The biology of magpies (Gymnorhina spp.) in New Zealand

Abstract

During the 1860s and 1870s the early settlers in New Zealand experienced insect plagues, especially armies of caterpillars, which created havoc amongst their crops (Drummond, 1907). Although some native birds were regarded as insectivorous, the clearance of native bush and settlement of land had restricted the distribution of these birds and the numbers remaining in crop areas were considered insufficient to control the insect numbers. Attention therefore turned to know insectivorous birds from other countries. Acclimatisation societies were formed and various birds were imported. Australian magpies (Gymnorhina spp.) and starlings (Sturnus vulgaris) were introduced during this period in the hope that they would help control the insect pests. Now, approximately 100 years later, both these birds are strongly associated with pastoral farming and are still commonly regarded as beneficial in keeping down insect numbers. However, with the restricted use of DDT insecticides for pasture pest control in recent years, the development of DDT resistant populations of grass grubs (Costelytra zealandica) and the resulting resurgence of grass grubs and porina (Wiseana spp.) as pests in certain areas, there is a need to evaluate the actual roles birds play in agriculture and especially as biologically controlling agents of insects.

Very few detailed studies covering most aspects of the ecology of introduced birds have been carried out in New Zealand. Marples (1942) is the only worker so far to have made a systematic study on the food of an introduced bird.

The objective of this thesis was to study the biology of magpies in New Zealand. The most important aspect of this was an investigation of the food to ascertain what magpies actually eat and to discover if any of these items were of any agricultural interest. Other aspects studied included possible interbreeding between the two, Gymnorhina hypoleuca Gould and G. Tibicen Latham, present in New Zealand and consequently their taxonomic classification: distribution of both species; taxonomic measurements and identification of sex and age classes; breeding biology; territorial behaviour and mortality factors.

A subsidiary objective was to review and if possible obtain fresh evidence on the aggressive behaviour shown by magpies towards other birds and mammals (including man).