I am not sure whether chickens really have all that much fewer deleterious recessive alleles (although it is *possible*, I just don't know).
One factor that separates chicken-breeding from dogs, horses etc is that it's generally considered a more trivial thing to cull 'defective' individuals, whereas even just a few 'messed up' puppies or foals is a much bigger concern to a typical breeder. Another factor is that you get more offspring, period, from chickens, and because of this, all you really need is a reasonable incidence of GOOD offspring, irrespective of how 'enh' the other ones are.
But really, inbreeding (or linebreeding - I am not trying to get into semantic arguments here
) is not generally
nearly such a terrible thing as most people imagine.
Unless you are for some reason starting with a population that already has a considerable incidence of deleterious recessives (in some cases, that can happen semi-accidentally as a result of breeding hard for certain extreme traits, e.g. in some dog breeds), you actually do not usually need very much nor very frequent injection of 'outside' blood to keep things ticking along quite happily in terms of healthy good-quality offspring. Like, as a *general* rule of thumb, something on the order of one new individual per hundred individuals of population every few generations (not years; *generations*).
Pat