Variability in foraging behaviour and prey of the Common Fiscal Shrike, Lanius collaris, along an altitudinal gradient in South Africa
AbstractAspects of the foraging behaviour along an altitudinal gradient of four subpopulations Durban, Merrivale, Estcourt and Harrismith) of colour-banded Common Fiscals (Lanius collaris) were analysed in summer and winter in South Africa. The shrike subpopulations showed significant differences in their attack, capture and success rates. The shrikes obtained more food during winter by increasing their attack and capture rates, and by taking larger prey items. Foraging success also increased from summer to winter in all except the Harrismith subpopulation, and was dependant upon the size of prey attacked.
Perching height, attack distance, encounter time and prey length were significantly different between the sites, but only prey length showed a significant seasonal difference. It is expected that the increased attacks and captures during winter were in response to greater food demands. Common Fiscals displayed significant variability in feeding strategies and foraging parameters along the altitudinal gradient. The foraging efficiency of Common Fiscals is similar to other shrikes that hunt in a similar manner.
Invertebrate prey was important (>90%) whilst vertebrate prey (frogs, small reptiles, birds and small mammals) was low (<10%). Durban birds took the greatest number of prey items during summer and winter, followed by the Merrivale, Estcourt and Harrismith birds. Subpopulations also displayed significant differences in the amount and size of prey items cached, but the number and length of cached items were independent of season. The wide variety of prey items consumed showed that Common Fiscals are highly opportunistic. This opportunism and their sit-and-wait predator method are important. Even if shrikes seek to maximise encounters with larger (higher energy) items, the low capture rates and short duration of foraging attempts when taking smaller items indicate that small items can be taken with little opportunity cost while waiting for better items to appear.
Ostrich 2004, 75(3): 133–140