We first blogged on this subject on April 30, 2014, but we’re asked about Mom’s behavior every spring. Wondering where the love went? This blog is for you!
We’ve had several questions and comments about our eagle Moms. Why are they so demanding? Why are they mad at their mates? Why are they so mean? While I can find snippets of behavior that seem loving to human observers – shared incubation duty, mutual nest defense, and tandem feedings, 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, days begin getting longer after the winter solstice. 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.
Both male and female eagles incubate eggs, 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 its reproductive cycle, an eagle’s behavior is mediated by hormones that stimulate courtship, mating, territoriality, sperm production, egg-laying, and lethargy. We humans are moved by the eagles’ devotion to one another and the tender care they give their young. We see love in bloom as adult eagles court, mate, bring and accept food gifts, and work together to keep egglets and hatchling eagles safe from all of the extremes that 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. Females may suddenly seem mean, snappy, or demanding to some watchers. Males care for their offspring but may seem less involved in day to day nest life once eggs hatch. In this narrative, our eagle couples are drifting apart – or maybe a female’s behavior will cause her mate 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. A female eagle’s whistling ‘tea-kettle’ makes its first appearance as vocalizations change, although it still stimulates food delivery and/or an appearance by her mate. 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 at our nests 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). This means that female birds determine the gender of their offspring. You can read more about that here: https://raptorresource.blogspot.com/2015/02/sex-determination-in-birds.html