Abstract: We investigated the activity rhythms and time allocation patterns of common terns (Sterna hirundo) and analyzed their behavioral patterns during breeding season from April to July 2019 in Dafeng Elk National Nature Reserve, Jiangsu Province, China. Infrared cameras were used to record the behaviors during the nest-building period of common terns. Twenty infrared cameras were deployed for a total of 320 d, and 100 nests were recorded, 96 of which recorded common tern activities. The behavior of breeding common terns was dominated by nest-sitting and calling (43.8%±7.7% and 30.1%±7.2% of the total behavioral frequency, respectively), followed by defense (6.2%±2.6%). Other behaviors accounted for a relatively low percentage. Common terns spent 16% more time sitting on the nest at night than during the daytime and 33% less time calling at night than during the daytime. The proportion of nest-sitting behavior was significantly higher in the mid-breeding period than that in the pre-breeding and post-breeding periods (60.0%±3.3%, 30.8%±6.5%, and 40.6%±5.7%, respectively). Nest-sitting had a rhythmicity, with peaks during 12:00-14:00 and 22:00-24:00 and troughs during 06:00-08:00 and 14:00-16:00. In contrast, the other behaviors showed no rhythmicity. Begging was significantly positively correlated with brooding and calling (P<0.01). Feather-grooming was significantly positively correlated with nest-sitting (P<0.01). Foraging was significantly positively correlated with nesting (P<0.01). Brooding was significantly positively correlated with nesting and defending (P<0.05). Foraging was significantly positively correlated with feather-grooming and nest-sitting (P<0.05). No significant correlation was observed for other behaviors. This study provides basic data for the conservation and management of breeding common terns and for subsequent breeding ecological studies.

Key words: Sterna hirundo, breeding season, activity rhythm, time allocation