Effect of variation in snowpack on timing of bird migration in the Snowy Mountains of south-eastern Australia

Abstract

The number of bird species present in subalpine woodland in the Snowy Mountains of south-eastern Australia was counted weekly in winter–spring for 6 years: for 3 years (2000–02) on an altitudinal gradient of 300 m extending from below to above the winter snowline (at ~1500 m asl) and for 3 years (2003–05) above the winter snowline on the western side of the same valley. The number of species was examined in relation to the number of open flowers on the shrub Grevillea victoriae, percentage snow cover, date, altitude and year. Flowering of shrubs and snow cover were better predictors of numbers of bird species than date, emphasising the complexity of environmental interactions at the beginning of spring. Bird species were attracted by shrubs projecting above the snow and flowering in response to increasing air warming but later snowfalls depressed both access to flowers and flowering itself. There was a significant reduction in bird species with increasing altitude, and fewer species were recorded in years of deeper, longer lasting snow. Climate change may account for earlier arrival of bird species in the Snowy Mountains. However, timing of immigration is not a simple consequence of incremental annual warming resulting in earlier snow-melt. The variability in climatic conditions in early spring, possibly increasing with global change, makes the situation more complicated.