The influence of the Southern Oscillation Index on the timing of breeding of a forest-bird community in south-eastern Australia

Abstract

ContextLong-term changes in the breeding phenology of bird communities have been widely studied. For many species, breeding appears to be starting earlier as temperatures increase. For south-eastern Australia, such a trend has not so far been demonstrated. AimsThe aim was to determine how the date of laying of the first egg (FE; for sedentary species) or arrival times (for migratory species) responded to climatic factors such as rainfall, air temperature and the Southern Oscillation Index (SOI), and whether FE or arrival time showed a trend through time. MethodsThe date of laying of the first egg (FE) for 13 sedentary species of birds was recorded over 18 (1975–1984 and 2007–2014) breeding seasons (August to January) at a single site in a coastal forest in south-eastern Australia. The arrival times for seven migratory species were also recorded for these seasons. Key resultsLinear mixed models showed that FE was negatively correlated with the mean monthly SOI for April to July (A-J SOI), the period directly before the breeding season. Eggs were laid earlier when A-J SOI was positive and later when it was negative. SOIs calculated over different combinations of months showed that those for the January to March period had no influence on FE. FE was not related to minimum or maximum temperatures during April to July, despite increases in temperature between 1975 and 2014, nor was it related to rainfall between April and July. Mixed linear models showed that arrival date for migratory species became earlier between 1975 and 2014, but was uninfluenced by A-J SOI or rainfall. ConclusionsMigratory species arrived earlier by 0.27 days per year. However, this was at least an order of magnitude smaller than annual temporal changes in FE for sedentary species (6–7 days) associated with cyclical SOI fluctuations. Changes in SOI dominated the annual breeding phenology of the community. ImplicationsThe mechanisms by which A-J SOI influences the timing of nesting may be related to the primary productivity of forests and the influence of this on insect abundance. There are few data on these factors.