wide range of invertebrates. Reduction in invertebrates within woodland may also have been brought about by a number of factors including climate change, deterioration in tree health, drainage, reduced availability of deadwood and impacts of adjacent agricultural management.
The potential effects of human disturbance on birds depend on the type of activity, the habitat and the sensitivity of the species.
Many birds will tolerate frequent background levels of human presence, eg walkers, cyclists or horse riders, and although recreational use of the countryside has increased enormously over the last 50 years, it is unlikely that this has been responsible for recent bird declines. However, local impacts may be significant and there is growing evidence that recreation can affect the composition and distribution of bird communities and bird behaviour, eg uncontrolled dogs can cause significant disturbance to ground- nesting birds. This potentially reduces territory quality in moderately or highly disturbed areas and can reduce breeding success. Furthermore, badly-timed events, eg car rallies, can cause serious disturbance to some rare breeding birds.
Predation is the largest cause of nest failure in most birds (Fuller 2005) and in some cases, it may be sufficient to suppress or reduce the breeding population. Several potential predators of woodland birds have increased over the last 30 years. The main problem in determining whether this may have contributed to bird declines is that, even if the level of predation is known, the impacts at the population level are often difficult to predict.
The grey squirrel has increased in both population density and range, and is almost ubiquitous in Wales. It has been suggested that it may be a major predator
Why are Welsh woodland birds declining?
of nesting songbirds in Britain (Hewson et al. 2004), and although ground-nesters and hole-nesters are potentially vulnerable, canopy nesters are likely to be most at risk.
The most significant avian predators in woodland are woodpeckers, corvids, owls and raptors. Great spotted woodpeckers are major predators of tit nests, and the most significant nest predator of lesser spotted woodpeckers. Some studies have suggested that nest predation is higher along the woodland edge than in the woodland interior, particularly by corvids. This may be worse where woodland meets farmland. Generally, there is little evidence of clear links between woodland songbird numbers and the abundance of avian predators, although it is possible that the effect of predators on declining bird populations may be more pronounced when these populations are already under pressure from other factors.
Interspecific competition may limit populations of some species when resources are in short supply, eg competition for nest sites and food supplies can affect local abundance and distribution of woodland birds. Marsh tits may be out-competed by the much commoner blue and great tits for nestholes, but equally have been known to usurp nest sites of willow tits. The decline of the lesser spotted woodpecker has occurred at the same time as an increase in the great spotted woodpecker, which competes for nest sites and deadwood invertebrates.
Migration and winter conditions in wintering areas
Some species considered here are long- distance migrants wintering in sub- Saharan Africa, eg wood warbler, pied flycatcher and tree pipit and it is possible that their declines may be driven by changing conditions within their winter ranges or on migration, eg climate, habitat