No 'black and white' case against magpies Thursday, Aug 14, 2003
Results from a four-year study suggest that while magpies do chase and sometimes kill other birds, they are far less of a threat to native species than pest mammals. As a consequence, many regional councils are unlikely to establish large ongoing magpie control programmes.
Landcare Research and Waikato University have been examining the effects of magpies on rural bird populations in five regions: Northland / Auckland, Waikato, Bay of Plenty, Wellington and Southland. In each area, regional councils established two study blocks covering nine hundred hectares each: one where magpies were killed and another where they were not killed. All types of birds were counted in all blocks in 1999 before magpie control began, with further counts in the next 3 years.
Landcare Research pest ecologist John Innes says by the end of the study, magpie populations in the kill blocks had decreased by 60 to 80%. As a result, there was a 130% increase in the very low numbers of kereru that were originally counted, and a weaker and more variable result for tui. Magpie control also caused definite but unspectacular increases in sightings of five introduced birds: songthrush, myna, starling, blackbird and skylark.
However, Mr Innes says these increased sightings do not necessarily mean there were more of these birds.
"At a farm scale, magpies seem to control the conspicuousness of the birds, rather than their abundance. It is likely that the number of birds was relatively unchanged, but individual birds were seen more often.
"Breeding pairs of magpies can be very territorial and will chase and occasionally kill other birds, probably to keep them away from food sources. These birds then have to seek food farther afield.
"If farmers want native birds to have access to particular places such as their gardens, controlling magpies around the gardens is probably worthwhile. With magpies in the garden, rural dwellers will see native birds less often, although this does not mean there are fewer native birds in total.
"However, when you remove territorial magpie pairs, you get constant re-invasion by previously non-territorial birds. It is an endless and expensive battle."
Mr Innes says based on the results of the study, the five participating councils will not invest significant amounts in large-scale magpie control.
"It is better to concentrate on controlling mammal pests. They are the ones that attack nests, and limit numbers of native birds. Magpies are clearly unlikely to eradicate another bird species from the wider landscape.
"Magpies are scapegoats, in a sense. They are conspicuous, noisy and active in the daytime, while mammals such as ship rats, possums, cats and stoats are secretive, nocturnal, silent and tree dwelling. Therefore, people do not see what mammals do. Inevitably, the role of mammals tends to be underrated, and magpies exaggerated.
"Pest mammal control increases tui and kereru populations in native forests. Now we are keen to try a new experimental mammal control operation with and without magpies, based in rural areas rather than large native forests, to get an even clearer picture."