Why is sexual reproduction useful if only half the animals can produce offspring and two separate cells must be combined to form a third?
Sexual reproduction produces a new combination of genes in the offspring that may better enable them to survive changes in the environment and assist in the survival of the species.
What determines which sex will result in offspring of birds and mammals?
The presence of the W chromosome in birds determines femaleness and the presence of the Y chromosome in mammals determines maleness. The absence of those chromosomes and the homogeneity of the offspring (ZZ or XX) leads to the development of the other sex.