Question

Why birds like sparrow lay only four or five eggs

Answer 1

The house sparrow ( Passer domesticus) (also called the
“English sparrow”) is, possibly, the most widely distributed
wild bird in the world. Its native range is Europe and much
of Asia, but it has been introduced, both accidently and on
purpose, into North America, South America, Australia, New
Zealand, Africa, and numerous, scattered islands. It thrives
almost anywhere that there are people. The only gaps in its
nearly worldwide distribution are extreme tropical regions,
deep forests, and extensive deserts.
Introduction of this species into North America during the
middle to late 1800’s occurred at multiple locations over a
25 or 30 year time period. Dozens, and hundreds, and even
thousands of birds were transported from Europe and
released into urban (Brooklyn, Boston, Cleveland, Cincinnati,
San Francisco, etc) and rural (Michigan, Iowa, etc) sites.
The rationale behind this widespread importation and
release is a bit murky. Control of insect pests on shade
trees, accelerated breakdown of street and road deposited
horse manure, nostalgia for familiar, Old World species, or
just a contagious mania concerning sparrows have all been
suggested. The house sparrow’s ability to nest in cavities
and protected spaces of almost any human habitation, to
consume a wide variety of stored seeds, grains, and
discarded human foods, to distribute themselves rapidly
through new environments, and to produce huge numbers of
offspring in a single mating season have all contributed to
the species’ explosive population growth in almost every
area to which they have been introduced.