European Wasps and Birds

A recent story in the local paper, The Mercury, caught my attention, and told of losses being sustained by Tamar area grape growers. This time they weren't aiming their ire at birds, such as the Silvereye, but at the European Wasp. (Most vineyards are now netted). Wasps were apparently accidentally introduced to Tasmania around 1959, possibly from New Zealand, and they are now frequently encountered around much of the state. Unlike bees, they are able to sting multiple times and, speaking from personal experience, it's much more painful! The growers complaint was that they are losing up to 30% of their crop to wasps (eating fruit) and they wanted a public campaign to combat them.
My thoughts about the story related to several species of our birds that actually regularly include this pest in their diet, including Noisy Miners, Yellow Wattlebirds, Grey Shrike-thrush and the ubiquitous Silver Gull, but I suspect there are several others. The miners and wattlebirds almost certainly don't target them, but encounter them when both are attracted to flowering
eucalypts. Shrike-thrushes and gulls certainly do, as I recount below.
While birding in the Meehan Range, I came across a pool of water that had collected in a wheel rut and flushed a shrike-thrush from the water's edge. Cursing that I had missed a photo opp. by my lack of vigilance, I was surprised when it quickly returned to the poolside. Over the next several minutes I photographed it on the ground and in the nearby scrub, and despite repeatedly flushing it, it was always drawn back to the pool, but didn't appear to drink. Later, looking more closely at the pool I noted that there was a procession of wasps visiting the edge of the water, presumably to drink. It was then that the penny dropped, the shrike-thrush wasn't drinking but had found a great source of 'easy' food--the wasps.
I had noted and blogged a previous occasion that I had witnessed Silver Gulls targeting wasps, and a few days ago I took the accompanying images of a similar event, this time on Bellerive waterfront. I suspect that it's a regular occurrence. Several gulls lined up, some few metres apart, on the river's edge, facing towards the sun, probably to silhouette the prey. Individual wasps were flying from the shore across the water. The gulls would spot them and give chase as the wasps climbed ever higher. To catch them, the gulls had to use all their flying ability, but I never saw them miss. They would usually descend back to the rocks to eat them, but I couldn't determine whether they removed the sting before swallowing. I'd estimate that they were catching maybe 20 or 30 an hour, so it's probably not going to make too much of a dent in wasp numbers, but every little helps.
From all this a few questions arise. Where are the wasps going to (they're heading off across the Derwent River), or for that matter where are they all coming from, as there was a fairly constant procession of them? Why do the gulls bother with them as they would appear to have very little nutritional value?