Importance of marine prey to growth of estuarine tern chicks: evidence from an energetic balance model

Abstract

We assessed the effects of quality and quantity of prey species on the growth of Little Tern Sterna albifrons chicks raised in salinas (salt-pans), Algarve, Portugal. An energetic balance model was developed, which estimated the total amount of energy ingested by all chicks in a nest and the energy expenditure by the chicks when either growing alone or in groups of two and three per nest. Energy intake per chick depended on the energy available per nest and a competition value for food between chicks. Energy expenditure was defined by the basal metabolic rate (BMR), which depends on chick\u2019s weight, plus a multiple of BMR, obtained by calibration and accounting for unspecified energetic losses. Prey species ingested by Little Tern chicks were mainly the fishes sand- smelt Atherina spp., Sardine Sardina pilchardus, Garfish Belone belone and mummichog Fundulus spp., but also two types of shrimp (Paleo- monetes spp. and Paleomon spp.). Although prey species more abundant in salinas (shrimps and Fundulus spp.) were delivered at a higher rate, and Fundulus spp. had the greatest calorific content, chick growth was more sensitive to alterations in the ingestion of typically marine fish species, such as Sardina pilchardus and Belone belone. Model predictions were in agreement with data obtained in the field, as adults breeding in salinas foraged regularly at sea. Practical considerations from the pre- sent results are: (1) birds breeding in salinas rely on a sufficient amount of typically marine prey to ensure the growth of their chicks; (2) in some years foraging conditions in the salinas can be favourable, appar- ently because high food abundance (plus proximity to nests) may com- pensate for relatively poor quality of food types encountered