Fat is good!

Fat is fuel for migrating birds. As I mention at least once every season in the blog, fat is what provides the best bang for your buck in terms of energy when you are a travelling bird. Birds during migration spend the non-travelling hours feeding as much as possible to accumulate fat, which they will burn during their long flights. Of course, there is a trade-off on how much fat one bird can put on: the heaviest one gets, the more energy must be expended to stay aloft. Different strategies can be used: at the extremes, a bird can put on very little fat reserves and do short flights with many stops to “refuel” along the way, or a bird can load on fat as much as possible before a ridiculously non-stop flight with no resupply. The Bar-tailed Godwits breeding in Alaska belong to the latter category: prior to their oceanic flight, they almost double their body weight before embarking on an eight- to nine-day (and night) non-stop flight to New Zealand where they spend the winter.

Most birds, however, fall into a happy middle ground. During banding at Cabot Head, we routinely check fat deposits (visible by gently blowing feathers apart on their bodies where they store fat: under the neck, under the wings, and on the belly)) of birds we band. Fat score is documented on a 0 to 7 scale. Many birds have a 0-fat score, which could mean they are not quite in migratory mode or they have used up all their fat and need to replenish their reserves. Often, birds with fat reserves have a 3- or 4-fat scores. Rarely, very, very rarely, a bird scores a 7-fat: that was the case for eight birds in 20 years of banding and 62,593 birds banded. This fall, on September 30, the 301st Red-eyed Vireo (REVI) banded was such a 7, with a weight of 24.8g. The following day, the 302nd REVI (and so far, last) was only a 1-fat with a weight of 17.5g. The average weight of the 302 banded REVI this fall (to date) is 17.3 ± 1.4g (range: 14.8 – 24.8), with the second heaviest weight being 21.2g and only 18 birds weighing 20g or more. So, our 301st REVI was 43% heavier than the average REVI this fall. In the previous fall seasons (2002 – 2021), the heaviest REVI was captured on September 26, 2019 and weighed 23.8g.

As forecasted, the weather this past week was unseasonably warm, very warm. Days alternated between being very quiet or relatively busy at the nets. Golden-crowned Kinglets have started to move in good numbers through Cabot Head and are now the most abundant bird being banded on any given day: 30 Golden-crowned Kinglets banded on September 30 and 48 on October 3, for example. On that last day, we banded a total of 78 birds of 17 species, which is the highest number of birds banded in a day for the season so far. The distant second and third species were Dark-eyed Junco (six birds banded) and Common Yellowthroat (five birds). We also captured the first Fox Sparrow of the season, as well as the first Yellow-bellied Sapsucker. The young male Pileated Woodpecker may claim the title of best bird captured for that day though, despite its ear-piercing calls. The largest of woodpeckers in Canada (and North America if we accept the sad and cruel extinction of Ivory-billed and Imperial Woodpeckers) is not often caught, not least because the small mesh size of our nets (i.e. designed to catch passerines of light weight) makes it easy for them to bounce off the nets. This species has been captured in 13 of the previous 21 Fall seasons, only one bird in each season, except in 2015 (two birds) and 2019 (three birds). 

In visual migration, we are still observing a few Northern Flickers and Blue Jays but the bulk of them are behind us now. Likewise, for Canada Goose. The first Horned Grebes, on the contrary, were seen on October 2. We are keeping an eye over the bay as October is the time when ducks and grebes and loons migrate in numbers.

Keeping an eye on the bay, keeping an eye in the sky and the trees, checking the nets every 30 minutes: it is a busy life at Cabot Head!