Growth development of bald eagle chicks in the Chesapeake Bay population has been a study subject by biologists at The Center for Conservation Biology. Here is a paper by Dr. Bryan Watts on bald eagle chick weight gain and food requirements.
Stages of Bald Eagle Chick Development – Bald eagle chicks progress through three general phases of development as nestlings including 1) structural growth, 2) feather growth, and 3) neurological and behavioral development. These phases overlap extensively but do exhibit a clear sequence from structural through neurological development. Structural growth includes primary tissue growth (e.g. skeletal and muscular systems). This phase of growth occurs early in the development period and accounts for virtually all of the increases in mass or weight. Different body elements grow at different times through the growth sequence. For example, feet reach their terminal size very early on in development whereas wings and flight muscles develop at a later stage. This sequencing coincides with the “need” for these elements. As nestlings, eagles progress through three different sets of feathers including natal down, thermal down, and juvenile feathers. Chicks are hatched with a coat of natal down. This down is very light in color and does not have much insulating ability such that chicks must be brooded by an adult for warmth. Natal down is replaced by thermal down beginning around 10 days of age. Thermal down has very good insulating qualities and by 15 days chicks are typically able to thermoregulate on their own. The emergence of juvenile feathers including contour and flight feathers typically begins on or before 27 days. These dark feathers are arranged in tracts and emerge in sequence beginning with the capital (head) and dorsal (back) tracts. The ventral (belly) tract is the last feather group to emerge. Flight feathers including the wing and tail continue to grow throughout the development period reaching their maximum lengths around the time of fledging. The last phase of development as nestlings is neurological or the learning of coordinated movements. This includes walking, feeding, and flying. The final month of development is consumed by acquiring the coordination needed for movement and flight.
Pattern of Growth – Like all birds, eagles exhibit an asymptotic pattern of growth meaning that they begin slowly, have a rapid phase of growth, followed by a slowing of growth to terminal size. Adult males are smaller than females such that they reach their terminal sizes at younger ages. In the Chesapeake Bay, males average 3,150 grams (7-8 pounds) and females average 4,225 grams (10-12 pounds). During their period of rapid growth, chicks gain 140-150 grams/day. T90 (time required to reach 90% of terminal weight) is reached between the age of 36 and 42 days depending on gender.
Peak Energy Demand – Depending on temperature and activity, etc. adult bald eagles require approximately 500 grams of fish/day. This includes energy for metabolism and maintenance as well as energy for flying and other activities. Energy demand for chicks includes both maintenance or metabolic requirements, as well as, energy for growth or the development of new tissues. Peak energy demand occurs in the later stage of rapid growth when metabolic demand is high due to large body size and when chicks are still growing at a rapid rate. After terminal size is reached, energy demand declines to that required for maintenance alone and resembles that of adults. In the Chesapeake Bay, peak energy demand occurs around 30 to 35 days of age. One of the most reliable outward signs that chicks have moved past the period of most rapid growth and peak energy demand is when they begin to appear mostly feathered. This appearance reflects that the transition between structural growth and feather growth has been made.
Provisioning – Provisioning of eagle broods within the Chesapeake Bay averages between 3 and 4 fish/day during the period of peak demand. Beyond this period, provisioning is closer to 3 fish/day.
Botanical Garden Brood – When the female was killed on 26 April, 2011, all of the chicks within the botanical garden brood had reached 42 days and had reached T90. This suggests that the brood was well beyond peak energetic demand. The brood had begun the transition to self feeding and the development of juvenile feathers was well advanced. Maintenance of this brood should not have required more than 3 fish per day through the period of fledging. Depending on fish size, it is likely that they could have fledged with an average provisioning rate of below 3 fish/day.
Markham, A. C. and B. D. Watts. 2008. The influence of salinity on provisioning rates and nestling growth in bald eagles in the lower Chesapeake Bay. Condor 110:183-187.
Markham, A. C. and B. D. Watts. 2008. The influence of salinity on the diet of nesting bald eagles. Journal of Raptor Research 42:99-109.
Watts, B. D. Assessment of body size for Bald Eagles of the Chesapeake Bay. Submitted to Northeastern Naturalist.