Gizzard Mass Dynamics of Nestling Rooks Corvus Frugilegus

Abstract

Developmental changes of the gizzard have rarely been studied in nestlings of non-domesticated birds or under natural conditions. Because nestlings would have variable amounts of stomach contents, we wanted to discover whether in natural conditions gizzard developmental changes are dependent on digesta mass. We examined the mass of gizzard and digesta in 56 nestling Rooks Corvus frugilegus aged between 1 and 13 days old. We found that gizzard mass increased nearly nine-fold, from an average of 1.25 g to 10.85 g. Gizzard mass expressed as a proportion of body weight significantly decreased with the age of nestlings. In hatchlings, the median ratio of gizzard mass to body weight was 18.4%; in 13-day old nestlings this value was only 5.8%. Tarsus length, a poor predictor of gizzard mass, explained 61% of its total variance. Nestlings (regardless of their age) without digesta or with small volumes of digesta had statistically significantly smaller gizzards (expressed as ratio of gizzard mass to body weight) than did nestlings with full gizzards, which suggest a rapid response of gizzard volume and musculature to variable food intake during post-hatch development.