The reliance of birds over insects for nutrition is an interesting ecological phenomenon. The reliance varies from being obligatory to facultative, but the major chunk of bird population are some way or the other depend on insects for nutrition.
Though its apparent that heavier birds prefer large insects and vice versa. Analysing the faecal content of birds could invariably answer their preference over the type of insect.
I do not know if there is a standard protocol. But usually the collected bird is placed temporarily (about 10 to 20 min.) in a bag made of cotton cloth and lined with paper towel for easy collection of faecal matter. After collecting the material containing the food items of the captured birds, its content was placed in Petri for stereoscope microscope analysis. The material found in good condition in the feces of the birds was fixed in alcohol 70%.
For adult insects it is interesting to try to find parts of the body that aid in identifying: parts of the head, wings and legs that differ for insect orders, which may facilitate the probable determination at more specific levels. For massed larvae it is more difficult.
Various sampling techniques are discussed in Southwood’s Ecological methods (currently Henderson & Southwood, 4th edition, 2016). I think it would be worthwhile to consult this excellent manual.
Tricky work! But you can "dilute" the fecal into water and the fecal will degenerated. Then, you can observe the undigested materials from the fecal, usually the hard insect's exoskeleton can be found, but in small pieces. Carefully find the distinguish character that may help you to identify the insect. Probably you can go only to Order level. But try to search for previous studies too! So you can narrow down your search. Good luck!
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