Bovine TB can take years to manifest itself, so depending on the last time he bought in stock it may well have come in with those beasts. I'd be pretty surprised if birds brought the disease in, more likely badgers, or possibly deer, but not birds.
Even if the beasts have been over wintering in sheds badgers poaching about the steading can introduce the disease, urinating on feed, such as cake, bruised barley or on the silage clamp. Which is in turn fed unwittingly to the beasts, exsposing them to the risk of infection. And all it takes is one animal to become infected, and as they are all is such close proximity whilst in the sheds, the disease can spread far more easily than if they were out in the field. As it is spread predominately vie respiration, so infected beasts coughing on each other will spread the infection. Bovine TB is a zoonotic disease, which means it can be passed from animals to humans, so personal hygine is paramount if you've been around animals that have/may be infected. Certainly stay well away from drinking troughs, even those out in fields, feed and obviously the animals themselves. But to keep it in prespective, the cases of humans becoming infected is negligible, less than 1% of reported tb cases in humans can be attributed to bovine TB, but it always pays to be careful. And in fairness if you're visiting a farm for the purposes of off-roading then there's no need for anyone to be anywhere need the stockyard. Hopefully the farm in question will have no more reactors and this has been an isolated case, but I fear otherwise, like fire engines, where there's one, there's two.
It's a good point to be made though, bio-security on or around farms is a serious buisness, as disease and the associated restrictions thus placed on the affected farm can have a huge effect on the enterprise concerned.