I'm posting this across for Susie after problems with the submisson pages:

I teach in a primary school and I am embarrassed to be asking this question 
as I probably should know the answer.

Birds mostly build nests. Chicks hatch. Chicks are fed by parents until they 
can fend for themselves. Why does this not apply to hens? We buy fertilised eggs, we incubate them, they hatch but don't wait to be looked after, immediately fend for themselves.

Is it because we have bred this into our stocks ?

I presume free range hens build nests and lay eggs into them. Do the chicks 
wander off straight away?

Your help would be greatly appreciated. Need to be able to explain this to a 
group of 5 year olds (and every adult I have asked!)

The scientific term for the relatively independent nature of these chicks is "precocial". The opposite term is "altricial", which implies that the young are pretty helpless when born/hatched. All passerines (http://en.wikipedia.org/wiki/Passerine) are altricial.

Having precocial young is actually fairly common in birds (e.g. in several duck species, most waders and rails) and is considered by many to be the primitive state. Some birds are even "super precocial" and can fly on the day of hatching (e.g. the Megapodes (http://en.wikipedia.org/wiki/Megapode)).Taxonomically, the Megapodes are part of the Galliformes, which includes domesticated chickens. The trait of precociality is shared among many of these birds.

With this view, it is altricial-ness that is the derived trait (i.e. a new trait that was not present in the ancestor). There are advantages/disadvantages for each strategy. Precocial young are usually less-vulnerable to predation because they have a shorter nesting period, and require less intensive care from the parents in early life. Thus, it could be seen as an adaptive trait - but then why would altricial offspring become the norm? There must be some disadvantages!
There are - in fact precocial young have a slower growth rate and actually take longer to become independent of their parents which can leave them vulnerable to predation for longer and make them "costlier" to raise.

There are still discussions in the literature about what determines whether a species will altricial or precocial, and in what situations each strategy is optimal. It is usually thought to be determined as a result of trade-offs among different investments. It may come down to predation risk, food availability (or the complexity of the task of food gathering), or constraints from the environment.

In theory, precocial young would be favoured:
(1) if the nest provides less protection than freely moving young (i.e. if the nest is easily accessible to predators)
(2) if food is easy, not complex to obtain (e.g. grain/ground-based foraging is easier than catching insects in flight)