My understanding was that the outer layer of the exoskeleton of an arthropod
was chitin, a protein carbohydrate compound secreted by the epidermis. There
is very little calcium (0.1 to 0.3 - if any at all) in the exoskeleton or
attached muscles and organs of most insects. The exception being the
crustaceans which reinforce the chitin structures with calcium.
Most growing chicks require calcium at a rate of approx. 1-1.5% by wet
weight. My understanding is that most adults supply calcium to chicks by
gape feeding (Passeriformes, Coraciiformes, et al) by being fed available
calcium carbonate (e.g. bone, crustacean shells, eggshells etc.) or it comes
in the supply of whole animals (i.e. skeleton contained) with birds such as
fish eaters and raptors.
There is probably more calcium in some bird feeder material (e.g. dog
kibble, some seeds) that most insect if the above is correct. This by no
means should be read as support for bird feeders in Australia - I don't do
it for a range of reasons not the least of which is the way it may distort
population structures.
Chris Lloyd
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