Daily and seasonal changes of vocal activity of the Common Crane Grus grus: implications for conservation and monitoring efforts

The vocal activity of Common Cranes Grus grus varies with time of day, season, population abundance, and nesting habitat area, and consequently affects the accuracy and precision of single survey estimates of breeding population size. To examine (1) how the vocal activity of Common Cranes varies during the day and seasonally, (2) how these changes affect the probability of observer detection of a pair during a single survey, and (3) which environmental variables influence vocal activity. Autonomous sound recorders recorded soundscapes (24 hours per day) in 24 occupied territories of Common Cranes, three times during the breeding season. Recordings were analysed by manual spectrogram scanning. The probability of detection was calculated for four survey durations: 10 and 30 min, one and two hours. Environmental characteristics were measured from satellite images. Common Crane vocal activity during a single survey significantly varied during the day, decreased with the progress of the breeding season, negatively correlated with the area of nesting habitat, and positively correlated with the number of neighbouring pairs within 2 km. 24-hour surveys recorded at least one vocalization in 96% of occupied territories in March, 67% in April, and 46% in May. The highest probability of detection during a single, one-hour survey ranged from 0.46 in March, 0.25 in April to 0.17 in May. Reducing survey duration from 1 h to 10 min caused a decrease of detection to 0.29 in March and 0.13 in April and May. We recommend surveying Common Cranes at the beginning of the breeding season, from one hour before sunrise to four hours after sunrise and from sunset to one hour after sunset, to ensure the highest (0.33–0.46) probability of detection during a single, one-hour survey. Alternatively, autonomous sound recorders may ensure detection of almost 100% of breeding pairs during a single, 24-hour survey at the beginning of the breeding season.