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

Abstract

Australian Magpies Gymnorhina tibicen sometimes attack and kill other birds in New Zealand. Here we assess how Australian Magpies influence the local distribution of other birds in New Zealand and identify the members of an Australian Magpie population that display the most agonistic acts. We conducted regular observations on six territorial breeding groups and three non-breeding flocks of Australian Magpies for 1 year to determine (1) if other birds avoid flying or foraging close to Australian Magpies, (2) the proportion of passing birds that are attacked and (3) which social subunits of the Australian Magpie population are most aggressive. In comparison with adjacent Magpie-free control areas, significantly fewer birds of a range of species (e.g. Common Blackbird Turdus merula , Skylark Alauda arvensis , Yellowhammer Emberiza citrinella ) foraged close (i.e. ≤ 50 m) to both territorial breeding groups and non-breeding Australian Magpie flocks; fewer birds were also recorded flying near (i.e. ≤ 50 m) territorial breeding groups but not non-breeding flocks. Excluding Australasian Harriers ( Circus approximans : see below), only 8% of birds recorded within 50 m of territorial breeding groups were observed being attacked. Attacks were most frequent when numerous birds occurred near Australian Magpies and species recorded in the highest frequencies were generally attacked most. Territorial breeding groups attacked 39% of passing Australasian Harriers. All attacks consisted of the victim being swooped at or chased; no physical contact was ever observed. Both adult male and female breeding Australian Magpies were seen attacking other birds; juveniles in breeding groups sometimes supported adults but never initiated attacks. Australian Magpies in non-breeding flocks were not seen attacking other birds, except Australasian Harriers (attacked in 17% of appearances). Our results suggest that some birds avoid foraging and/or flying close to Australian Magpies because they are sometimes chased by breeding adults of both sexes; however, the proportion of passing birds actually attacked was small. The numerous published observations of Australian Magpie attacks are apparently biased heavily towards sensational events that are rare. Possible reasons why Australian Magpies attack other birds are discussed.

Invasive birds: global trends and impacts

“…Conversely, there is also anecdotal evidence that invertebrate prey abundances in some areas are actually higher where there are magpies, which may be the result of other predators being driven from these areas by magpies (McCaskill, 1945). While this hypothesis has not been tested, rural bird species were shown to avoid being in close proximity to territorial magpies in one New Zealand study (Morgan et al, 2006a; but also see Borowske et al, 2012).…”

“…McIlroy, 1968). Swamp Harriers appear to be regarded as a serious threat to magpies and have been attacked more frequently than any other bird species in New Zealand (Morgan et al, 2005(Morgan et al, , 2006a. Furthermore, Morgan et al (2006b) reported that Swamp Harriers were an important predator of bird nests, predominantly the Eurasian Blackbird (Turdus merula) and Song Thrush (Turdus philomelos), in rural New Zealand and accounted for 36% of recorded predation events.…”

“…For example, in New Zealand, 45 bird species have been observed being attacked by magpies but many of these reported attacks are on birds that are neither predators of, nor have resource overlap with, magpies (Morgan et al, 2005). Furthermore, magpies do not appear to consider other birds as prey, as they rarely kill adults (Morgan et al, 2006a), nestlings or eggs (Boulton and Cassey, 2006;Morgan et al, 2006bMorgan et al, , 2011.…”

Threats from introduced birds to native birds

“…There was some strong evidence for a lack of competition in interactions with the Common Myna (Parsons et al 2006;Lowe et al 2011;Borowske et al 2012), Common Starling (Williamson and Gray 1975;Vierling 1998) and Mute Swan (Conover and Kania 1994). There was correlative evidence that the Australian Magpie reduced the abundance of native species in New Zealand (Morgan et al 2006). There was correlative evidence that the Common Myna is a threat to the Seychelles Magpie-Robin (Copsychus sechellarum) and that the Common Myna and Red-vented Bulbul are a threat to the Tahiti Monarch (Pomarea nigra) ( Table 1; see 'Evidence for threats involving introduced species', below).…”

Do heterospecific size and demeanour influence visitation behaviour of birds at urban foraging patches?

“…Both are assigned to demeanour class A due to reputations for adversely impacting other birds through interference competition or, more rarely, predation of adults and/or nestlings (Barrington 1995(Barrington , 1996Tindall 1996;Morgan et al 2005). Documented impacts include avoidance of magpies by New Zealand birds (Morgan et al 2006) and aggressive dominance of mynas over native Australian parrots (Pell and Tidemann 1997). Pigeon (33 cm; 400 g) and dove (28 cm; 140 g) (also exotics) are comparably sized to magpie and myna respectively but were assigned to demeanour class B because they are not considered aggressive toward other species (Heather and Robertson 1996).…”