Friday, April 19, 2013

What Makes Birds Incubate?

Red-tailed hawk Stitch incubating eggs
Looking out my window this morning, it seems unlikely that spring will ever arrive. But most of the birds we watch are incubating eggs or, in the case of bald eagles, feeding young. Clearly, they aren't waiting for the ice and snow to melt.

Daylight length has a profound effect on the physiology of birds. As the length of daylight increases, light receptors deep in the brains of birds detect the changing day length and trigger the seasonal development of the reproductive system. Hormones flow and reproductive organs increase in size. Courtship begins and pair-bonds are formed or re-established between male and female birds. Nest work kicks into overdrive: eagles and hawks fetch in sticks and greenery, while falcons create a scape, digging into gravel or substrate with their breast and pushing with their legs to create a depression for the eggs.

So what makes active birds like hawks, falcons, and eagles spend so much time sitting on eggs? Like the rest of bird reproductive behavior, egg-sitting is triggered by hormones. Initially, luteinizing hormones trigger the production of testosterone in males and progesterone in females. Testosterone triggers sexual behavior and aggression in male birds, while progesterone induces egg production in female birds. Birds are really louder and brighter in the spring, when bird song, plumage, and courting behavior are in full swing. However, shortly before incubation, female birds (and male birds that share incubation duties) experience another big hormonal change. Prolactin, a hormone which promotes incubation in birds, rises sharply while other hormones decrease. Opioid peptides stimulate prolactin secretion, which may explain why even active birds become lethargic while incubating their eggs.

The eagles have young and Stitch and Spot's eggs are ready to hatch. Over the next 30-40 days, the peregrine falcons will enter what Bob calls the incubation doldrums. We'll watch them sit, and fritter with gravel, and sit, and sit some more. The induced lethargy will end quickly once eggs begin hatching, and prolactin (and presumably opioid peptides) begin declining.

Prolactin plays an interesting role in bird parenting behavior. We'll return to it a little later in the season. See the bottom of the page for my links.


This chart shows the relationship between testosterone and prolactin in a male emu. Note the sharpness of the curves.

Things that helped me learn and write about this subject:


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