A very windy week! And still very REVI!

Over the past week (from August 30 to September 5) we experienced strong and persistent winds for four days; winds that were fierce enough to completely preclude banding on two days and allow only a subset of the nets to be open on the other two. As a consequence, the number of mist-net hours realized for this week has been the lowest on record, accounting for only 51% of the potential. On August 30, there was a ferocious North wind under an overcast sky that kept the nets closed and most birds safely tucked away in the trees and shrubs. On September 2 and 3, the wind was warm and from the South blowing furiously without respite, slightly less so on the second day, permitting the most sheltered nets to be open. It was another morning of strong and warm South wind on September 5, albeit diminishing enough towards the end of the morning to open all the nets.

Nonetheless, when we could open the nets, we captured good numbers of birds, notably on the perfectly calm day of September 4 we banded 70 birds of 27 species and 14 recaptured of four species. Once again, the stars of the day were REVI (Red-eyed Vireos) with 31 banded and nine recaptured. Now, you would recall, if you’re an attentive and assiduous reader of the blog, that a day with 31 banded REVI already happened on August 24 this year, making it the absolute one-day record, smashing to pieces the 18-year-old record of 19 REVIs on one day in the fall of 2005. So, yeah! It happened again! What can I say? It’s a REVI season. It will be very interesting to compare these two fall seasons (2005 and 2023) in details once the (feather) dust has settled and we have time to examine the similarities and differences. 

With REVI banded on every day of banding this week, even with a paltry two birds on September 3, the season total now stands at 211 birds, getting ever closer to the all-time-record of 239 REVI in the fall of 2005. The question now is not if we will break the record, but when! The migration of Red-eyed Vireo through Cabot Head takes place throughout September, with the occasional bird seen and/or captured in October. It’s only a matter of time!

There’s a saying that ‘a bird in the hand is better than two in the bush’. This is certainly true when it comes to be able to evaluate its physical conditions (wing chord, fat level, and weight, notably). By that same token, a bird captured twice is better than a bird captured only once, as it enables comparisons between days. And so it was that a young Red-eyed Vireo was captured for the first time on August 31, where its fat level was noted at 0 (zero; on a scale of 0 to 7) and its weight was 17g. Four days later, on September 4, the same bird was recaptured, this time with a fat level of 6 (‘greatly bulging’) and a weight of 21g. In this short span of time, it increased its weight by 23.5%, a remarkable physiological feast and a sure proof that this bird is getting ready for the long and sustained effort of migration. Fat is the preferred fuel for migrating birds, providing more energy per unit than proteins or carbohydrates. It’s energy they need in order to fly 10 or 12 hours non-stop at night, over a series of multiples flights to reach their final winter destination.  

With its bountiful energy stores (and a good measure of luck!), our young REVI will fly all the way to the Amazon in Brazil, with no assistance from its parents, guided by an internal compass and clock. Unfortunately, this bird – along with all long-distance migrants – will encounter many dangers on its way: bad weather, lack of food at stopover sites, predators like Sharp-shinned Hawks or feral/outdoor cats, collisions with windows, disorientation from artificial lights of ever-expanding cities, etc. It’s hard not to wish them luck as they embark on this, at times treacherous, journey.

While certainly remarkable, it was not all REVI this past week. We noted a marked surge in numbers of Swainson’s Thrush as well as the first Gray-cheeked Thrush of the season. The first (two) American Pipits were heard and seen flying overhead on September 2, with another one the following day. A Blue-gray Gnatcatcher was briefly seen on the windy day of August 28. This species – rare on the Bruce peninsula – is seen almost every spring but not as regularly in the fall: it was detected in 11 fall seasons from 2002 to 2022. 

We also banded not one, but two, Broad-winged Hawks this week!!! (yes, I believe that three exclamation marks are warranted on this occasion) On Sunday, September 3, and on Tuesday, September 5, in both cases, a young Broad-winged Hawk flew into one of our nets, only to be plucked out by an extremely happy bander in charge. In 22 years of migration monitoring, they are the third and fourth Broad-winged Hawks ever captured. The first one was in the spring of 2007, when we released it unbanded: they take special ‘lock-on’ bands that we didn’t have at the time. The second one was on September 7, 2021, which, this time, was duly banded. So, to get not just another one, but two, in a span of three days simply defies the laws of probability. Although the wise would say that any event with a probability of not zero can happen (see, lotteries…). 

For comparison purposes, a large REVI might have a wing chord of 80mm and a heavy one a weight of 21g, a Broad-winged Hawk, on the other hand, has a wing chord of about 270mm and a weight of 330g for one and 321g for the other. (The wing chord is the measure of a folded wing (i.e. held against the bird’s body), from ‘wrist’ to wingtip, which is different from a wingspan that is the distance from one wingtip to another)

Like REVI’s, Broad-winged Hawks migrate to the southern hemisphere. But unlike REVIs,  Broad-winged Hawks are soaring raptors, using thermals to gain altitude, barely flapping their wings, and gliding effortlessly in their intended direction in search of another thermal. Since no thermals develop over large bodies of water, this creates bottlenecks that concentrate Broad-winged Hawks and other soaring raptors in vast numbers. One such site is Holiday Beach (Ontario) at the western end of Lake Erie, on the north shore, where counts have been done every fall since 1974 (https://hbmo.ca). On September 23, 2022, for example, 33,000 Broad-winged Hawks were counted! Of course, further South, hawks pile on as they encounter the huge Gulf of Mexico and veer along its northern shore. With mountains rising on the west and the shimmering blue expanse of the gulf on the East, birds get pinched in the bottleneck of Vera Cruz, Mexico, where some fall seasons, over five million birds of prey (of various species) are counted, peak days of Broad-winged Hawks reaching into 200,000 birds (https://pronaturaveracruz.org/vrr/pgs_conteo.php). A ‘hawkwatcher’s’ dream (for when I finally retire from Cabot Head!).