The breeding ecology of whimbrel (Numenius phaeopus) in Shetland : with particular reference to the effects of agricultural improvement of heathland nesting habitats
The effect of re-seeding of heathland on the breeding ecology of whimbrel in Shetland was studied from 1986-1988, on five study sites located on the islands of Unst and Fetlar. A total of 36-45 pairs bred on these study sites in each year, and a large proportion of the breeding adults were individually colour ringed. Heathland was the main nesting habitat for whimbrel. Established pastures and areas ploughed or harrowed before re-seeding were avoided as nesting habitats, but some pairs nested on areas subjected only to surface re-seeding. Avoidance of ploughed or harrowed re-seeds was associated with changes in vegetation composition and structure, and such re-seeds lacked attributes important in the selection of nest-sites by whimbrel. Habitat change was less marked in surface re-seeds. Ploughed or harrowed re-seeds were used extensively as feeding habitat by adults during the pre-laying period, and evidence was obtained to indicate that re-seeding improved feeding conditions for adults. Measures of breeding success were positively correlated with egg volume, and negatively correlated with laying date. However, the improved feeding conditions on re-seeds for females prior to laying were unlikely to have provided major benefits for breeding success via effects on either egg volume or laying date. Approximately 30% of all broods studied used re-seeds at some stage prior to fledging. Although some broods did show preferences for this habitat, there was little evidence that this was associated with either, decreasing the risk of predation on chicks, or improving chick food supply. The survival of chicks to fledging did not vary according to the habitat-use of broods. Both nesting densities and productivity varied between study sites. The possibility that areas of heathland differ in their suitability as breeding habitat for whimbrel is considered, and the implications of this in assessing the effects of re-seeding heathland are discussed. Over the study period the overall production of fledglings from study sites was probably in excess of that required to balance adult mortality. This result is consistent with the current increase of the whimbrel population in Shetland.