The disclosure provides a method for administering an agent to an avian species by in ovo delivery of an implant releasably containing the agent. In one embodiment, the method is particularly advantageous for stimulating an immune response in a bird by in ovo administration of a biocompatible implant releasably containing an immunogen. The implant can provide for sustained or delayed release of the immunogen or both. The amount of immunogen that is released from the implant into the bird is preferably sufficient to effectively stimulate a primary immune response to the immunogen. Other agents which can be administered according to the method of the invention are disclosed.