Also, changes in surveillance and reporting confounds interpretation of existing data. Nevertheless, with the exception of rare catastrophic events, available information suggests that substantially greater numbers of wild birds are dying from diseases now than in earlier periods of the 20th century. Emerging diseases of wildlife. The number of avian die-offs in the NWHC data base with reported mortality of 1,000 or more is sufficient to support the contention of increased numbers of birds dying from disease compared with the period before 1965. Habitat and Human Interactions Causes of major bird die-offs during the past decade and their geographic distribution are shown in Table 3.
Composite data indicate a relation between bird concentrations and the occurrences of avian disease.
Those states with large concentrations of migratory birds on migrational staging and wintering areas tend to have the most disease outbreaks. Preliminary assessments suggest that habitat quantity and quality are important factors in this relation. The association between wetland losses and spread of infectious disease is due to the interactive factors that aid disease transmission. Movement patterns of infected birds can spread the disease to other locations and populations and help establish the disease as a recurring problem. High concentrations of birds for prolonged periods of time on limited habitat often degrade the quality of habitat through fecal contamination and damage to vegetation.