Estimating occupancy and nest survival of cliff-nesting raptors in an open population framework

Nest survival is a key demographic parameter for assessing the viability of bird populations and is frequently responsive to management. While nest survival is often monitored alone, its joint monitoring with abundance permits a more thorough understanding of breeding productivity and the mechanisms of population change. However, nests are subject to a time-to-event process that presents a challenge for joint modeling of these processes. That is, availability for detection is conditional on nest survival until the survey occasion, which, if ignored, results in negatively biased estimates of nest abundance or presence/absence (breeding occupancy). Cliff-nesting raptor abundance and reproductive success have been the subject of intense conservation concern with the recognition of widespread population declines and manifold persecution. Inferences about changes in cliff-nesting raptor abundance and reproductive success are often based on the unadjusted occupancy rates and apparent nest success. Here, we developed methods for joint estimation of breeding occupancy and nest survival in these populations, thereby extending occupancy models to the case in which the occupancy states are subject to an explicit time-to-event process. Our approach accommodates false negatives in the occupancy data due to nest failure prior to the sampling occasions. A simulation study with varied detection probability, nest success, and Markov properties in occupancy showed our model to generally have low to moderate bias. We applied the model to data from American Peregrine Falcon (Falco peregrinus anatum) monitoring in Alaska (1987–2021) with conspicuous observer and stage-specific heterogeneity. Breeding occupancy increased over time while nest success decreased, suggesting potential density-dependent effects. Our approach will allow the discarding of the untenable assumption of constant detectability common to cliff-nesting raptor studies while also helping preserve spatial replication through more efficient sampling. The capacity to explicitly estimate nest success together with breeding occupancy should lead to improved understanding of breeding productivity and the mechanisms of population change.