Behavioral hypotheses suggest that RSD allows the larger females to better distribute food for the nestlings, defend the nest against predators, and maintain pair bonds. Smaller males may also be better able to protect the nest because they are more maneuverable.
This explanation focuses on the benefits of RSD during the nestling phase: 1. Since male raptors provide most of the prey for their mates and offspring during the breeding season, they must develop quickly to become adequate hunters and therefore breed successfully.
It is true that male nestlings generally develop more rapidly than their sisters, and leave the nest sooner. Given their faster growth rate, if male nestlings were as large as females, there would be a higher risk of siblicide in which males would kill the females. The following table provides a breakdown of these ideas and offers even more hypotheses things to ponder!
The true reasons for RSD may be species-specific and involve a combination of these factors. What we do know is that female bald eagles are larger than males and their system seems to be working! Why do you think the Hanover male is larger than the female? What benefits could arise from their difference in size? Do you think the Hanover nestling is a female or male? As a parting challenge, take a look at the following photos and see if you can tell which bird is the leading lady.
Photo Credit: Sara Hysong-Shimazu. Photo Credit: April Ryan. Bortolotti, G. Journal of Field Ornithology , 55 4 , Dzus, E. Does sex-biased hatching order in bald eagles vary with food resources? In many bird species, it's easy to tell males from females because the plumage is different.
Not so with eagles! However, we can use size measurements from different parts of the eagle's body to determine the gender of the eagles we capture. Female eagles generally have larger wings, feet, talons, beak, etc. A biologist by the name of Bortolotti discovered that 2 size measurements show the greatest separation between male and female.
The two measurements are:. These measurements can be used in the following formula:. Here are the key measurements from 6 of our eagles, plus their weights. Can you determine the gender of these birds yourself?
Click here for student worksheet. Females cover more of the incubation duties and incubate when they choose. In effect, males fill in for the female when she wants to be relieved. This dynamic is readily apparent when observing shift changes. If the male returns to the nest to relieve the female without being called she may or may not accommodate the male regardless of how vigorously he attempts to replace her.
By comparison, if the female returns to the nest she will supplant the male regardless of how long he has been incubating. Average division of incubation duties between male and female bald eagles in the Chesapeake Bay. Composite data from seven eagle nests monitored with video cameras. Data from CCB. The gender disparity was driven primarily by the female taking the night shift.
Night shifts averaged 13 hours and 20 minutes, or more than half of the hour cycle. During the daylight hours between AM and PM, the pair split incubation duties relatively evenly. A male bald eagle rt along the James River waits patiently to relieve the female of her incubation duties after the long night shift. Photo by Bryan Watts. One of the more interesting aspects of the team effort is that the length of the night shift imposes a basic structure on the daily pattern of incubation.
The most predictable shift change occurs around dawn after the long shift performed by the female. The male is punctual in relieving the female and often performs his longest shift of the day.
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