It’s hard to keep your eyes on the road sometimes, crossing Dyarubbin – the Hawkesbury. For those not entranced by the scene of early-morning fog spilling down the gullies in Marramarra National Park, there’s the raptor action. If you were heading north on the Peat’s Ferry Bridge about 6.30 on Sunday morning, for instance, you would have seen these two young sea-eagles eagles just a few metres above the freeway.
I caught sight of them first from afar, tumbling and whirling. The cartwheeling argument was brief but emphatic – a couple of lunges at each others’ talons – one bird upside-down – as they fell from the sky. Then abruptly it was all over. The two flew off companionably through the mist, pulling up in a tall eucalypt on the shores of Spectacle Island.
Needless to say, I did a U turn in my kayak and went to have a better look.
Two youngsters. Siblings, I thought – hanging out together, just like the collared sparrowhawk fledglings we watched grow up in our backyard a few years back. What a wonderful omen for the new year! The plentiful rains of La Nina and their fecundity – an explosion of spring wildflowers, new growth on all the trees, insects everywhere – and also this – two chicks from one raptor nest! Sea-eagle parents is lucky to have one chick make it each year. Two fledgelings defeating death – what a thing to see!
But then I noticed their bellies: the tan-coloured torso of the eagle on the left – much lighter than the brown chest feathers of the other youngster. Fledgelings are a dark brown colour and over three or four years, as they mature, their plumage slowly changes to the eponymous white belly and crisp grey wings of adults. These two were definitely not nest-mates.
So what was going on here? Training flights? Teen romance? Territorial aggro?
A cursory read of accounts of sea eagle behaviour suggested that the mid-air argument I saw by the freeway is a characteristic courtship display. But sea eagles are only mate at adulthood – some distance away for this two. Plus there’s some controversy over the received wisdom about romantic cartwheeling flights. Some researchers say raptors are more likely to be aggressive than sexy when they get into these mid-air tumbles. And sometimes – although less often – this kind of in-flight wrestling is simply play (Simmons, 1993, 17). There’s not much aggro going on here – or it doesn’t seem like it to my untutored eye.
So if they weren’t courting and didn’t seem to be competing over territory, what were these two eagles doing hanging out together?
I wondered at first about co-operative breeding. Quite a few Australian birds have offspring that don’t disperse straight after they’ve left the nest, but stay with their parents and help to raise their siblings in subsequent years. A few species raptors do it too, at least occasionally – including some hawks and eagles – with immature helpers and sometimes even unrelated adult birds helping build nests, defend territory or feed chicks.
There’s some debate about whether young white-bellied sea eagles leave the territory of their parents at around six months old or hang around for a few years before heading off somewhere new. When I say controversy, I mean a mild-mannered, ornithological debate conducted on paper – not a gun fight at ten paces. But regardless it seems a bit unlikely that I saw co-operative breeding in action. . Unlike other co-operative breeders raptors tend to have adult helpers. And teaching younger raptors aerial skills isn’t something I’ve read in the chores of “helper” in cooperative breeding set ups, though maybe it happens!
Big mixed groups of adult, immature and juvenile sea eagles have been seen in some places, like Jervis Bay, south of here, particularly in autumn. Researchers have have compared this behaviour to the large congregations of Bald Eagles in the US, which gather to take advantage of an abundance of prey. Jennifer Spencer and her colleagues conclude groups of white-bellied sea-eagles are “unlikely to be permanent associations [but] they may have an important social role as conspecifics were frequently observed engaging in mock attacks and courtship displays (Spencer, 217). Perhaps something like this – though a bit more socially distanced – was going on here on Dyarubbin.
A while back, Steven Debus, who really does know his raptors, observed “for its size, appearance, and abundance on the densely human-populated coasts of south-eastern Australia, the White-bellied Sea-Eagle Haliaeetus leucogaster is remarkably little studied” (2008,166). But I bet someone out there knows these two youngsters were up to. If you’re a sea-eagle watcher and can fill in the gaps – drop me a line! I’d love to know.
And, in the absence of an evidence base of any kind, I’m going to take the cartwheels and the company of eagles as a sign of good things for the year to come.
More about eagles I’ve met on Dyarubbin
References
S J S Debus, (2008) “Biology and Diet of the White-bellied Sea-Eagle Haliaeetus leucogaster Breeding in Northern Inland New South Wales” Australian Field Ornithology 2008, 25, 165–193
T.E. Dennis, G.J. Fitzpatrick and R.W. Brittain (2012) “Phases and duration of the White-bellied Sea-Eagle Haliaeetus leucogaster breeding season in South Australia and the implications for habitat management Corella, 36(3): 63-68
Terry E. Dennis, Rebecca R. Mcintosh & Peter D. Shaughnessy (2011) Effects of human disturbance on productivity of White-bellied Sea-Eagles (Haliaeetus leucogaster), Emu – Austral Ornithology, 111:2, 179-185
Rebecca Kimball, Patricia Parker and Janes Benarz (2003) “Occurrence and evolution of cooperative breeding among the diurnal raptors”
The Auk 120(3):717–729, 2003
R. E. Simmons & J. M. Mendelsohn (1993) “A critical review of cartwheeling flights of raptors”, Ostrich, 64:1, 13-24,
Jennifer A. Spencer & Tim P. Lynch (2005) Patterns in the abundance of White-bellied Sea-Eagles (Haliaeetusleucogaster) in Jervis Bay, south-eastern Australia, Emu – Austral Ornithology, 105:3, 211-216