Development of Methods for Bioacoustic Monitoring of Leach's and Fork-tailed Storm-petrels at Castle Rock National Wildlife Refuge: 2007-2009 Report

Storm-petrels are small, nocturnal seabirds that spend their entire life on the ocean, except when nesting on offshore islands. In northern California, breeding populations of Leach’s Storm-petrels (Oceanodroma leucorhoa) and Fork-tailed Storm-petrels (Oceanodroma furcata) have not been monitored since the last state-wide breeding survey of seabird colonies in 1989. The objective of this study was to investigate the efficacy of a novel technique, bioacoustic monitoring, to confirm the presence of Leach’s and Fork-tailed Storm-petrels at their breeding colonies. Also, a standardized sampling regime was developed to monitor population trends for Leach’s Storm-petrels using vocalization rates. This research was conducted at the largest seabird breeding colony in northern California, Castle Rock National Wildlife Refuge. During 134.5 hours where audio recordings were reviewed from 2007-2009, a total of 12,654 Leach’s Storm-petrel vocalizations and 10 Fork-tailed Storm-petrel vocalizations were detected. For Leach’s Storm-petrels, calling peaked between the hours of 02:00 and 03:00 and the rate of vocalization was consistently high during the month of May and, as a result, this time was selected as the most appropriate time for recording Leach’s Storm-petrels vocalizations to monitor long-term and large-scale population trends. Traditional techniques to monitor storm-petrels are infrequent and often lack precision. However, if population trends are not assessed at regular intervals, corrective conservation and management is not possible. Unless techniques are developed to monitor storm-petrels more frequently and precisely, catastrophic declines could go unnoticed for decades. For storm-petrels, bioacoustic techniques can provide frequent information regarding species presence and relative changes in abundance.