Why are Welsh woodland birds declining?
3 Why are Welsh woodland birds declining?
Although research is ongoing, recent findings have shown that changes in woodland structure, due to changes in types and extent of management, may be one of the main causes behind woodland bird declines. There are several other potential factors, and it is probable that multiple factors have exerted a combined effect. The changes discussed below are summarised in Table 7.
Changes in woodland structure
One of the main factors determining the distribution and abundance of birds in woodland is woodland structure; the Repeat Woodland Bird Survey (RWBS) revealed that changes in woodland structure, due to changes in management, are the most likely drivers of many woodland bird declines.
Key issues affecting woodland structure and likely to impact on priority bird species are:
development of a closed canopy, and simplified shrub and field layers
lack of diversity of woodland age structure
changes in grazing and browsing pressure, affecting the shrub and field layers, and woodland regeneration
grey squirrel damage, affecting establishment
abundance and extent of clearings, glades and rides
availability of wet features
insufficient abundance of deadwood
woodland edge management.
Agricultural practices can impact on both the woodland itself, eg grazing, and on habitats outside the woodland used by woodland birds, eg hedgerows and scrub.
Woodland edge can be strongly modified by agricultural chemical drift, trimming of wood edges, heavy grazing by livestock and cultivation up to the woodland edge, all of which can affect the food and nesting resources available to woodland birds.
Farming practices on land adjacent to woodland can potentially limit opportunities for some woodland edge species. For example, turtle dove are likely to be affected by a loss of arable habitats. Land drainage can reduce the water table in some woods thus affecting foraging opportunities for species such as woodcock, which require damp earth in which to probe for invertebrates. The loss
of scrub, hedgerows and hedgerow trees could help to explain the decline of species, eg spotted flycatcher, bullfinch. Woodlands connected to dense hedges with trees are utilised by more species than those connected to hedges without trees. This may reflect the use of hedges as movement corridors as much as a requirement for a combination of woodland and hedgerow habitats.
The expansion of agricultural activity and urban and industrial development has resulted in the isolation and fragmentation of woodlands. The dispersal behaviour of birds is relevant to understanding the effects this has on bird communities; marsh tits are highly sedentary and have been shown to have a negative correlation with woodland fragmentation, while lesser spotted woodpeckers have large home ranges suggesting that landscape-scale changes in tree abundance may be significant.