Dataset for the manuscript entitled: Cold nests, warm choices: selective use of anthropogenic nest materials in closely related raptors

<p>Anthropogenic debris is increasingly incorporated into bird nests, yet the functional factors shaping this behaviour remain uncertain. Because synthetic materials can modify nest insulation, their use may be influenced by thermal demands. We examined two closely related birds of prey that differ in breeding phenology – the earlier-breeding Red Kite (<em>Milvus milvus</em>) and the later-breeding Black Kite (<em>Milvus migrans</em>) – to test whether phenological differences translate into contrasting use of insulating anthropogenic materials. By analysing debris from 45 naturally built nests, we found that Red Kites incorporated a higher proportion of thermally beneficial items than Black Kites, and that the contribution of insulating materials declined as the breeding season progressed. This temporal and interspecific pattern is consistent with the hypothesis that thermal pressures promote selective debris use rather than opportunistic uptake alone. Our results show that litter can become functionally incorporated into nests of rare, free-living raptors, linking human activity to nest microenvironments and highlighting a previously underappreciated route through which human-derived debris may shape breeding conditions.</p>