When Sea Eagles snatch up swan cygnets I momentarily hate them and threaten to cancel my subscription to Sea Eagle Cam but I had no idea they’d feed on seagull. So why then, are there so few Sea Eagles and an abundance of Gulls???? ALSO:
The Black Swan Communal Nesting Site at Paynesville, (Gippsland Lakes) has its first occupants for this season with 3 nests occupied. At nest #1 both mum & dad were in the water today and I saw a white (???????) seabird swoop down and steal an egg and started eating the yellow contents in about 6 inches of water by the lake edge. Dad came by and after having a taste of the contents himself, washed the contents out of the egg shell and threw it into deeper water.
Obviously my first question is, What is the name of the white seabird that looks like a plain seagull on steroids, (about twice the size) like a dwarf Albatross ???? Also, why do black swans leave their nests to chastise harmless juvenile swans yet pay no attention to egg stealing preditors ??? Is it an experience thing? I notice the other two nests are never left unattended. ===============================
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I was curious enough to do back-of-the-envelope calculations from detailed estimates I found for Bald Eagles.
Wintering Bald Eagles require about 2000kJ of food/day.
With smaller size and warmer climate that might be 1500kJ of food/day for a WBSE.
A silver gull might weigh 350g, which might equate to 2500kJ.
So 1 Silver Gull day looks to be enough for a WBSE.
Bald Eagles are estimated to use ~10kJ/minute in flapping flight and ~3kJ/minute when soaring/gliding. Active pursuit must burn more energy, but it looks like an WBSE could easily afford to spend more then an hour pursuing each gull caught.
Andrew ===============================
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It should be nuptial.
Here some of Raja’s photos documenting this awesome behaviour: http://www.adarman.com/Birds/Raptors/Peregrines
Cheers,
Nikolas
The energetics of pursuing, capturing and digesting prey also needs to be taken into account by the predator.
A potential prey species might be locally abundant, but the predator may have to spend a lot of energy to chase and eventually capture it. Therefore, a predator needs to select prey items that require the expenditure of as little energy as possible to capture and digest, and which provide maximum nutritional and energy benefits.
If gulls were a common food source for WBSEs, my guess is that a WBSE soaring overhead or nearby would cause the gulls to take to the skies to escape potential danger. Gulls are quite aerobatic and, when they are in a flock, I would imagine it would be difficult for a WBSE to capture one quickly. Therefore, there is a risk of the WBSE using up too much energy to capture a gull in comparison with the energy it would gain from consuming one (if it is lucky enough to capture a gull).
I remember many years ago observing a Peregrine Falcon harassing a small flock of airborne Welcome Swallows at Kanidal Beach (Eyre Bird Observatory). The swallows swooped and swirled in a tight flock, and just aerobatically out-manouevred the diving Peregrine. This continued for several minutes before one swallow made the fatal error of separating from the flock and flying in a straight line out over the ocean. Seconds later, it was taken successfully by the Peregrine. The question that immediately came to my mind: was it worth the Peregrine doing this? I don’t know, but it would have spent a lot of energy harassing the swallows and I suspect the reward (one swallow) did not provide much net energy and nutritional benefit in return.
Stephen Ambrose Ryde, NSW
Penny et al
My wife, Jackie and daughter, Isabel, saw three White-bellied Sea-eagles just above the old tollgates at Berowra (between Cowan and Berowra Creeks just south of the Hawkesbury) the other day. What was unusual was two of them engaged in a downward spiraling talon-locking flight. Would this be territorial (with the third a female) or nuptial? They’re seen fairly regularly in the area as are Wedge-tailed Eagles, Collared Sparrowhawks, Grey/White Goshawk, Peregrine Falcon and the very occasional Little Eagle.
Paul
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Regarding numbers (up or down) of White-bellied Sea-Eagles on the NSW Central Coast and whereabouts, on a recent trip up the Hawkesbury River, 5 – 7 May from Brooklyn to Peat’s restaurant and beyond, I saw a pair at Brooklyn, a pair with 2 juveniles opposite Milsons Island and another pair further up river not far from Borowra.
Perhaps there is an excellent source of prey in the river and the air – certainly quite a few Silver Gulls around and there were many people fishing.
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Patrick,
A basic rule in ecology is that the prey controls the predator, not the predator the prey.
In other words: Much prey – good time for the predator.Shortage of prey – bad time for the predator. If predators would control their prey, this cycle wouldn’t work. Unfortunately there are exceptions; certain species in man-made or man-influenced habitat and – of course – man itself…
In other words again: There are White-bellied Sea-eagles around as long as there is abundance of its prey. It is not the White-bellied Sea-eagles who reduce their prey – it is us!
I hope the real ecologists out there are not going to punish me for this very simplified statement!
Cheers,
Nikolas