Wednesday, April 30, 2014

Decorah: Why is Mom Being So Mean to Dad?

We’ve had several questions and comments about Mom’s behavior lately. Why is Mom mad at Dad? Why is Mom so demanding? Why is Mom so mean? While I can find snippets of behavior that would seem loving to human observers – shared incubation duty, mutual nest control, and Dad feeding Mom, to name a few – both adults are going through hormonal changes and their behavior reflects that.

Daylight length, or photoperiod, strongly influences hormone production in birds. In the northern hemisphere, our story begins shortly after the winter solstice in December. As daylight length increases, a cascade of hormones causes birds’ gonads to swell, increasing the production of testosterone in males and progesterone (plus a small amount of testosterone) in females. Testosterone is associated with aggression, territoriality, courtship, nest-building and, in males, testicular development and spermatogenesis, while progesterone, the “pregnancy hormone”, induces egg production in females.

Mom and Dad share incubation duties, so both of them experience another hormonal change once incubation begins. Production of prolactin, a hormone that induces incubation and stimulates brood patch development, rises sharply, while testosterone and progesterone production rapidly decrease. Opioid peptides help stimulate prolactin production, which may be another reason that normally active birds suddenly want to spend the entire day sitting on eggs.

So in the first part of their reproductive cycle, Mom and Dad’s interactions with one another and their young are mediated by hormones that stimulate courtship, mating, territoriality, egg-laying, and lethargy. We humans are moved by their relationship with one another and their tender devotion to their offspring. It’s hard not to see hearts everywhere – I know I did! – as Mom pursues Dad around the nest, Dad brings food gifts to Mom, and both eagles work together to keep their eggs safe from all the extremes Iowa’s winter and early spring can bring.

And then they start shaking the prolactin off.

If the eagles’ earlier behavior added up to love, it’s hard not to see this as its opposite. Mom suddenly seems mean, snappy, or demanding to some watchers. Dad still loves his offspring but seems more distant. In this narrative, our eagle couple is drifting apart – or maybe Mom’s behavior will cause Dad to reject her for a less snappy, more appreciative mate. While compelling to human watchers, this scenario isn’t true.

So what is happening to our eagles? It isn't eagle divorce, but it isn't entirely our imagination, either. As their gonads begin shrinking, they decrease courtship and pair bonding behaviors. As prolactin ebbs, their metabolisms speed up, they become more physically active, their body fat drops slightly, and they probably become hungrier. Mom’s whistling ‘tea-kettle’ makes its first appearance as vocalizations change, although it still stimulates food delivery and/or an appearance by Dad. What we interpret as a falling out is simply a pair of mature, active bald eagles beginning to resume the non-reproductive phase of their lives. To paraphrase Scott Weidensaul, sex hormones pull many strings in a bird’s body. We are seeing that in Decorah right now.



Things that helped me write this post:



Did you know? 

In humans, females are xx (homogametic) and males are xy (heterogametic). But in birds, females are zw (heterogametic) and males are zz (homogametic). Unlike humans, female birds determine the gender of their offspring. http://en.wikipedia.org/wiki/ZW_sex-determination_system

28 comments:

Roberta said...

Great post, thank you!

foxfire7352 said...

Thanks for all this information, very interesting!

Lotus C said...

Thank you, so facinating!!

Nan Carol Manley said...

Thank you Amy. Great information.

Carole Brandt said...

Very interesting and informative! Thank you! Amazing!

Flamingo Tina said...

Thanks so much. It is hard not to 'humanize' these two as we spend so much time watching and learning.

JZ said...

Always interesting posts, thank you. We watch Eagles all year here on Lake Charlevoix in Mi.

RuthMItchell said...

Thank you so much for the wonderful information..I have so enjoyed learning more and more about these magnificent animals!!!
I can see a parallel in human marriages...I have been married for 45 years and I can tell you that things are NOT the same as they were when we first fell in love!!! Of course we don't go through the cycle every YEAR ( thank goodness) but I can see similarities!!!

Patricia Oshkosh said...

Very informative thank you!

Just Me said...

Very interesting. Thank you so much.

l.a.mari said...

Illuminating info. Thank you so much.

Hnldude said...

Super informative!! Thanks and aloha from Hononlulu from a former Iowan.

LilPupsMom said...

I am so glad you guys post information like this! Thanks for all you do to keep us educated!

SharonDoesBaskets said...

Great post. :)

marney said...

this is very good information to have thanks

beacher creature said...

Thank you so much, Amy. Interesting information.

beacher creature said...
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Pam Eastridge said...

Thank you! Very interesting information!

Gloria said...

What great information!!!

drchachere said...

Thank you for teaching us, Amy. I have learned so much from the blog and cam, and it makes me want to learn more.

Hillarie said...

Do the parent eagles remain in contact year-round or just during the reproductive phase?

goodwish said...

Very informative. Ruth Mitchell - may be pure biological information would prevent so many emotional divorces!

goodwish said...
This comment has been removed by the author.
Braillists RRISD said...

Does anyone know what the eaglets are? Boys or girls?

Sharon said...

Thank you Amy, and to all the other Mods (who I often refer to as Gods as well), for your information, education and continual willingness to put up with repeated questions. You and all the other mods/bloggers on other sites I follow have no idea how much you are appreciated especially when things do not go well. Personally, Bob Anderson is one of my heros. Sharon

Chuck Lantz said...

When friends ask for advice on how to deal with a broken heart, either theirs or the "other half" of the equation, I usually try to lighten their mood by saying that, some day, someone will invent a pill that will cure this common malady. After reading this article, I'm convinced that the invention of such a pill is much closer to reality than I thought.

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