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Growth and energy requirements of captive Great Blue Herons (Ardea Herodias) Bennett, Darin Chris

Abstract

The energy requirements of Great Blue Herons (Ardea herodias) during critical stages of their life-history when food is considered limiting were investigated. The metabolizable energy coefficients (MEC) of herring (Clupea harengus), mackerel (Scomber scombrus) and trout (Oncorhynchus mykiss) when fed to Great Blue Herons were determined. Estimates of apparent MEC of mackerel and trout were affected by the level of energy intake. Correcting for endogenous energy losses in the excreta yielded estimates of true MEC that were independent of energy intake. True MEC of mackerel and trout averaged 0.866 ± 0.014. Correcting for nitrogen retention did not improve the estimates of MEC. Estimates of MEC of herring were highly variable, possibly due to rancidity of the herring. A continuous feeding trial was used to determine the energy requirements of 12 hand-reared Great Blue Heron chicks. Male (n=8) and female (n=4) chicks did not differ in their hatching mass, but, by day 30 the mass of male chicks was significantly greater than the mass of female chicks. Male chicks were about 13% heavier than female chicks at fledging. Metabolizable energy intake (EIME) and energy requirements did not differ between male and female chicks. EIME was maximum between days 26 and 41 at 2027 ± 25 kJ/d. Total energy required for maintenance (kJ/d) was greatest between 30 and 39 d of age. Total energy required for gain (kJ/d) was greatest between 10 and 29 d of age, the time of maximum growth. The maintenance energy requirements (EMAIN) of captive male Great Blue Herons were determined by a continuous feeding trial through two autumn and winter periods (year 1, n = 4; year 2, n = 10) that differed in the severity of their winter months. Body mass increased through the autumn to a maximum in early winter and declined during late winter. Herons gained about 650 g between August and the time of peak winter mass. This reserve could potentially meet a heron's total maintenance energy requirements for about 6 d. The lower critical temperature for energy intake and EMAIN were 7.9°C and 7.6°C, respectively. The findings in this thesis provide evidence to support field observations that indicate the importance of food limitations as determinants of life-history traits in herons.

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