Methods

The ledge was monitored using a Canon XL2 video camera with a Canon 100-400mm L lens. This camera records Standard Definiton 25p PAL video on to 60 minute miniDV tapes, in practice these tapes produce just over 62 minutes of usable video. The tapes were captured onto computer using Sony Video Capture 6.0e and edited using Sony Vegas Pro 8.0b. Corrupt video at the beginning or end of a recording was deleted. For those measurements where a standard length of time is needed (provisioning rate, parental attendance, etc.) only the first 60 minutes of video was used.

Focal sites were identified by the presence of an incubating or brooding adult, or by the presence of a pair of birds. Pairs were identified by mutual interactions; particularly mutual preening, or displays. For transcription purposes, adults were noted with their focal site letter and a number. For each day, the initially brooding bird was number 1, and the non-brooding bird was number 2.