So yesterday I noticed a flock of birds, flying in a rather tight formation and doing some weird back and forth flying...thing. I had never seen birds behave such way (granted, I rarely even pay attention to them) so I got my phone and filmed a bit of their flying behavior.

Can any of you tell me why those are flying like that and, if possible, what species are they.

Sorry about the quality of the video, I only had my phone available.

https://www.youtube.com/watch?v=QWsiX-PEOxE

I'm not an ornithologist, but their call sounds similar to a jackdaw's to me:
https://www.youtube.com/watch?v=8d_l2q2fXnI

As for the formation and behavior, maybe just playful enjoyment of the moment?

No, but I've got two birds for ya rrriiight here!!

Hmm, you may be right. The sounds are very similar! Thank you!

Starling?

I wonder if it has something to do with defense? I am totally taking a shot in the dark here but when certain types of fish (yes I realize those aren't fish) encounter sharks/seals they do almost the exact same thing. The predator aims to focus on one target and go in for the kill but if they ball up and then move like that it makes picking out a target VERY hard and thus confuses the shit out of the hunter.

Do you have any birds of prey in your area? Are they out and about this time of year?

Those arent jackdaws - by any means.
Jackdaws are related to crows and have a similar caw.

They're more likely starlings and trying to get laid (as some femailes are coming into season), or perhaps chasing some early insects.
I'll check with my dad tomorrow to be certain.

Edit: also jackdaws dont flock

My thought exactly (had to check for the translation of "étourneau")

if they were evading a hawk they would fracture every now and then, fly low and also hit the first evergreen tree possible for cover (we have 100s here in the summer - along with a rather large sparrowhawk who swoops in every now and again when the crows arent looking).
if they were chasing a predator (such as a magpie after eggs) it would make sense, though they would be making a different kind of cacophony,

Okay there goes that theory.

Was Tippi Hedren in the area?

The first time I met some British Marines, on shore leave, they asked me where they might find some birds.

What outdoors men :-)

They sound like jackdaws, not starlings, to me. Jackdaws do flock, though usually in hundreds rather than the millions of the largest starling flocks. Here's another video where the birds are making the same calls and are accurately identified as jackdaws. http://www.youtube.com/watch?v=Rc6nVkDnFn4

The usual thinking is that these birds do this for defense, but they're also very social, and this is their way of congregating, for example before they roost for the night. Sachys is right: it isn't just defensive, because they'll do it in the absence of predators. The individual birds in the flock (so characteristic that it gets its own name, a "murmuration" or starlings or a "clatter" of jackdaws) move by rules that fascinate the scientists who are trying to figure out how the movement of the flock arises from them.

Well.. don't you know about the bird?!

Well, everybody knows that the bird is the word!
A-well-a, bird, bird, b-bird's the word
A-well-a, bird, bird, b-bird's the word..

XD

Jackdaws do like to flock. As mentioned and [url=http://www.britishwildlife.wikia.com/wiki/Jackdaw]here:

"The jackdaw is a highly sociable species outside of the breeding season, occurring in flocks that can contain hundreds of birds."

And another mentioning here:

"Jackdaws are often seen flying together in large flocks. [...] Despite the flock’s loose ranking, Jackdaws still look like they have fun together as they tumble and dive and chase each other in the air."

YouTube has several videos displaying this.

On top:

Audio of Starlings calls

and in comparison, Audio of Jackdaw calls

Hey, now that I'm intrigued, if an ornithologist or wildlife expert could chip in, I would love to hear an expert opinion :)

Perhaps I should have been clearer: I meant flocking in terms of the flight pattern. Not gathering sociably.