Comparing plumage, movement, and summer locations in three dark morphs

We are fortunate that three of our birds have settled in summer locations that are in range of a cellular tower, so we are receiving regular updates on their movements. Two of these individuals took an exciting spring migratory route that we haven’t seen until now. Sefi and Ja’Marr moved along the coast of British Columbia and southeast Alaska, hopping from island to island and passing over cities such as Juneau. Eventually, both birds ended up in south central Alaska. Comparing these two individuals against Ragnar, who ended up at a breeding site in central British Columbia, creates an opportunity to look at the similarities and differences in their plumage, age, and purported sex to provide some interesting context to the movement patterns we are seeing.

These individuals are similar in many aspects of their plumage. They are all dark morphs, with fully red tails. Ja’Marr and Sefi each have harlani like streaking in the breast, while Ragnar is a very deep dark and solid brown throughout the body. The other striking difference is in the tail pattern, where Ragnar has rather thick and regular tail banding while the other two birds have irregular barring that is mostly restricted to the base of the tail, and includes harlani-like spotting. Although Ja’Marr is not breeding (as indicated by it’s more irregular and widespread movement patterns), it is interesting to compare the plumage similarities between Ja’Marr and Sefi, and the proximity to their summer locations, to Ragnar’s breeding location. These individuals are an important contribution to understanding whether or not birds like Sefi and Ja’Marr that occur well within the distribution of harlani represent the phenotypic diversity of that population, or represent plumage traits that come from calurus to the south, and occur because of contact and interbreeding.

The image above illustrates the home range pattern in a heat map for each individual for the month of June. The red clusters indicate areas of high use. Sefi seems to have nested in a site across the bay from Valdez, however there is some spread to the points. Contrasted against Ragnar, who also seems to have bred, the difference is striking. Ragnar has a very centralized, almost circular cluster pattern suggesting that the bird is tied tightly to a nest location. The difference between these two nesting patterns could be explained by sex. Based on morphometrics at the time of capture, Ragnar was tentatively sexed as a male while Sefi was tentatively sexed as a female. At face value, the patterns in their movements don’t make much sense considering their likely sex, since the female is more tied to the nest during the first part of the nesting period, while the male is more random in his movements as he works to provision the incubating female, and later, the young brood.

Ja’Marr’s movements are widespread, with multiple areas of high use. The lack of a centralized home range pattern suggests that Ja’Marr did not breed this summer. This makes sense, since the bird was a second-cycle at the time of capture this past winter. Generally, Red-tailed Hawk do not breed until the end of their third or fourth-cycle. Ja’Marr represents what is termed as a ‘floater’, or an individual that has not yet joined the breeding population. These floaters drift around until they find an opportunity to breed. If all goes well, we may capture when this occurs for this individual. Taken together with other birds we tagged as second-cycles, we can provide some very interesting insight into patterns of movement during the floater period, as well as the timing of recruitment into the breeding population.

Although at the moment we can only mostly speculate about the patterns we are seeing, we are situated to confirm most of our suspicions. For instance, we can confirm the sex of each individual using molecular techniques, and take a more systematic approach to analyzing the data to investigate what factors (such as sex, age, etc) best explain the movement patterns we see. There is a lot of exciting things to dig into, so stay tuned for more.

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