The nesting season is a risky time for birds, both for the adults who have to keep returning to the same spot to feed their nestlings and for the nestlings themselves who huddle there unable to fly and exposed to all sorts of danger.
One of the hazards is the weather: a cold spell can kill the young birds, particularly when they are recently hatched and featherless. The parents brood them to keep them warm, but they cannot do that all the time as they also have to go in search of food.
Conversely, a very hot spell and drought can overheat them and cause dehydration. Rain too is a danger when heavy and prolonged, increasing the hypothermia problem and also the risk of drowning - especially among ground-nesters like lapwing and other waders, partridges and other gamebirds, larks and pipits.
Birds in nestboxes can also drown - which is why it is a good idea to have drainage holes in the bottom of the box, have an overlapping lid and give the box a slight forward lean.
Predators are another problem, so nestboxes should not have a perch by the entrance hole for them to cling to (the birds do not need it anyway). If cats are a threat, put prickly twigs below (not round) the nest.
Another way to help birds is simply to keep away - they may desert if you take too close an interest by peeping in the nest - and this applies to all stages from choosing a nest-site to incubating the eggs and rearing the young. And remember that potential predators – crows, magpies, jackdaws for example – are always on the watch, while others may follow tracks up to the nest.
Finally, in drought conditions birds like swallows, martins and blackbirds cannot get mud for their nests, so occasionally watering their dried up mud supply can keep them going.
Birds need all the help they can get!