The effects of logging on understorey birds in Borneo

Studies have established that birds are affected by logging and largely adversely. However, which species are affected, and to what degree, remains unanswered. In particular there is a need to establish why birds decline. Studies show insectivores to be the most vulnerable, but factors affecting insectivorous bird survival are unknown. The purpose of this study is to confirm which species are vulnerable, and the reason why. This study demonstrates the high proportion of understorey bird species which are affected by logging and the large extent by which most species are adversely affected. Not only did the majority (about 80%) decline, but two-thirds of species were severely affected (declined by 50% or more), and a third exhibited acute decline (of over 75%). Only generalists and nectivores are not adversely affected by logging. The abundance of understorey invertebrates also declines following logging. Howwever, pertinent to insectivorous birds, this decline is more significant for the prey groups. It is dependent on substrate, and becomes more severe in drought. For this reason temporal changes in prey availability are likely to be as important as determinants of local bird densities as mean prey availability in logged forest. The lower range of prey availability is likely to be a critical factor in bird persistence in logged forest. Results indicate that drought is the mechanism through which habitat disturbance causes a decline in prey abundance. This decline in arthropods appears to directly impact on insectivorous birds such as babblers. Arthropods (at least those that are bird prey) and babblers are associated with microhabitats that are uncommon in logged forest. The response of three arthropod communities (foliage, ground and suspended litter) to disturbance depends upon how the logged microhabitats available approximate the natural microhabitats they prefer. Skids and gaps are avoided, and arthropod abundance depends directly upon the availability of preferred and usable microhabitats. Such microhabitat becomes increasingly rare with increased severity of disturbance. Insectivorous birds decline because optimium foraging sites are rarely available, and they experience a decline in prey availability. The decline in density of babblers is due at least in part to prey scarcity. The foraging success of babblers declines in logged forest, on average by 80%. Logged attack rates are universally lower (for all 15 species) than those of undisturbed forest. Arboreal, low level and terrestrial babblers all have a lower attack yield in logged forest, which suggests that babblers consume less prey in logged forest than in primary forest. Foraging preferences also explain the relative decline of babblers. The foraging strategy of babblers dictates how vulnerable they will be to logging, and it is clear that it is specialisation specifically that causes species decline. This graduated response to logging can also be predicted on the basis of microhabitat use, and is also explained by the magnitude of disturbance. Understorey birds show greatest decline in abundance where the degree of disturbance is most severe. This implies that forest management could greatly affect bird survival.EThOS - Electronic Theses Online ServiceGBUnited Kingdo