from second broods being at higher risk partly because of the hawk's greater need for food for its own developing young. The nests of great tits are raided by great spotted woodpeckers, particularly when nesting in certain types of nest boxes. Other nest predators include introduced grey squirrels (in Britain) and least weasels, which are able to take nesting adults as well. A species of biting louse (Mallophaga) described as Rostrinirmus hudeci was isolated and described in 1981 from great tits in central Europe. The hen flea Ceratophyllus gallinae is exceedingly common in the nests of blue and great tits. It was originally a specialist tit flea, but the dry, crowded conditions of chicken runs enabled it to flourish with its new host. This flea is preferentially predated by the clown beetle Gnathoncus punctulatus, The rove beetle Microglotta pulla also feeds on fleas and their larvae. Although these beetles often remain in deserted nests, they can only breed in the elevated temperatures produced by brooding birds, tits being the preferred hosts. Great tits compete with the pied flycatcher for nesting boxes, and can kill prospecting flycatcher males. Incidences of fatal competition are more frequent when nesting times overlap, and climate change has led to greater synchrony of nesting between the two species and flycatcher deaths. Having killed the flycatchers, the great tits may consume their brains. Physiology Great tits have been found to possess special physiological adaptations for cold environments. When preparing for winter months, the great tit can increase how thermogenic (heat producing) its blood is. The mechanism for this adaptation is a seasonal increase in mitochondrial volume and mitochondrial respiration in red blood cells and increased uncoupling of the electron transport from ATP production. As a result, the energy that would have been used to make ATP is released as heat and their blood becomes more thermogenic. In the face of winter food shortages, the great tit has also shown a type of peripheral vasoconstriction (constriction of blood vessels) to reduce heat loss and cold injury. Reduced cold injury and heat loss is mediated by the great tits’ counter-current vascular arrangements, and peripheral vasoconstriction in major vessels in and around the birds’ bill and legs. This mechanism allows uninsulated regions (i.e., bill and legs) to remain close to the surrounding temperature. In response to food restriction, the great tits’ bill temperature dropped, and once food availably was increased,