Subtropical Roseate Terns (Sterna dougallii) of Santa Maria (Azores) deliver more appropriate sized prey to their chicks than Common Terns (Sterna hirundo)

Abstract

We compared chick food provisioning between Roseate Tern (Sterna dougallii) and Common Tern (Sterna hirundo) on Vila, an islet offshore Santa Maria Island (36.9ºN, 25ºW), Azores in 1996. Twelve nests of each species were fenced and prey deliveries to chicks of both species were observed during three diurnal periods each day for 35 days. We identified all prey offered to chicks, registered whether chicks ingested prey or not and weighed chicks daily. Blue jack mackerel (Trachurus picturatus) and Atlantic sauri (Scomberosox sauri and Nanicthys simulans) were the main prey items offered to chicks by Roseate Terns, whereas Common Terns offered mainly blue jack mackerel and boarfish (Capros aper). The number of Atlantic sauri offered to chicks with more than 6 days old by both tern species decreased markedly, and the inverse occurred for blue jack mackerel and boarfish. Overall, non-ingested prey items were larger than those that were ingested, especially for chicks aged 1-12 days. Roseate Tern chicks showed a higher acceptance rate (frequency of prey ingested/frequency of prey offered) than did the Common Tern chicks. Acceptance rate of the chicks increased with chick age for both tern species but, overall, Roseate Tern adults made a better adjustment of prey delivered to chicks (in particular those aged 1-12 days) than did the Common Terns. The breeding strategy of the Roseate Tern might reflect a greater specialization on favourable marine fish species.