The impact of Desert Locust Schistocerca gregaria swarms on pre-migratory fattening of Whitethroats Sylvia communis in the western Sahel.

Abstract

The spring migration of Sylvia warblers across the Sahara is preceded by a period of fattening and increased fruit consumption (Fry et al. 1970. Stoate & Moreby in press). British breeding Whitethroats Sylvia communis are known to occur in Senegal in winter (Dowsett et al. 1988), and winter ringing records reveal a movement of birds through the northern part of the country on the spring migration in March and early April. A study conducted in mid-March 1993. in the Parc National des Oiseaux du Djoudj, northern Senegal, showed that the berries of the Saltbush Salvadora persica, a locally abundant shrub, comprise the main component of Whitethroat diet in March, with berries exceeding 5 mm diameter being selected (Stoate & Moreby in press). Subalpine Warbler Sylvia cantillans. Blackcap Sylvia atricapilla and Orphean Warbler Sylvia hortensis also were present, also feeding mainly on the fruit of the Saltbush.
The study site comprised a belt of shrubs and scattered trees (mainly Acacia spp.) lying between the River Gorom (a tributary of the River Senegal) and an area of semi-desert. Shrubs cover 25% of the area and comprise two berried species. Saltbush (79% of shrub area) and Nitraria retusa (3%), and a species which does not produce berries Tamarisk Tamarix senegalensis (18%). Tamarisk, however, does provide a source of invertebrate food. Seasonally flooded areas nearby support pure stands of Tamarisk. Ripe berries of the Saltbush are most abundant in March.
In January and February 1994, the region was severely affected by swarms of Desert Locust Schistocerca gregaria, which occurred throughout a western belt approximately 150 km in width and extending south to Gambia and north through Mauritania (Anonymous 1994). The swarms defoliated Saltbush and Acacia trees.
Locust abundance can be considered in terms of discrete 'plague' and 'recession' years, swarms being virtually absent in the latter (Steedman 1990). In plague years, swarms can occur across the western Sahel from May to November, but in Senegal and Mauritania they can occur until February (Steedman 1990). Locusts defoliate many trees and shrubs; as well as destroying leaves of the Saltbush, they also consume flowers and developing fruit. The possible impact of defoliation by locusts on migratory passerines has not been considered previously. An assessment of the impact of the late winter locust swarms in Senegal was made at the 1993 study site in late March and early April 1994, 5-6 weeks after the end of the locust swarming.