few mature trees with natural tree holes, and hence an impoverished hole-nesting bird community – pied flycatcher and redstart in particular suffered. Thousands of nestboxes were erected in the 1970s and 1980s, and both species responded positively, occupying woodland with nestboxes at up to twice the density of neighbouring woodland without nestboxes.
However, nestboxes are not always beneficial. They can potentially upset the balance of species, eg by giving an advantage to commoner species that compete with scarcer ones that do not derive the same benefit. Blue tits, for example, are aggressive foragers, and readily take to nestboxes, even evicting existing occupants, and could compete with less aggressive species like the marsh tit for nest sites and food. It is therefore important to assess (a) the relative populations of the bird community in the wood and (b) the availability of natural nestholes. There should be no need to provide nestboxes in woods with reasonable populations of a range of hole- nesting birds, and an evident supply of natural holes. If a particular hole-nesting species is rare or declining, and nestholes are apparently lacking, then a targeted supply of nestboxes may be beneficial.
Nestbox types and applications
A variety of nestbox designs have been developed to imitate natural nest sites; either with a hole of a specific size (depending on which species are targeted) or open-fronted boxes. In general, nestboxes should be located away from ground predators, and sheltered from direct sunlight and bad weather.
Different designs can be used, eg a ‘standard’ box constructed from timber, or a box constructed from a standing or cut tree stump. The BTO Nestbox Guide (du Feu 2003) should be consulted for further details.
Nestbox details for priority species
Spotted flycatcher An open-fronted box with a low (60 mm) front, sited 2 m–4 m up with a clear
Managing woodland for birds
outlook, eg above a glade or ride. Some cover for the box, eg ivy or other foliage is important. In the absence of cover, a chicken-wire cover (30 cm diameter hemisphere of 50 mm wire) for the box front could be constructed.
Pied flycatcher Pied flycatchers will readily take to standard nestboxes with a 28 mm hole set at 2 m–4 m height. These should be located overlooking a glade, or in woodland with sparse shrub and field layers. If competition from tits is likely to be a problem, boxes can be placed in clusters as tits will not nest close to each other, alternatively box holes may be blocked with cork until pied flycatchers arrive.
Marsh tit Boxes with a 25 mm hole placed close to ground level are preferred. Where competition with other tits may be a problem, provide an excess of boxes with 28 mm and 25 mm holes at 1 m–5 m. Many natural marsh tit nests have relatively long access holes, compared to typical nestboxes. An experiment in Sweden had some success with adding a block of wood to the box front before drilling the standard hole-size.
Willow tit Standard nestboxes with a 25 mm hole need to be tightly filled with sawdust or shavings, and the hole plugged with sawdust mixed with a little wood glue. Non-biodegradable fillers like expanded polystyrene should be avoided. Nestboxes made from sections of tree log are virtually cost free and have been successful, as have those made of other material but clad in bark to mimic natural nest sites. They need to be set low, as willow tits usually excavate holes less than a metre above the ground.
Male willow tits need to excavate several nestholes, and a scarcity of suitable rotting stumps may contribute to their population decline. Rotting timber also offers little protection from predation or damage and providing several nestboxes could provide relatively secure nesting habitat. The need to excavate several nests each year might be in response to eviction by blue tits, so a combination of willow tit boxes and