One of the greatest images of bird migration I’ve ever seen is of a flight of geese crossing a full moon, the birds in silhouette.
Nothing could best portray the wonder of migration, in which many birds are guided by the moon and the stars on their epic journeys that can encompass the far ends of the earth.
A supermoon on the night of September 18 came right at the height of the spring migration and I was out in the garden not just contemplating the remarkable sight of an extra-bright moon but anticipating the arrival of visiting birds the next day in the wild spaces surrounding Hobart.
This spring, because of unfavourable winds and some unseasonal low temperatures, the inter-state migrants had been slow to arrive. Although I had ticked off welcome swallows, I still awaited what I call the song of spring, that of the fan-tailed cuckoo.
I hear the cuckoo some years In late August but come the night of the supermoon I had only heard the descending trill of the cuckoo on one occasion, and even then I could not be sure that it was my first cuckoo of spring because it was too distant and competing with the drone of a passing aircraft.
The portents for what birders call a “fall” of migrants landing after an overnight flight were good the morning after the supermoon. Not only was there a relatively clear sky, the winds were also favourable. The migrants will wait for a northerly or north-westerly wind to help them on their way, in contrast to the south-westerly winds which had brought hail and snow to Hobart in the early part of September.
Birds exploit both the sun and stars to plot their journeys, using celestial cues to navigate, much as sailors of yore used the night sky to guide them. But unlike humans, birds also detect the magnetic field generated by the earth’s molten core and use it to determine their position and direction.
The night of September 18 was the year’s second supermoon – the term for when the full moon’s orbit is closest to the earth. The supermoon was about 357,000 km from Earth – 27,000km closer than normal. There are normally between two and four supermoons a year and they do not always coincide with bird migration so, for birders, the opportunity for a post-full moon outing in early spring and autumn must be seized.
Among migratory birds missing from my checklist in the second half of September were all the four species of cuckoo found in Tasmania during the spring and summer and the black-faced cuckoo shrike, appropriately also called the summerbird.
A walk on the lower slopes of kunanyi/Mt Wellington next morning confirmed the supermoon had lit the migrants’ way. Fan-tail and shining bronze-cuckoos were bringing music to the woods.