Birds possess a system for eye lubrication and protection, which includes structures analogous to human tear ducts and glands. While they produce tears, their mechanism and purpose differ from the emotional crying observed in humans.
The Avian Lacrimal System
Birds have a specialized lacrimal system for tear production and drainage. This system involves two types of glands: the lacrimal gland and the Harderian gland. Unlike in mammals, the Harderian gland is often the main source of tears in birds, playing a more prominent role than the lacrimal gland in many avian species. These glands produce a complex tear film, a multi-layered coating that covers the ocular surface. This tear film consists of a mucin layer closest to the eye, an aqueous (watery) middle layer, and an outer lipid (oily) layer.
The tear film moistens and lubricates the eye’s surface, protects it from foreign bodies and infectious agents, and provides oxygen and nutrients to the cornea. Tears also contain components that contribute to immune protection. After production, tears are distributed across the eye by the nictitating membrane, often called the third eyelid. This transparent membrane sweeps horizontally across the eye, spreading the tear film and removing debris.
The lacrimal system also includes a drainage pathway to drain fluid and debris from the eye. Tears drain through small openings called puncta lacrimalia, located at the nasal corner of the eye. From there, the fluid enters a lacrimal duct, which directs secretions into the choana, an opening in the roof of the bird’s mouth, bypassing the nasal cavity. This drainage system explains why tears do not spill down a bird’s face.
Understanding Avian Eye Secretions
Birds do not produce tears as an emotional response to sadness or distress. Their tears serve physiological purposes, primarily for eye lubrication, cleaning, and protection.
If a bird exhibits watery eyes or excessive eye secretions, it typically indicates a physical issue rather than an emotional state. Such symptoms can be a sign of irritation, the presence of foreign objects, or an underlying health problem.
Common causes for increased eye secretions in birds include infections (bacterial, viral, or fungal), trauma to the eye, or even allergies. Conditions like conjunctivitis, characterized by inflammation of the eye, can lead to increased tear production and crusting around the eye. Respiratory diseases can also manifest with eye symptoms, such as foamy or sticky eyes and facial swelling. Observing watery eyes in a bird suggests a need for veterinary attention to determine the cause and appropriate treatment.