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.