often occupy a breadth of habitats and have broad geographical ranges, yet many individual species are specialised in their requirements and have narrow distributions. Birds are mobile and responsive to environmental changes. There are enough bird species to show meaningful patterns, yet not so many as to make identification itself a significant challenge. Birds have real economic importance in their own right—a useful attribute in an indicator. However, birds are generally less specialised within micro-habitats than, say, insects or plants. Importantly, the extent to which they reflect patterns in unrelated taxonomic groups remains disputed.

The evidence so far suggests that:

• on a local scale, patterns of bird distribution may not always match well the distribution patterns of other taxa (Prendergast 1993, Pearson 1995, Lawton et al. 1998); nevertheless a network of sites selected as important for birds will capture most other biodiversity (Howard et al. 1998, Brooks et al. 2001). Birds are likely to work better as biodiversity indicator taxa in terrestrial habitats (especially well-vegetated ones) than in either freshwater or marine habitats

• on a larger scale, birds are very useful (although still imperfect) indicators of species richness and endemism patterns (Bibby et al. 1992, Burgess et al. 2002)

• changes in bird populations tend to integrate a set of ecological factors. Given adequate ecological knowledge, they can provide a useful indication of environmental change (Bennun and Fanshawe 1997, Donald et al. 2001, Gregory et al. 2003). For instance, the UK government has adopted an index based on wild bird populations as one of its 15 headline Quality of Life indicators.