Zapdos doesn't seem to be based on one particular type of bird. It just appears to be a bird in general. If I had to categorize it, then I would place it as a mix of a type of Falcon and some sort of sparrow like creature.
Zapdos has the form of a large bird with black and yellow plumage. Its
spiky feathers make it appear to be made of lightning. Its tail
feathers stick out in several directions, furthering this illusion. A
large crest of feathers decorates its head, and distinctive black
rings encircle its eyes. Its beak is very long and thin. While Zapdos
is the smallest and lightest of the legendary birds, it is still a
Description by Bulbepedia.
Hope I helped xD