Diet

Numerous nest watches, both physical and photographic, have produced a great deal of information about the diet fed to nestlings. What adults ate at the same time can only be assumed to be similar plus maybe some fruit items if any were available. It is obvious that the food fed to young is quite catholic in its variety of invertebrates. The bulk consitsts of caterpillars of many species with a predominancy of Herald moth Scoliopteryx libatrix. Amongst other varieties brought in were Poplar Hawk moth Laotho populi caterpillars, which were fed head first in the manner of Kingfishers feeding scaly fish to their young. Hawkmoth caterpillars have a spike protruding at an upwards and backwards angle on the last segment of the body and so could possibly cause swallowing difficulties. Brightly coloured and sometimes hairy larvae are fed without any skinning and in poplar situations this applies particularly to larvae of the White Satin moth Stilpnota salicis. It is a mixture of black, red and white and is moderately hairy, all of which usually send out warnings of unpalatability to many other potential eaters. This variety is also quite numerous in poplars in the GOG study area. Amongst many other unidentifiable invertebrates, bumble bee species formed quite a large proportion.

Adult moths were also part of the diet, and in the second week of the nestling period the size grew markedly larger. Hawkmoths especially were used and surprisingly were not de-winged, nor were any others, even when smaller food was fed to smaller young.

Literature on the subject does mention Vespidae species being used but GOG research did not substantiate this. However it was found that a confusion species, the Hornet moth Sesia apiformis, was being fed. This species employs Batesian mimicry, which orioles seem to have penetrated.

An investigation was undertaken to determine what food items are available to orioles in poplars during the foraging period of June. In particular, the research was designed to establish how the three main poplar varieties differ in their insect species, how the new varieties compare, and whether the food supply in nesting trees explains site preference of East Anglian orioles. To this end Andrew Foggo from the University of Oxford was engaged to carry out the work and the result is published in (Foggo 1993). It found that some 145 types were available but this did not mean the birds actually used all of them.

Orioles clearly have a great capacity for seeing through camouflage as many moths taken were of types that use tree trunks to rest on in the hope that they blend with their surroundings. Hawkmoths and Prominents are just two types that do this. In 1998 in particular birds were seen hovering over and dropping into low herbage such as sugarbeet and investigation found large numbers of Silver Y larvae Autographa gamma being harvested.

References: Comparative studies of the invertebrate fauna of poplar varieties, and their value as a food resource for the golden oriole Andrew Foggo Dept of Zoology, Oxford 1993.