If you stare at them they won't swoop you
I've mapped in plenty of places where there are magpies and that holds true. They come at you from behind.
You can't stare at them if they come from behind?
I learned a trick that I like to call 'turning around'. Maybe you don't know how it works but what you do is you just turn around.
On a bike? You never know what will pop out at you from in front...
When I got to the far end of the valley yesterday I could hear giggling coming from higher up the Pony Ridge switchbacks. A bit later I was overtaken by 4 cyclists - on road bikes - with very muddy rear ends. Can't believe they rode road bikes down the Pony Ridge singletrack!
If I'm on a bike then my helmet will protect me. I do most of my mapping on foot and tend to have a clipboard with me, which works wonders against swoopers.
CX bikes down the switchbacks maybe? It would be pretty extreme to take road bikes down there and I'd think most road cyclists wouldn't risk damaging their bikes!
I mistook some CX riders for roadies at the last MTBO I rode in. They were on the gravel but had drop bars.
You could put a big eye on the back of your helmet - apparently they think you are looking at them.
Oddly also, magpies don't seem to swoop if they know you, such as where they are nesting in your garden.