Fat fuels the journey!

Fat is the primary fuel for bird migration. To sustain the immense energy demand of sustained flight, fat provides, weight for weight, 7-9 times more energy than alternative fuels, and thus ensures the maximum energy storage for the minimum weight gain. An added advantage is that its oxidation yields water, contributing to another need during long-distance flight. Before migrating, birds accumulate large reserves of fat by eating almost constantly, a behaviour called hyperphagia. As always in nature, there are tremendous variations between species and even between populations of the same species. Depending on their migration, birds may need to stop and “refuel” during so-called stopovers, pointing to the need of finding good foraging habitats along the entire path of their travel. During bird banding, the amount of food deposited by migratory birds can be easily evaluated from their body weights and their fat ‘scores’ based on the yellowish fat that can be seen through the skin when the feathers are blown aside. The fat score is on a scale of zero to seven: at Cabot Head, very often, birds have a fat score of 3 or 4, but birds with no fat at all are also captured. They will need to replenish their stores before embarking on the next leg of their journey. The highest score, 7, is extremely rare to encounter. Of course, it is a visual score and there could be variations in how banders interpret the scale but a “seven” bird is fondly nicknamed a butter bird: the bird is bulging with fat from the furculum (the cavity under the neck) to the wing pits and the abdomen. In the previous 19 years of banding, only seven birds have been given a score of 7, including two Swainson’s Thrushes.

This spring, on May 21, we caught another butter bird, a Swainson’s Thrush with a fat score of 7! Its weight was 40.1g, an astonishing 30% more than the other Swainson’s Thrush caught on that day (weight of 31.3g with a fat score of 4). The average weight of the 445 Swainson’s Thrushes banded in the spring between 2002 and 2020 is 30.9g (± 2.8g) and only three previous thrushes weighed more than 40g (with a record of 43.2g in spring 2017). Pictures of the heavy and normal thrushes being weighed were taken and can be seen on Instagram at @brucepeninsulabirds.

This past week we experienced lots of unsettled weather, mostly with strong South wind precluding most nets from being open and one morning of much-needed rain. As a result, migration was not very active, although the FOY Cedar Waxwings were seen on May 20. This species is always detected quite late at Cabot Head. Large flocks of Blue Jays were also a daily occurrence: the northern fringe of this mostly sedentary species is migratory and Blue Jays can be seen sometime in impressive flocks of over a hundred birds milling around in the wind along the shoreline at Cabot Head.

A few Common Terns have taken residence in Wingfield Basin and the adjacent shallow lakes, thrilling us with their acrobatic flight. I do have a soft spot for terns and am always delighted to see them, even on a daily basis. 

Red-eyed Vireos have finally returned from wintering in the Amazon Basin, with two birds first detected on May 20. A few individuals are now incessantly proclaiming their presence by their perpetual singing. If one bird can be the definition of persistence, it is the Red-eyed Vireo: no afternoon lull in singing, no lunch break, no rest! The Cree name of the Vireo translates loosely to “the one who counts the leaves in the forest” because it never stops singing. 

Many other species have also started singing and establishing territories in the fading spring, the ubiquitous American Redstarts chief among them but also Ovenbird, Black-throated Warbler, Black-and-white Warbler and the newly arrived Black-billed Cuckoo.

I also briefly heard the singing of a Mourning Warbler on May 22: it is one of the latest species of warblers to arrive, although with large variations across the years. We are certainly entering the last few weeks of spring migration, waiting for the late species (Yellow-bellied Flycatcher and Gray-cheeked Thrush, for example) to make an appearance. Very soon, it will be summer and breeding time.

Leave a Reply

Your email address will not be published. Required fields are marked *

*