by Dr. Dale Mierau
November 25, 2014
I observe the behavior of Bald Eagles (Haliaeetus leucocephalus) during the summer breeding season on Lac La Ronge, a large lake in the pre-Cambrian shield of Saskatchewan. At that location, Bald Eagle eggs hatch during the third week of May after an incubation period of 35 days. The two, sometimes three, Bald Eagles eggs hatch three days apart. The young that hatch first have a significant advantage for survival over subsequent hatchlings and the norm (on Lac La Ronge) is for just one hatchling to survive past July 1. Bald Eagles are known to practice infanticide and siblicide. Such radical behavior is likely associated with the availability of food or the ability to provide it.
A Bald Eagle hatchling develops the ability to regulate its own body temperature (thermoregulation) by the fourth week. Before that, one adult, usually the female, must preside over the nest. After week-four, the nestling no longer needs the adult to provide shelter and it can be left alone in the nest. My first glimpse of this three-week-old hatchling was on June 8, 2014
The primary food source for Bald Eagles on Lac La Ronge is fish with a preference for burbot (Lota Lota), sucker (Catostomus commersonii) or cisco (Coregonus artedi). Bald Eagles are not known to eat Walleye. Bald Eagles will take waterfowl and other small game but I have not seen Bald Eagles on Lac La Ronge feed their young anything but fish. Both adults can provide food. However, it is my experience that the male does most of the feeding of young until mid-summer.
In 2014, I was lucky to observe a breeding pair that raised two hatchlings that survived to leave the nest in September. The relatively small nest was in a coniferous tree on a small island approximately six kilometers across some open water from my home island.
The sequence of images of the pair of Bald Eagles feeding their four-week old hatchlings was taken on June 20, 2014 at 4 PM. The weather was dreary with heavy overcast, spotty rain and light winds. On this occasion the female adult brought a live fish to the nest, killed it, tore it into small chunks and then fed one hatchling at time. She always fed the older, darker-colored male hatchling first. When he was finished eating he moved away, or was moved away, before the adult would feed his younger sister. Food brought to the nest was always fed to the young before the adults ate. In mid-to-late June, mealtime seemed to be a pleasant affair with all being treated fairly as long as there was enough food to go around
The mood at mealtime wasn’t nearly so congenial on July 25th (Figure 10).
The older, more aggressive nestling took the entire Cisco for itself and was not interested in sharing. This image was taken under horrible weather conditions and in my view is just barely salvageable. However, I like it because, to me, it does a better job of illustrating the mood in the nest than other images of higher quality.
The fledgling in figure 11 is the same bird that is seen in figure 1 above as a hatchling. He demonstrated defiance and aggression on August 17th when he arrived at the nest and started a fight seconds after the adult landed with a fish.
I travel the large, sometimes dangerous, lake in a Lund Alaskan 20 DC, equipped with a 90 hp Honda four-stroke outboard motor (Figure 12).
This photograph of the boat was taken on the blustery day in 2013 when wind gusts topped 70 km/hr. In these conditions I’m forced to move the boat to a sheltered area on the home island to avoid damage from wind and waves.
The images in this article were taken with an smc Pentax 300mm DA Star F/4 telephoto lens on a Pentax K3. I use a Pentax 1.2x-magnifying eyepiece to assist with manual focus. The camera was in aperture priority mode with the f-stop at f/5.6. The iso was set at 400. The images of the four-week-olds being fed were taken at 1/320 second, except the last one in the series, which was taken at 1/250 second. The image taken in July was taken at 1/180 second and the one in August at 1/1250 second. The camera and lens were mounted on a Seven Oaks shoulder mount with a Sirui TY-50 quick release. I’ve modified the Seven Oaks apparatus so I can operate the camera with one hand leaving the other free to maneuver the boat. The images were processed on an iMac with Adobe Lightroom 4.
Dr. Dale Mierau
479 First Avenue North
See previous articles by Dr. Mierau on the Canadian Nature Photographer