During courtship E2 is very active but now Dorothy has to plead with him to get up off the eggs. Dorothy herself is able to sit for 12 hours in a snow storm. How do they do it?
How do birds instantly switch gears from the frantic activity of courtship to sitting on eggs all the time?
They’re cued by hormones. Here’s how:
- As day length increases after the winter solstice, a bird’s hypothalamus releases LHRH (luteinizing hormone releasing hormone).
- LHRH triggers the pituitary gland to release LH (luteinizing hormone).
- LH increases production of testosterone in males and progesterone in females.
- Testosterone triggers aggression, territoriality and sexual behavior. It’s good at the start of breeding but doesn’t help raise a family.
- Progesterone is the “pregnancy hormone” that induces egg production. It’s only needed for a short time since female birds are only ovulating and pregnant until they lay the eggs.
- On the day before incubation begins the hormones switch. Prolactin, the hormone that promotes incubation behavior, rises sharply while the other hormones suddenly decrease. In females, LH and progesterone drop off. In males, testosterone has been dropping since egg laying began. If the male shares incubation he has a sharp rise in prolactin, too. On a graph this hormone switch looks like a sine curve. There’s a moment where all these hormones are low, then prolactin takes off.
In peregrines, both parents have to be ready to incubate at the same time. Their courtship rituals help get the couples’ hormones in synch.
This whole process may sound as if birds are at the mercy of their hormones but in every species reproduction is chemically tuned for success. In humans for instance, progesterone and prolactin switch after delivery so that the mother’s body produces milk to feed the baby. Individual animals whose hormones malfunction do not have live offspring.
So how do birds incubate so nicely? In a word, prolactin.
(photo of Dorothy and E2 from the National Aviary falconcam at the University of Pittsburgh. Today’s Tenth Page is inspired by page 448 of Ornithology by Frank B. Gill.)