Nest usurpation and adult mortality in a secondary cavity-nesting songbird

Competition for limited nest sites among secondary cavity-nesting bird species is intense and may result in loss of nests, physical conflict, injury, and occasionally death. The Prothonotary Warbler ( Protonotaria citrea ) is a secondary cavity-nesting species that has experienced a 38% population decline over the past five decades. In the northern portion of their range, Prothonotary Warblers are sympatric with two cavity-nesting species known for their nest usurpation behaviors: the House Wren ( Troglodytes aedon ) and Tree Swallow ( Tachycineta bicolor ). From 2017-2023 we monitored Prothonotary Warbler nests. House Wrens destroyed and usurped 38% of Prothonotary Warbler nests, which represented the greatest cause of nest failure, while Tree Swallows usurped only 1.5% of Prothonotary Warbler nests. We also documented two instances of adult Prothonotary Warblers likely killed by Tree Swallows. Interference competition from House Wrens is likely a significant factor limiting Prothonotary Warbler reproductive success where it is sympatric with these nest competitors.