Nesting at the edge: breeding behavior and nest microclimate of the Glacier Finch (Idiopsar speculifer) on the Quelccaya ice cap, Southeastern Peru

• Abstract: All suggested changes have been fully incorporated, including the specification of nest type, monitoring details, sampling description, and the replacement of subjective terminology. • Introduction: The term “most closely associated” has been clarified in the Introduction to more precisely describe the species’ relationship with glaciers. • Methods: The corresponding paragraphs in the Field Methods section were revised to address the suggestions regarding the clarification of “attachment type,” the use of “quantified,” and the description of abandoned nests. We have now clarified the likely origin of the abandoned nests. However, we decided to retain the sample size information in the Results section, as moving it to Methods could affect the logical structure and clarity of the manuscript. • Results: All reviewer comments were addressed by clarifying in the Results section the origin and use of data from each nest. Table 1 was modified as indicated, a brief explanation was added for why vegetation from three nests was not analyzed, and the wording regarding camera trap records was improved for better understanding. • Discussion: All reviewer comments were addressed. The finding of the early nest built on a rock was added to the abstract, specifying how much earlier it occurred compared to previous records. The location error was corrected, and several lines were added to the first paragraph of the discussion to expand on the explanation regarding sample size, the relevance of the collected data, and the justification for long-term monitoring. High‐elevation glacierized environments pose extreme nesting challenges for birds, yet the Glacier Finch ( Idiopsar speculifer ) regularly nests inside ice cavities, but data on active nests and their microclimates remain scarce. We documented these aspects across two breeding seasons (2024–2025) in three sectors on the western side of the Quelccaya Ice Cap, southeastern Peru, above 5300 m asl. We characterized nest structure and composition, parental behavior, and the microclimate within glacier cavities used as nesting sites. We located six nests (four abandoned and two active) and described the structure and composition of three only once they were inactive (one abandoned and the two formerly active). Nests were open-cup structures composed primarily of two high-Andean grass species ( Deschampsia ovata and Cinnagrostis nitidula ). The active nests were monitored with trail cameras and microclimate sensors. One nest was found in April 2024 and the other in February 2025, the latter representing the earliest breeding record to date, occurring four days before the previously proposed egg-laying period, and the first documented nest built on rock rather than directly on ice. Daily biparental care was observed at one nest, with 8–9 visits per day and no nocturnal activity, whereas the second nest experienced predation by an Andean fox ( Lycalopex culpaeus). Glacier cavities acted as thermal buffers, maintaining near-freezing temperatures (∼0°C), high relative humidity (90–100%), and low evaporation rates (0.2 kg m⁻² h⁻¹), providing more stable conditions than outside the cavity. These findings enhance understanding of reproduction in extreme glacierized habitats.