Bird Migration

The Autumn bird migration and a Regent at the Bower

The first tentative movements of honeyeaters are usually evident from the middle of March when little groups of Yellow-faced can be seen meandering northwards, keeping in touch with their trademark 'quick quick' calls. The migration peaks in late April, round about Anzac Day. However, it was on Friday 13 May, an auspicious day, when a rare Regent Honeyeater was seen at The Bower, feeding on lerps in a Spotted Gum and in the company of a small flock of migrating Red Wattlebirds. It was one of those delightful moments of birdwatching: a rarity feeding placidly in the canopy and allowing a leisurely inspection.

These autumn travellers, most of them honeyeaters, are the summer-migrant breeders; their job is done, their young raised and with the fat of good summer feeding laid down to sustain them on their journeys, they go north where the milder climate ensures a good supply of insects and nectar. Their precise destinations remain a mystery, or perhaps they have no destinations; in the 1960s, 44,000 Yellow-faced Honeyeaters were banded in the ACT and only 18 were ever recovered. This suggests the birds might simply disperse at will to suitable sites along the coast and uplands where food is plentiful.

Reasons for Bird Migration
It is worth looking at the reasons for bird migration. Most movements are governed by the necessity to secure a regular supply of food, specially in severe weather, so it is understandable that the insectivorous Silvereyes leave Tasmania and the southern offshore islands in April and head north. But why would thousands of Yellow-faced Honeyeaters, and others of their ilk, come south to breed in the spring when food is quite as plentiful in the northern parts of the country?

Consider the summer of the north: it is hot, wet and humid and subject to ferocious storms. In conditions where nests and brooders are wet and wind-tossed for days on end, bacterial and fungal infections thrive and feeding flights are curtailed. Nestlings fall victim to a variety of ills when cleanliness cannot be maintained. So the Yellow-faced, and other species too, choose to breed in the south where the climate is temperate and the general housekeeping necessary for successful brooding is easily accomplished.

When we come to butterfly migrations, the reasons for their movements are foggy in the extreme and, to be honest, nobody really knows why they do it. This autumn saw an influx of Black Jezebels Delias nigrina; at first sight it could have been a large hatching, though the insects descended in numbers and with the abruptness of a migration. There was no build-up, just 30 to 60 feeding on the blossoming callistemons on 11 March. The numbers fluctuated daily and almost hourly till about 24 April when only three or four remained. Unlike the Caper White Belenois java migration, where the insects travel steadily north while feeding, the Black Jezebels simply milled about the food sources, wafting northward at times, singly or in small, random groups.

Puzzling creatures !

by Jill Whiter, author of The Eurobodalla Natural History Diary

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