Between-lake differences in the diet and provisioning behaviour of Black-throated Divers Gavia arctica breeding in Scotland

Surveys at 23 Black-throated Diver breeding lakes in Scotland showed large between-lake differences in the species and size range of potential prey. The study lakes were classified into four types according to the main size-taxa prey classes present. Type 1 lakes lacked small fish, Type 4 lacked salmonids < 300 mm and Types 2 and 3 both had all four of the main prey classes. Diver diet for 30 families at the study lakes was quantified from 7943 prey items seen fed to chicks, and 153 items seen eaten by adults, during 662 h of observations. At all lake types adults mostly ate salmonids of 120–240 mm in length, especially Brown Trout Salmo trutta , and European Eels Anguilla anguilla up to c.  350 mm. The prey eaten by chicks ranged widely, from c.  0.03 g to 300 g, with important implications for feeding behaviour. Young chicks (days 1–8) rejected items greater than 70 mm long. The diet of young chicks consisted mostly of small fish, either Three-spined Sticklebacks Gasterosteus aculeatus (Type 2 lakes) or Minnow Phoxinus phoxinus (Type 3 and Type 4 lakes). At other lakes (Type 1) it consisted mostly of mayfly larvae (Ephemeroptera); invertebrates were the only abundant potential small prey in these lakes. At all lakes the chick diet became more similar to that of adults as chicks grew. Adults provisioning chicks used two foraging strategies. In 'excursion foraging', adults hunted away from the chicks and carried prey back for them; in 'attendance foraging', the chicks accompanied the foraging adult(s). Excursion foraging was used mostly before day 4 and was strongly and positively correlated with the abundance of fish < 80 mm in length. This behaviour is probably adaptive, but is probably tenable only where small fish are relatively abundant. Young chicks fed mainly on small fish had higher survival rates than those fed on invertebrates.