New Zealand Garden Bird Survey – analysis of the first four years

The New Zealand Garden Bird Survey started in 2007 primarily to monitor long-term trends in common garden bird populations. The method was based on the Big Garden Birdwatch in the UK. Volunteers spent one hour in midwinter each year recording for each bird species the largest number of individuals detected at any one time in their gardens, as an index of abundance. A large number of species was recorded, the two most numerous being house sparrow (Passer domesticus) and silvereye (Zosterops lateralis). There was regional variation in species occurrence and abundance; more species and more individuals of most species in rural than in urban gardens; more individuals of some species and fewer of others in gardens where supplementary food was provided; and changes in the abundance of some species over the 4 years. Potential problems with the methodology and interpretation of the data are discussed. As a consequence of convenience sampling the results apply only to the gardens of participants, not necessarily New Zealand as a whole. The survey has the potential to alert authorities to changes in garden bird population trends, and to provide circumstantial evidence of the success or otherwise of management actions such as restoration planting.