I've had a couple of messages so far suggesting collecting food
disturbed by the swans. This was my initial thought also, but
I didn't want to jump to conclusions.
At times the gull dove while (or immediately after) a swan was feeding,
but it also dove while the swans were busy with each other above water.
Possibly their shadows were disturbing small fish...
In reply to Dean's question below, no I didn't notice the gull
consuming anything, but I could easily have missed that in the
action. Dean's observation of Silver Gulls diving after dusk is
interesting: my photos were taken just prior to the sun setting.
Has anyone else made similar observations?
BTW, there were a fair number of gulls around the lake (as well as
swans, pelicans, Chestnut Teal and Black Duck) but this was the
only one I could see that was acting in this way.
__
David Burren
> From: "Dean Portelli" <>
> To:
> Subject: Re: [BIRDING-AUS] Silver Gull feeding(?) with Black Swans
> Date: Wed, 29 May 2002 16:48:55 +1000
>
> I wonder if David noticed whether the Silver Gull ever appeared to
> have caught anything and consumed it?
>
> In my mind the most likely explanation for the behaviour is that
> the gull was utilising the disturbance of the black swans to attempt
> to capture prey that were driven out from their hiding places at
> the bottom of the lake (where the swans are feeding) and thus visible
> to the gull. I have observed Silver Gulls hovering about a metre
> above the water and then diving down into it in a similar manner
> to the gull David observed (judging from the photos). These
> observations took place after dusk, although the water was lit by
> traffic lights as it was next to a road. I couldn't see exactly
> what the purpose of the behaviour was, although I assumed it was
> attempting to capture something, an alternative explanation may be
> some sort of bathing behaviour but this is not the typical way
> Gull's bathe (and it is an unlikely explanation for the black
> swan/gull interaction David observed).
>
> A further note: experiments done with Herring(?) Gulls have shown
> that success rate of prey capture increases with gull group size
> (ie. individual gulls benefit by foraging in groups). This was
> attributed to the response of the prey to the behaviour of the
> gulls. Single gulls caught most fish from the rear (ie. while fish
> was facing away), but in groups the gulls had higher success rates
> of capturing fish either from the front or the side. In other words
> disturbing the fish makes them more vulnerable to being captured,
> this may apply to the Black Swan/Silver Gull behaviour, the gull
> may be benefitting from the effect of the swans on the behaviour
> of whatever prey the gull may be attempting to capture (most likely
> fish I would have thought). But I am getting ahead of myself and
> we haven't yet established that this was the gull's intentions (ie.
> to capture prey), but I thought I would just provide some points
> to be taken under consideration.
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