There are a number of different avian species which may be carriers of Salmonella Gallinarum, it has been found in free-living corvids, pigeons, psittacine birds, ducks, and doves.
Exposure to droppings made by infected birds which have contaminated the surrounding environment, feed, or drinking water, is a large source of exposure. Other sources of infection can be exposure to other animals who serve as vectors---wild rates, red poultry mites, wild birds, and ticks.
Wild rats can shed S. Gallinarum in their feces for up to 3 months. The red poultry mite (Dermanyssus gallinae) from infected poultry houses can harbor S. Gallinarum for months, and is the main route for carry-over between flocks. Infected red mites can be carried between farms on equipment or the clothing of workers or visitors, as well as being carried by wild birds moving between farms.