Loss of habitat through deforestation, agriculture and pollution have certainly reduced availability of natural nest sites and prey, but by far the greatest problem for Purple Martins has been the release and spread of European Starlings and English House Sparrows in North America. Both Starlings and House Sparrows are very aggressive cavity-nesters. Since their introduction in the late 1800’s their population has increased dramatically and numbers of native cavity-nesting birds have drastically decreased.
Starlings build their nests in Purple Martin housing and drive the Martins away. If the Martins nest first, Starlings will destroy their nests and eggs, and kill both young and adults.
English House Sparrows are known to investigate a Martin colony and proceed to fill the compartments so full of nesting material that Martins are unable to enter them. If an individual pair of Sparrows nests in a Martin house, they often enter the compartments occupied by Martin nests and peck holes in the eggs to prevent the Martins from raising their young.
The battle against these aggressive and destructive non-native species is ongoing. To keep Purple Martins you must keep Starlings and Sparrows out.
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