Why are woodland birds declining?

First published in BTO News 253 (July-August 2004) by Rob Fuller

The BTO has recently completed a review of the possible factors behind recent declines in woodland birds.This article outlines the main conclusions.

Until recently, woodland bird populations rarely made conservation headlines in Britain. A clear exception is the Capercaillie, which maintains a tenuous existence in the Scottish pine woods. It is now clear, however, that several other birds of woodland and scrub are not faring well. The Common Birds Census (CBC) shows that a diverse collection of species has declined substantially since the late 1960s. Figure 1 shows those species that have declined by more than 30% on woodland CBC plots between 1966 and 1999. These declines have generally been accompanied by contractions in range between 1970 and 1990, as shown by the BTO’s two breeding bird atlases. In addition to the species shown in Figure 1, other sources of data indicate declines in Capercaillie, Nightingale, Wood Warbler and Hawfinch. The last revision of Birds of Conservation Concern in 2002 added several species of woodland birds to the Red and Amber lists.

Unlike the farmland bird declines that were mainly a feature of the mid 1970s to mid 1980s, there is great variation in the timing of the woodland declines. Some have been steadily declining since the 1960s but some started more recently. Furthermore, the species concerned differ enormously in ecology and behaviour. They include migrants and sedentary species, habitat specialists and habitat generalists, open nesters and hole nesters, foliage insectivores and ground insectivores. The lack of a common theme suggests that there are no simple answers. Possible causes of the population changes were reviewed to help focus research into the most likely areas. Four broad types of factors were identified: (i) Large-scale environmental change, (ii) Large-scale trends in land management, (iii) Fine-scale trends in habitat quality, (iv) Interactions with other species.

(i) Large-scale environmental change

Two factors are likely to be highly relevant. First, changing climate and habitat conditions in Africa south of the Sahara are probably significant in the declines of several long-distance migrants, for example Willow Warbler and Spotted Flycatcher. There is a major gap in knowledge, however, about exactly where some of our populations of migrant birds spend the winter. This is important because conditions in Africa vary considerably in space and time. Relatively small differences in winter distribution, between species and between different breeding populations of the same species, could translate into large differences in overwinter survival.

Second, climate change on the breeding grounds may be affecting some species. Are birds finding it difficult to adjust their breeding seasons to keep pace with the changing phenology of their invertebrate food supplies? This could be a particular problem for migrant birds whose migration patterns and arrival on the breeding grounds are driven by internal rhythms that are not readily altered. Climate change could also benefit species that are competitors or predators of declining species. Food supply of some birds may also be affected, for example through the drying out of woodland. Indeed, widespread reduction of woodland invertebrates may have occurred, due to a combination of changing climate, diffuse pollution and surrounding land management. Unfortunately there has been no long-term monitoring of most groups of woodland invertebrates and it is possible that large changes in availability of invertebrates for birds have gone undetected.

Changes in tree health have been evident in recent years. Of particular interest is the deterioration of crown density in oak, which became apparent in the early 1990s. Oaks provide rich feeding sites for several species of insectivorous birds, although two of the species that forage most commonly in oak canopies, Blue Tit and Chaffinch, are not showing any signs of decline. Tree condition seems unlikely to be a driver of the declines in woodland birds, but the ecological effects of reduction of crown density in oaks would be worth looking at more closely.

(ii) Large-scale trends in land-use and habitat management

During the 20th century there were huge shifts in the overall tree species composition of British woodlands. This was dominated by the planting of non-native conifers, most strikingly by Sitka Spruce in upland Scotland. Furthermore, since 1960 the amount of oak planting has halved. In the immediate decades following World War II, considerable areas of ancient woodland in the lowlands were either replanted with conifers or cleared for agriculture. These changes are unlikely, however, to be driving the current declines in woodland birds for several reasons. Most of the changes in tree species composition relate to the establishment of new forest. It is revealing that large declines have been evident within many woods that have undergone little change in management approach for decades. It appears that the problem cannot be related in any simple way to changes in woodland management or to habitat loss. Nonetheless, the
increased coniferisation and fragmentation of lowland woodland over the last 50 years may have reduced the carrying capacity of some landscapes for two broadleaved woodland specialists with relatively weak dispersal ability – Marsh Tit and Nuthatch. Any such effects must be purely local, however, because Nuthatch is increasing strongly at a national scale.

Intensification of silvicultural practices has not been evident in recent years. In fact, forest policy and management in Britain have generally become more sympathetic to wildlife. Nevertheless, two management trends may have contributed to some of the observed declines. The first is the gradual maturation of many of the forests planted during the last century. The extent of young conifer forest has decreased in some regions leading to less habitat for species such as Lesser Redpoll and Tree Pipit. The second concerns the neglect of substantial areas of lowland broadleaves. This has been caused by the collapse in traditional markets for coppice products and by the deteriorating economics of timber production. Canopy closure and the loss of open space such as rides has reduced the quantity of field and shrub layer upon which many bird species depend. Increased shading also reduces plant diversity in the low vegetation, effectively reducing the year-round food supply for species such as Marsh and Willow Tits.

Woodland birds are not insulated from changes in the countryside surrounding the woods in which they live. The intensification of farming has potentially been highly significant for birds that spend most of their lives within woods. Although extremely little is known about how birds use the resources available to them at a landscape scale, it is clear that some woodland birds use resources from a combination of habitats, especially outside the breeding season. For instance, loss of hedgerows and reduction in their quality as bird habitats may be important in the period when juveniles are dispersing. The whole pattern of land use has changed profoundly around many woods and this may have affected food availability for birds that frequently feed outside woods, or at the woodland margin e.g. Song Thrush and Bullfinch. Woodland edges are critical habitats for many birds. The food and habitat resources they provide are potentially vulnerable to external pressures including trimming, grazing by livestock, cultivation, drainage and spray drift. There is a need to understand how agriculture interacts with the quality of woodland edges.

(iii) Habitat quality within woodland

Increasing numbers of deer in parts of Britain are causing some of the most profound changes in the ecology of our woodlands for many decades. For example, in East Anglia Red, Fallow, Roe and Reeve’s Muntjac Deer have all increased over the last 20 years, with widespread impacts on woodland regeneration and structure. The general effects of intensified grazing and browsing include a reduction in density and height of woody stems which, coupled with the removal of other low palatable vegetation, leads to an open understorey. As many species of woodland birds depend on the low vegetation for nest sites or food, the effects of high densities of deer are potentially serious. It is likely that deer have contributed to local declines in some species including Nightingale, Dunnock, Song Thrush, Willow Warbler, Marsh Tit, Willow Tit and Bullfinch.
The effects of deer on habitat structure are similar to those of canopy closure, arising from lack of woodland management, and in many woods these two factors will have acted jointly to reduce habitat quality for birds.

For some woodland birds, dead and decaying wood is an important habitat component as a source of food or nest sites. Two of the declining woodland birds are especially dependent on dead wood — Lesser Spotted Woodpecker and Willow Tit. It is unclear whether there has been a decline in this resource for these birds but, in general, it is very unlikely that a general reduction in dead/decaying wood has occurred. The severe storms of 1987 and 1990 in the south of England resulted in a large increase in dead wood. The drought of 1976 also caused widespread tree death, as did Dutch Elm disease. Nonetheless, it is possible that subtle changes have occurred in the types rather than the quantity of dead wood. This could be important because dead wood occurs in many different forms and states of decay, not all of which are valuable to birds.

(iv) Interactions with other species

Predation is the largest cause of nest failure in birds and may sometimes be sufficient to reduce a breeding population. Two potential predation issues are especially relevant to woodland birds. The first is the expansion and increasing population density of the Grey Squirrel. A recent review by the BTO concluded that potentially they are major predators of nesting songbirds in British woodland but there is a lack of hard evidence. A wide range of species is vulnerable, including hole-nesters and ground-nesters, but canopy-nesters particularly Hawfinch and Chaffinch seem most at risk. This new predator is now virtually ubiquitous in lowland woodland and a study of its potential effects is highly desirable. The second predation issue concerns the increase in corvid populations. While this has occurred mainly outside woodland, birds nesting at the edges of woodland may have been become more vulnerable to predation by crows and Magpies. This is another reason why more work on woodland edges is a high priority.

Several woodland species overlap in their foraging and nesting requirements to such an extent that an increase in one could lead to a decline in another through competition. Could the declines in Marsh and Willow Tit, for example, be linked with increases in Blue Tit and Great Tit, possibly even with Nuthatch? Although analyses of BTO population trend data do not support this idea (BTO News 235), intensive studies in southern Scotland have shown that the recently excavated nests of Willow Tits are frequently taken over by Blue Tits (BTO News 244). The decline of Lesser Spotted Woodpecker has occurred at the same time as a large increase in Great Spotted Woodpecker. The two species use similar nest sites, but it is unknown whether competition is a problem for the smaller woodpecker.

Conclusions

The potential causes of declines in our woodland birds are diverse. Nonetheless, certain factors stand out as especially strong candidates (Box 1). Each of these factors could affect several different species. Individually they may be insufficient to cause the observed population changes. It is possible that combinations of factors or interactions between them are critical. For example, birds could become more susceptible to predation as nesting cover is reduced by deer. Changes in food supply, predation, inter-specific competition and deterioration of habitat could be acting simultaneously on some species. The issues listed in Box 1 are all ones where more research would be extremely valuable in helping to identify solutions. The BTO is planning several projects over the next few years that will help to improve our understanding of the needs of woodland birds.

Acknowledgements

This article is a highly condensed summary of a review published as a BTO Research Report authored by Des Vanhinsbergh, David Noble and Rob Fuller. The review was undertaken under the partnership between BTO and JNCC. The review benefited greatly from discussions within the Woodland Bird Group, a forum of UK government agencies, NGOs, research bodies and universities.

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