Why do cocks take fewer breaths per minute than hens?

[centrarchid]

Producing eggs is a lot of work, like rapid growth. Check out ventilation rate of Cornish X chickens while growing fast, they breath like humming birds (you do not have those so look at swallows). I keep a lot of chickens in a maintenance mode year round. It allows seeing how they are all about energy conservation. When they are working to scrounge up a meal their metabolism ramps up. When they have crop fill they ramp metabolism / ventilation rate again. Fish follow same pattern, enough to locally depress oxygen levels at times.

Pregnant mammals also ventilate heavily. Egg production in birds has a lot in common with late pregnancy in mammals.

 

[centrarchid]

When I check crop fill of my free-range harem masters, they go into night with very little. Same with hens covering chicks on roost. In contrast hens in lay and fast growing immature chickens go into night with very full crops. More food requires more oxygen, like a fire. Fire of life.