Why is the Pigeon Different to All Other Birds?
We see pigeons bathing in pools of water and perhaps the more observant amongst us have noted that whilst most birds fill their beaks with water and to swallow will lift and tilt their heads, pigeons and doves drink without raising their heads. The Rabbis of the Talmud noted this and established Halacha that reflects this characteristic. Chullin 62a, b
כל העופות פוסלין מי חטאת חוץ מן היונה מפני שמוצצת
Why is this bird, the pigeon, different from all other birds?
The Red Heifer is slaughtered and incinerated. Its ashes are sprinkled onto water, which is then, as a step in the purification procedure, sprinkled upon those who have become defiled by contact with a corpse.
This water is known as Mei Chatos (מי חטאת).
This water, if contaminated, is no longer fit for purpose. Accordingly, if a bird other than a pigeon, drinks from the bowl holding this water, it becomes disqualified because some water drips from the beak into the bowl.
Professor G.A. Zweers from the zoological laboratory at the University of Leiden published what is surely the definitive paper on the topic, Drinking of the Pigeon (Columba Livia L.).
Zweers notes that birds drink in many different ways. “Most of them drink like waterfowl; they walk to or through the water, move their beak open and close their beak several times, take some water, tip head and let the water run down by gravity.”
The way in which pigeons drink however, was debated amongst academics which prompted Zweers to analyse their drinking using high tech systems including high speed cameras which allows observing the activity with each frame capturing a moment that is only a fraction of a second ahead of the previous frame.
Zweers concluded that pigeons and a few other birds, use “a double-suction or vacuum-pump model” to drink, i.e. a series of repetitive cycles, each transporting one dose of water into the oesophagus. The cycle comprises five phases:
1, capillary action of the beak tips;
2, lingual suction
3, pharyngeal preparation
4, pharyngeal suction; and
5, oesophageal collection.
By retracting its tongue, the pigeon creates low air pressure (like a piston in a cylinder) in the buccal cavity which 'sucks' water into the mouth.
Another zone of low pressure is generated by depressing its pharynx floor.
Rather than relying on gravity to move water from its beak, pigeons drink water somewhat like we drink from a straw.
Consequently, water it ingests does not drip back and therefore does not disqualify the Mei Chatos.
The Talmud would likely not have known that some parrot species like the parakeet (also known as budgerigar) and the fig parrot also suck to swallow.