A matter of timing: changes in the first date of arrival and last date of departure of Australian migratory birds

摘要

Although there is substantial evidence that Northern Hemisphere species have responded to climatic change over the last few decades, there is little documented evidence that Southern Hemisphere species have responded in the same way. Here, we report that Australian migratory birds have undergone changes in the first arrival date (FAD) and last date of departure (LDD) of a similar magnitude as species from the Northern Hemisphere. We compiled data on arrival and departure of migratory birds in south-east Australia since 1960 from the published literature, Bird Observer Reports, and personal observations from bird watchers. Data on the FAD for 24 species and the LDD for 12 species were analyzed. Sixteen species were short- to middle-distance species arriving at their breeding grounds, seven were long-distance migrants arriving at their nonbreeding grounds, and one was a middle-distance migrant also arriving at its nonbreeding ground. For 12 species, we gathered data from more than one location, enabling us to assess the consistency of intraspecific trends at different locations. Regressions of climate variables against year show that across south-east Australia average annual maximum and minimum temperatures have increased by 0.17 degrees C and 0.13 degrees C decade(-1) since 1960, respectively. Over this period there has been an average advance in arrival of 3.5 days decade(-1); 16 of the 45 time-series (representing 12 of the 24 species studied) showed a significant trend toward earlier arrival, while only one time-series showed a significant delay. Conversely, there has been an average delay in departure of 5.1 days decade(-1); four of the 21 departure time-series (four species) showed a significant trend toward later departure, while one species showed a significant trend toward earlier departure. However, differences emerge between the arrival and departure of short- to middle-distance species visiting south-east Australia to breed compared with long-distance species that spend their nonbreeding period here. On average, short- to middle-distance migrants have arrived at their breeding grounds 3.1 days decade(-1) earlier and delayed departure by 8.1 days decade(-1), thus extending the time spent in their breeding grounds by similar to 11 days decade(-1). The average advance in arrival at the nonbreeding grounds of long-distance migrants is 6.8 days decade(-1). These species, however, have also advanced departure by an average of 6.9 days decade(-1). Hence, the length of stay has not changed but rather, the timing of events has advanced. The patterns of change in FAD and LDD of Australian migratory birds are of a similar magnitude to changes undergone by Northern Hemisphere species, and add further evidence that the modest warming experienced over the past few decades has already had significant biological impacts on a global scale.