Influence of environmental factors and energetic value of food on Little Tern Sterna albifrons chick growth and food delivery

Abstract

Capsule Chick diet and early growth parameters differed between birds breeding on natural (sandy beaches) and alternative (salinas) habitats. Aims To describe diet and growth of Little Tern chicks, to examine the influence of tide and wind speed on food delivered to chicks in southern Portugal, and to determine whether diet and early chick growth parameters differed between birds breeding in natural (sandy beaches) and man-made alternative (salinas) habitats. Methods Nests were marked and visited regularly. Chicks were ringed and weighed at hatching. Between 12 and 14 nests were enclosed by a fence in order to measure and weigh chicks every day. Prey items delivered to fenced chicks were observed daily from hides, in different tidal periods and wind speed conditions. Prey items dropped in the colonies were collected. Foraging efficiency of adults was evaluated in relation to tide and wind speed. Fresh prey items were collected to determine their energetic content using a calorimeter. Results A chicks grew faster than B chicks, but both attained the same weight, tarsus-length and wing-length at 19 days old; C chicks attained the same values at day 21. There were significant differences in early growth parameters. A and B chicks grew faster on sandy beaches than on salinas. The main prey items were Atherina spp., Fundulus sp. and shrimps in salinas and Sardina pilchardus, Atherina spp. and Belone belone on sandy beaches. There was no correlation between ingestion rate and chick age, but mean length of prey ingested by chicks increased significantly with age. Conclusion Tide and wind speed influenced both size and type of prey delivered to chicks. Parents met the increasing energy demands of the growing chicks by adjusting prey size rather than prey delivery frequency. Differences in chick growth between salinas and sandy beaches could be related to both parental quality and diet. The presence of a high energy-content euryhaline fish species in the salinas, together with more sheltered conditions and no tidal influences, may be important in explaining the use of salinas by Little Terns.