Page 633 - Hand rearing birds second

634 Hand-Rearing Birds

Figure­41.4­ Syringe-feeding a Black-billed Cuckoo nestling. Source: photo courtesy of Diane Winn/Avian
Haven.

Comparative­Analyses­and Evaluation

As previously noted, primary nutrients of concern in a rehabilitation setting should be calcium,
vitamin A, and, if adequate sunlight or full-spectrum lighting is unavailable, vitamin D. In addi-
tion, moisture levels of diets can be a critical component for success of nestling chicks, and other
nutrients, including fatty acids, fiber, and carotenoids are worthy of consideration in nestling diets.
Moisture content of the insect-substitute diets, and insects themselves, ranges from 55 to 85%
moisture. Water is a critical component of the diets of hatchling and nestling birds, as evaporative
losses in small birds can be very high. Very small birds (e.g. less than 50 g) can lose up to 25% of
body weight as water per day (Bartholomew and Cade 1963), and Psittacine hatchlings (Nymphicus
hollandicus) have been shown to require ~ 80% moisture for the first few days of life for optimal
survival and fledging (Roudybush and Grau 1986). The hydration level of the chick is often appar-
ent in a smooth, “plump” appearance of skin versus the wrinkled, “tight” appearance of the skin
of a poorly hydrated chick.
The calcium content of insects is generally low, as previously noted, but can be modified by sup-
plementation (e.g. gut-loading, dusting), resulting in insect-based diets containing close to 1% cal-
cium (dry matter basis). Insect-substitute diets are generally higher in calcium, resulting in
~1.4–2.1% dietary calcium (dry matter basis, Table 41.4). Unfortunately, almost nothing is known
about the daily calcium requirement for most species of nestling wild passerines. In very occa-
sional reports of the accretion of body calcium with age as the skeleton mineralizes, less than
30 mg calcium per day has been estimated, depending on age and species (e.g. Common Blackbird
(T. merula), Bilby and Widdowson 1971; Eastern Bluebird (Sialia sialis) Hungerford et al. 1993).
Using the blackbird as an example, and based on predicted basal metabolic rate (Aschoff and Pohl
1970), consequential daily energy intake, and relative bioavailability of calcium at 68% (based on
work in domesticated poultry using a variety of sources) (Soares 1995), the reported deposition of
calcium into body tissues during growth would require ~ 1.08% dietary calcium. Obviously, this is
merely a calculation and should be considered as such, but it does correspond well to the recom-
mendations made for nestling passerines (i.e. 1.5–2.0% calcium; Duerr 2007), and with formulated
levels of calcium in various insect-substitute diets. However, users of daily supplements should
remain mindful that excessive calcium intake can cause several problems, including visceral gout,
kidney nephrosis, and secondary deficiencies of several other minerals (see Klasing 1998).
Dietary fatty acids, while not defined in each of the insect-substitute diets, are critical not only to
chick development via energy provisioning, but also to immune function. The recent focus on