Effects of volcanic ash on the insect food of the Montserrat Oriole Icterus oberi Lawrence 1880

Abstract

The Montserrat Oriole, Icterus oberi Lawrence, endemic to the West Indian island of Montserrat, has grown critically endangered since volcanic eruption began on that island in 1995. The Soufriere Hills Volcano has devastated much of the oriole's native habitat, and populations within intact forests have plummeted in recent years. One hypothesized cause for the Montserrat Oriole's decline is that low insect prey numbers during the nesting season, as a result of volcanic ash in the environment, is resulting in increased nest failure. The hypothesis of a negative effect of ash on canopy arthropods was tested. Four sites, varying in the level of ash deposition they typically receive, were sampled via canopy fogging over a 14-month period. Results indicate that ash is having a significant negative impact on canopy arthropods, particularly at the sampling sites closest to the volcano, but that the decline is limited to a few insect taxa.

To investigate whether the arthropod taxa utilized by the Montserrat Oriole were among those negatively affected by volcanic ash, observational studies were conducted to identify the main insect prey types and sizes brought to oriole nests, and to examine whether nestling feeding rates have declined since the onset of volcanic eruption. Ortoptera, which were not significantly affected by volcanic ash, were the most important nestling food resources utilized in 2002 and 2003. The most frequently delivered size of prey item was calculated at bill length long (approx. 2 cm), and were not significantly affected by ash. Orioles appear to be selecting their prey from the portion of the insect fauna that is least affected by ash in the environment. Oriole nestling feeding rates appear to have declined since 1995, but this may not be strictly due to reduced insect prey numbers. Montserrat's Orthoptera (including Phasmida and Blattaria) were catalogued. Thirty-seven species were reported for the island, including several new species and at least 16 new distribution records for the island.