West Nile Virus has been found to cause mortality in a number of avian species both in Michigan and across the United States but during the spring and summer months it is not unusual for mortality unrelated to a West Nile Virus infection to occur in a variety of avian (bird) species. In the spring, as adult birds migrate north, they are exposed to many changes in weather, availability of food, and competition with other birds of their own species as well as other species. This is an extremely stressful time of the year for all adult birds and any change in the weather or habitat may result in malnutrition, exposure, and dehydration. The main activities of adult birds in the spring revolve around establishing territories, courtship, and breeding, As a result of these activities the animals are often-times more susceptible to injury or death due to trauma (flying into a house, being hit by a car, etc), predation (either mammalian or avian), intraspecies competition (injuries inflicted by a member of the same species), or interspecies competition (injuries inflicted by a member of another avian species).

The summer months bring about vast quantities of young birds which, because of their age, are susceptible to many of the same problems faced by the adults during the spring as well as some additional problems. Young, naive birds are very susceptible to traumatic injuries (falling out of their nest, injuring themselves as they fledge from the nest, and competition between siblings), predation (either while still in the nest or during fledging), diseases due to an undeveloped or underdeveloped immune system (lack of resistance to bacterial and viral diseases), malnutrition and/or exposure, and developmental problems. Adult birds may also be more susceptible to mortality as they attempt to incubate, brood, care for, and protect their eggs and young.

In conclusion, it is possible that birds found dead in the spring and summer may have died from a West Nile Virus infection, but it is more likely that for the majority of species of birds in the state that death is due to some other factor. Please continue to use the On-Line WNV Sick and Dead Bird Observation Report to report instances of bird and mammal mortality. This data will be used to detect trends, over time, to determine a normal baseline wildlife mortality figure.