Conclusion
The differences in the morphology of the upper, lower and third eyelid
seem to strongly correlate with the lifestyle of the birds reflecting an
adaptation to the habitat and feeding. The structure of the skin and
conjunctiva of both upper and lower eyelids contains many defensive
immune cells that maintain the safety of the eye and the hydration
moreover, the lower eyelid’s CALT constitutes the majority of CALT
tissue in Elanus caeruleus , and is considered part of the mucosal
immune system.