A method of altering the phenotype of a bird comprises introducing a DNA molecule into the muscle tissue of a bird contained within an egg during in ovo incubation. The DNA molecule is selected to be effective to cause a change in phenotype, such as an increase in growth rate, feed efficiency, immune response, or a combination of these and other phenotypes in the bird after hatch. A DNA molecule may further be selected to increase disease resistance, induce disease prevention, or neutralize maternal antibodies by the expression of an antigen over a period of time.