A death trap in the nest: anthropogenic nest materials cause high mortality in a terrestrial bird

Abstract

The impact of anthropogenic debris on wildlife, particularly in marine environments, has garnered significant attention in recent years due to the severity of its effects. In terrestrial environments, the incorporation of human-derived materials into bird nests is well documented, yet most studies primarily focused on quantifying their prevalence rather than systematically examining their direct impacts, particularly on entanglement mortality. This study investigates the use of anthropogenic nest materials (ANMs) by a long-lived bird species and their impact on nestling mortality. Over four years, we photographed 568 white stork ( Ciconia ciconia ) nests to quantify ANMs and nestling mortality, while in 2023, we monitored 93 nests weekly from hatching to fledging to identify vulnerable developmental stages and high-risk materials. Habitat characteristics and proximity to landfill sites were analysed to infer ANM sources. ANMs were present in 91% of nests, with ropes – the main cause of entanglement − found in 42%. Entanglements, often fatal due to injuries such as necrosis and limb loss, were recorded in nearly one-third of the nests, affecting 12% of all nestlings. Most entanglements occurred in early nestling stages (mean age: 2 weeks ± 1.38), highlighting that such fatalities may go undetected without early monitoring of nests. Entanglement risk increased with the number of ropes in the nests which were associated with agricultural areas. Baler twine, a slow degrading polypropylene rope used in agriculture, accounted for 63% of entanglements. This study reveals the widespread use of ANMs, their harmful effects on terrestrial birds, and the urgent need to mitigate baler twine pollution through management actions that minimise the impacts on the species affected by ANMs.

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