Long-term adaptations of a migratory bird (Little Tern Sternula albifrons) to quasi-natural flooding disturbance

• A migratory bird species Little Tern Sternula albifrons responded to the hydrological fluctuation.
• Wavelet transformation identified time-delayed relationship between estuarine discharge and Little Tern abundance.
• Korean Monsoon timing of yeart-1 influenced on the next year's newly arriving Little Tern individuals.
• The migratory bird species is thought to adapt to quasi-natural disturbance caused by a regional climate characteristic.

Population size of migratory birds responds to various types of environmental factors, which affect different stages of bird population. In the current study, we analyzed a long-term avifauna monitoring database to investigate population changes in the migratory bird Little Tern, Sternula albifrons, with respect to a quasi-natural disturbance, flooding, induced by a regional characteristic climate pattern, the Korean Monsoon (KM), in the Nakdong River Estuary of South Korea. We scrutinized the time-delayed influence by means of wavelet transformation and year-to-year comparison. Little Tern individuals started to grow in April, and reached its maximum generally in May or June, and an abrupt decrease of individual number was observed after KM occurred. Sequential time-series analysis based on wavelet transformation revealed that the changing pattern of the population size of Little Tern in the estuary was linked to the previous year's flooding (a 9- to 10-month delay), which is regarded as the time difference between Little Tern breeding season and previous KM period. Stronger flooding waters during KM season expelled comparably more individuals of Little Tern from the estuary (r2 = 0.595; p < 0.05; n = 9), and the more the individuals left, the smaller the bird arrived at the estuary in the next year. Further examination revealed that earlier initiation and longer duration of KM in yeart − 1 negatively affected the newly arriving Little Tern individuals in the current year (i.e., yeart; r2 = 0.809 for impact of KM onset, r2 = 0.909 for impact of KM duration; n = 10, respectively; p < 0.005). The Little Tern population gradually increased when summer flooding was not strong in the previous successive years, from which we concluded that the population of the migratory bird Little Tern tends to adapt to quasi-natural disturbance (flooding) to maintain their population size.