Kestrels and jackdaws are natural enemies. Jackdaws are scavengers that will gladly steal and eat kestrel eggs, while kestrels are aggressive birds that will fight anybody. And yet, somehow, these two would-be foes have carved out a shockingly efficient truce.
Researchers at the University of Palermo in Italy and Canada's University of Manitoba noticed that the two bird species were actually roosting together in old buildings in the Italian countryside. That's particularly strange because that means they are keeping their eggs right next to each other - and a jackdaw's foraging diet could easily include a bunch of kestrel eggs. For these two species to form such a close alliance, there must be some serious benefits to be had.
That definitely seems to be the case here. The researchers discovered that both species were able to reduce their time spent patrolling for predators. The kestrels showed less concern in the face of potential threats, and the jackdaws were able to spend more time away from their nests than if they were by themselves. Taken together, this adds up to some serious energy savings for the bird species in this communal living arrangement.
This sort of relationship, in which two species that share a predator-prey relationship put aside their natural inclinations and work together, is known as the "predator protection phenomenon." The pugnacious kestrels keep away species that would potentially prey on the jackdaws, and the jackdaws repay the favor by not eating their children, which seems only fair.
That said, it seems the jackdaws are aware they are only there at the kestrels' pleasure, as the researchers noted an increase in alarm signals from the jackdaws when living in these shared broods. If I didn't know better, I'd say the jackdaws are worried that the kestrels will suddenly remember some of their nest-mates' old dining habits.