We analyzed variability in growth of Red Kite (Milvus milvus) chicks using biometrical measures and urea and alkaline phosphatase blood levels. We used variables estimating relative mass and tarsus length growth rates during the 5-7 days before every blood extraction (short-term growth) and deviations of size (mass and tarsus length) from population averages at the age of blood extraction (long-term growth). We found a significant relationship between blood levels of alkaline phosphatase and short-term tarsus growth rate but not with long term tarsus growth. Urea levels were correlated with long-term mass growth but not with short-term growth. We found some chicks with relatively rapid mass growth, slow tarsus growth, and low levels of alkaline phosphatase after periods of fasting. Chicks of Imperial Eagles (Aquila adalberti) showed less variable nestling periods but higher and more variable blood urea levels. These differences agree with theoretical predictions about the evolution of growth regulation abilities in relation to feeding ecology; the species with a more variable growth and more resistance to food deprivation can experience a more irregular and unpredictable food supply.

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