To improve existing marsh bird survey protocols, we need to evaluate closure assumptions

Abstract

Abstract Our collective understanding of secretive marsh birds has increased in the past decades due to the development and implementation of the North American Standardized Marsh Bird Monitoring Protocol (hereafter, Protocol). The Protocol proposes call broadcast surveys to increase vocalization and detection rates within 3 standardized survey periods aimed at surveying peak breeding activity for a suite of secretive marsh birds. We noted a trend in the literature linking occupancy modeling with the survey design from the Protocol, despite some evidence that vocalizations decline across survey periods, which could indicate lack of population closure. An underlying assumption of occupancy modeling is closure, and the Protocol was designed to focus on only birds which will remain in an area throughout the breeding season and not migrants that may only be present in the first survey period. Including migrating marsh birds, especially if a large percentage of marsh bird detections are migrants, can bias occupancy estimates and lead to erroneous density and population size estimates that may affect conclusions about habitat resource and bird associations. We urge researchers and managers to carefully consider the analytical and field techniques when designing studies for marsh birds and to not simply pair the biweekly survey design within the Protocol with occupancy modeling and ignore closure assumptions, turnover rates, and potential differences in resource use by migrating and breeding marsh birds. Specifically, we suggest that researchers consider short survey interval times (e.g., ~2 days rather than 2 weeks) or continuous call monitoring using automated recorded devices deployed for 1–2 weeks per survey location when using occupancy analysis on secretive marsh birds. We also call for future study of turnover rates, stopover duration, and vocalization rates during migration and breeding periods to better inform study designs and increase the appropriateness of statistical analysis.

Keywords

• call‐broadcast surveys
• closure
• detection
• marsh birds
• migration