Knowledge of energy requirements for growth, maintenance and reproduction at a species-specific level is limited. Where information is available, values are used specific to the species in question. In other instances, assumptions are made in calculations based on general knowledge of nutritional requirements of birds of similar species.
Basal Metabolic Rate
This refers to the minimum energy required by an animal in a post-absorptive state, at rest in an environment that is within the thermoneutral zone.(27)
The BMR of the osprey is estimated to be 3.2kcal/kg/hour(5). For an osprey weighing an average of 1.5kg, the BMR could be calculated as follows:
1.5kg X 3.2kcal/kg/hour = 5.12kcal/hour x 24 hours = 115kcal per day
115kcal is equal to 481.16kJ per day
The BMR of a bald eagle is estimated to be 1252.3kJ per day(20).
Maintenance Energy Requirement
The maintenance energy requirement of an animal includes not only the BMR, but also the energy produced in the form of heat increment of feeding, energy required for thermoregulation, and the energy required to perform regular daily activities essential for life, such as foraging and hunting(27). The maintenance energy requirement can be roughly calculated by multiplying the BMR by a factor of 2.5(27).
Osprey: 2.5 X 481.16kJ per day = 1202.9kJ per day
Bald Eagle: 2.5 X 1252.3kJ per day = 3130.75kJ per day
Reproductive Energy Requirement
An osprey’s egg are small, lightweight and relatively affordable in terms of the cost of energy required for production, and there does not appear to be a correlation between caloric intake and egg size or clutch size.(5) It is estimated that egg laying increases the daily energy requirements by only 18% of the BMR(5).
Female Osprey: 0.18 X 481.16kJ = 86.61kJ
86.61kJ + 1202.9kJ = 1289.51kJ
The energy cost of egg production for bald eagles is significantly more, with egg laying being estimated to increase daily energy expenditure by as much as 39% of the BMR.(27)
Female Bald Eagle: 0.39 X 1252.3kJ = 488.40kJ
488.40kJ + 3130.75kJ = 3619.15kJ
The reproductive energy requirements of male birds is considerably less. It is estimated that the energy required for testicular growth & semen production only increases the daily energy expenditure by 1% of the BMR.(27)
Male Osprey: 0.01 X 481.16kJ = 4.81kJ
4.81kJ + 1202.9kJ = 1207.71kJ
Male Bald Eagle: 0.01 X 1252.3kJ = 12.52kJ
12.52kJ + 3130.75kJ = 3143.27kJ
Energy Requirements for Growth
Hatchlings have a considerably lower BMR, typically only around 43% of the BMR requirements of an adult.(27) However, the maintenance energy requirements are significantly greater than an adult in order to support growth, maintain core body temperature etc, as much as 150% of the adult MER.(27)
Osprey hatchling BMR: 0.43 X 481.16kJ = 206.89kJ
Osprey hatchling MER: 1.5 X 1202.9kJ = 1804.35kJ
Bald eagle hatchling BMR: 0.43 X 1252.3kJ = 538.49kJ
Bald eagle hatchling MER: 1.5 X 3130.75 = 4696.13kJ
The Osprey | The Bald Eagle 