Humans are responsible for most of the threats to birds. Expanding and intensifying agriculture and forestry, the biggest problems, cause habitat destruction, degradation and fragmentation. Fisheries degrade the marine environment and kill seabirds through accidental bycatch. The spread of invasive alien species, pollution and over-exploitation of wild birds are also major threats. In the long term, human-induced climate change may be the most serious threat of all.
Habitat destruction and degradation are the most serious pressures on the world’s birds (). Habitat destruction renders areas entirely or almost entirely unsuitable for their original fauna and flora. Agricultural and forestry activities, and large-scale infrastructure development, can all have this effect (). Habitat degradation is much harder to define and its impacts often less easy to discern but they can undoubtedly be very important. Intensification of agriculture and fisheries, slash-and-burn crop cultivation, logging of hardwoods in tropical forests, nutrient enrichment of fresh- and saltwaters and the spread of invasive species are just some of the factors that can alter habitats, sometimes drastically, and affect their ability to support populations of particular species. Of all the ways that humans can affect habitat, fire can have some of the most far-reachng consequences. Fire is a natural phenomenon in many habitats but since pre-history humans have drastically altered the fire regime wherever they have spread, often with major impacts on local biodiversity (, ).
Fragmentation—the division of formerly contiguous areas of natural or semi-natural habitat into smaller, more or less isolated patches—affects the ability of habitats to support populations of constituent species. This is because the ecological characteristics of small fragments of a habitat are different from those of larger areas, principally owing to so-called ‘edge effects’ and to the inability of small areas to support viable populations of species that have large territories or home ranges. For these reasons, small fragments of a habitat almost invariably support fewer species than large areas (). Nevertheless, remaining fragments of natural or semi-natural habitat can serve a hugely important role in maintaining biodiversity in otherwise heavily modified areas.
Direct mortality has the greatest impact on bird populations after habitat destruction and degradation. Birds are exploited for food, their plumes and for the cage-bird trade. They are also hunted for sport, and as pests, and many are accidentally killed by pollutants, as bycatch in fishing gear or through collision with structures such as power lines and lighthouses. Introduced predators can inflict heavy mortality on populations, particularly on island species that have evolved in the absence of native predators. Invasive predators can also reduce the reproductive success of native species through predation of eggs and nestlings. Several other factors stress species’ populations, and become particularly prevalent in small populations: hybridisation, disturbance and competition are often apparent in bird populations but their impacts become more marked as the number of individuals decreases.