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628 Hand-Rearing Birds

on a dry-weight basis (Reichle et al. 1969; Gist and Crossley 1975; Graveland and van Gijzen 1994;
Oonincx and Dierenfeld 2012; Spranghers et al. 2017). Earthworms from soil in chalky areas con-
tain significant amounts of calcium for nestling common blackbirds (Turdus merula) and song
thrushes (Turdus philomelos) (Bilby and Widdowson 1971).
While calculations suggest that insects contain insufficient calcium for most insectivores, in the
wild, insectivorous animals seek out either prey unusually rich in calcium or other sources of cal-
cium, especially while breeding. For example, Collared and Pied Flycatchers (Ficedula albicollis
and F. hypoleuca) preferentially feed on calcium-rich isopods during reproduction (Bureš and
Weidinger 2003). Isopods were also found by to be unusually prominent in the diet of nestling
European Starlings (Sturnus vulgaris) (Moore 1986). Birds may also obtain extra calcium via the
ingestion of eggshells, snail and clam shells, calcareous grit, bones, crawfish exoskeletons, lime-
stone gravel, or mortar (St. Louis and Breebaart 1991; Graveland and van Gijzen 1994; Graveland
1996; Dhondt and Hochachka 2001).
Most insects and other invertebrates appear to be good sources of magnesium, sodium, and
potassium, as well as the trace minerals iron, zinc, copper, and manganese. Invertebrates with a
mineralized exoskeleton have higher amounts of magnesium and manganese, while flies appear to
be a rich source of iron (Oonincx and Dierenfeld 2012; Finke 2013). Mineral composition in gen-
eral probably largely reflects the food sources of the insect, both that present in the gastrointestinal
tract and that incorporated into the insect’s body because of the food it consumed. Studies of wild
insects show seasonal variation as well as variations between different populations of the same
species living in the same general area (Reichle et al. 1969; Studier and Sevick 1992; Graveland and
van Gijzen 1994).
Vitamin A is provided by a group of compounds composed of both retinol (preformed vitamin
A) and various carotenoids, such as β-carotene, α-carotene, and β-cryptoxanthin. Like most verte-
brates, insects obtain retinoids via the cleavage of these carotenoids (Von Lintig 2012). However,
unlike vertebrates in which cleavage takes place largely in the intestine, insects convert carote-
noids to retinoids only in the compound eye (Von Lintig 2012). This explains why adult insects
contain very low levels of vitamin A/retinoids and why holometabolous insect larvae, which lack
compound eyes, do not contain retinoids (Pennino et al. 1991; Barker et al. 1998; Giovannucci and
Stephenson 1999; Finke 2002, 2013; Oonincx and Dierenfeld 2012).
The conversion of carotenoids to vitamin A (retinol) varies widely among animal species, but
conversion takes place in several species of birds, including chickens, turkeys, quail, and geese
(Olson 1989). High levels of carotenoids, including those that can be converted to vitamin A, are
found in various wild insect species, whereas commercially-produced insects contain far lower
quantities (Finke 2002, 2013, 2015b; Isaksson and Andersson 2007; Eeva et al. 2010). While most
commercially-produced insects contain few carotenoids, insect carotenoid levels can be enhanced
by feeding them a carotenoid-enriched diet (Finke 2015b).
Data on vitamin D in insects are limited. Most commercial insects contain low levels of vitamin
D (typically <400 IU/kg dry matter) (Finke 2002, 2013; Oonincx et al. 2010). More data is needed
to better understand the vitamin D content of wild insects and its application to feeding
insectivores.
The vitamin E content of commercial feeder insects varies widely (Pennino et al. 1991; Barker
et al. 1998; Finke 2002, 2015b; Hatt et al. 2003). This variation is likely due to diet resulting in dif-
ferent amounts of vitamin E being incorporated in the insect’s tissues as well as from food remain-
ing in the insect’s gut. In contrast, the limited information available suggests that wild insects
contain higher levels of vitamin E than those typically found in commercially-raised insects
(Pennino et al. 1991; Barker et al. 1998; Cerda et al. 2001; Finke 2015a).