Nest monitoring.
Fieldwork to locate breeding pairs and nests began approximately 25 May (+/- 2) each year. Parental behavioral cues and rigorous nest searching are both important techniques to find nest locations (Martin and Geupel, 1993). Searching effort was relatively evenly distributed among sites. Once found, nests were marked with a GPS and flagged no less than 8m away from the active nest so as not to disturb the nesting pair. We monitored each nest every 2-3 days until they were close to fledging (~11 days old), and then every day after until fledged. If presumed fledged, we searched the area meticulously to take notes such as provisioning behavior from parents (i.e., food in their bill, alarm calling, etc.) or visual sightings. A successful nest yielded at least one fledged bird, and a failed nest was depredated or abandoned. Nests were considered depredated if an active nest was observed with cracked or missing eggs, dead young nestlings (<11 days old) and/or adults, or a nest that was pulled apart. On occasion, nests were abandoned and confirmed as such if adults were absent for ≥3 visits. We collected nesting data including initiation date (i.e., date the first egg was laid), clutch size, hatch date (i.e., date the first egg hatched), and nestling condition (i.e., how old the nestlings were at the date checked and what their appearance was). When hatch date was unknown, we back-calculated nest age based on the nestlings’ condition. Finally, if nest initiation date was unknown, we backdated from known or estimated hatch date, assuming one egg was laid per day, incubation occurred for 12 days, and nestlings were at least 11 days old before fledgling (Hałupka et al. , 2018). Nests were binned in four groups (“BEF” = 200-300m; “Low Jeff” = 500-700m; “Mid Jeff” = 701-900m; “High Jeff” = 901-1,250m) to correspond with the matching spatial precipitation and temperature data that was collected along the elevation gradients.