Predation by Australian magpies (Gymnorhina tibicen) on pasture invertebrates: Are non‐territorial birds less successful?

Abstract

Diet composition and prey intake rates of Australian magpies in a New Zealand population were investigated to determine whether flocking, non-territorial birds were less successful predators of pasture invertebrates than territorial birds. Time-activity budgets showed that flock magpies foraged throughout the day during February and March, indicative of a prey shortage at that time. Prey abundance was therefore measured in the autumn months but there was no evidence of a large quantitative difference in the food supply of flock and territorial magpies. Significantly fewer invertebrates were collected from pitfall traps in the flock foraging area, but these were counterbalanced by significantly more scarab beetle larvae and more flying insects, obtained by sweep-netting. When compared with territorial magpies, flock birds exhibited similar peck rates, prey intake rates, and earthworm intake rates. While there were qualitative differences in the diets of flock and territorial magpies, determined by faecal analysis, flve of seven prey items analysed were captured in the same relative proportions by the flock and territorial birds. Earthworms exceeded their relative availability in the diets of flock birds, and more scarab larvae were found in the diets of territorial magpies. Non-territorial magpies were therefore no less successful than territorial birds during the autumn, and it is suggested that the function of flocking behaviour may eventually be discovered through consideration •Present address:

Invasive birds: global trends and impacts

“…In New Zealand, results reported by Veltman and Hickson (1989) and McIlroy (1968) were similar to the Australian study in terms of invertebrate prey items taken by Australian Magpies; however, the relative importance of prey groups varied greatly.…”

“…Australian Magpies predominantly consume invertebrate prey located on or just under the ground surface (McIlroy, 1968;Vestjens and Carrick, 1974;Veltman and Hickson, 1989;Kaplan, 2004) using visual, olfactory or vibratory stimuli (Floyd and Woodland, 1981). Vestjens and Carrick (1974) identified and enumerated prey items from 1319 stomach samples from Black-backed Magpies at a site close to Canberra, Australia.…”

“…Variability in the relative importance of prey groups may reflect a functional response to the availability of vertebrate prey (Holling, 1965). Veltman and Hickson (1989) assessed prey remains from faecal samples (in addition to using other techniques) to quantify the relative abundance of different invertebrates in the diet of magpies; most prey were taken in the same relative proportions by both flock and territorial magpies (Veltman and Hickson, 1989).…”

Do territorial and non‐breeding Australian Magpies Gymnorhina tibicen influence the local movements of rural birds in New Zealand?

“…Magpies mainly consume pasture invertebrates supplemented with seeds. However, they can be opportunistic feeders and sometimes consume plant material, other birds (and their eggs), frogs and lizards (Vestjens & Carrick 1974, Veltman & Hickson 1989). Accordingly, most rural birds, including all birds that were seen attacked by Magpies in the current study, could compete with Magpies for food.…”

The use of supplementary foods by Australian magpies Gymnorhina tibicen: Implications for wildlife feeding in suburban environments

“…For different food types (natural or supplementary food), the foraging amount per minute was ascertained by counting the number of items consumed for a particular type of food item within a minute. Head tossing and swallowing (Veltman & Hickson 1989) distinguish the successful capture of prey, and most food items could be determined before being swallowed. Some small food items could not be identified and were categorized as 'unknown'.…”