Bit of a new concept for me, something that I've assumed occurred previously, but was really clearly demonstrated by the satellite tracking of Lesser Black-backed Gulls from Pennsylvania... A strong impression that birds are more than willing to travel huge distances both north and south (particularly in spring?) in response to local weather conditions... Some of these Lesser Black-backs flew hundreds of kilometers back and forth (north and south), presumably following warm/cold fronts, before finally settling in more northerly latitudes. After years of birding Point Pelee in May, I had definitely noticed that cold fronts would bring a "return" of species more associated with colder weather, with one of the best examples being Iceland Gulls (including adults), which would suddenly be observed in flight over Lake Erie at the tip in mid-May, after being generally absent. I had (presumably falsely) assumed that these adult Iceland Gulls had been lingering on some offshore island and the cooler weather simply convinced them to "fly around more" and pass by the point, when in reality they may have just been driven hundreds of kilometers southwards (off of Hudson Bay?) by cold/stormy weather, and would then presumably fly right back back north again when the next warm front arrived. Here are some screenshots of LBBG which went both north and south, often hundreds of kilometers, multiple times during a single spring migration (note the "loops"):
Given that a species like Iceland Gull becomes (potentially) detectable in mid May, I can't help but wonder how many "non-detectable" birds have also flown south during the same weather event (i.e., how many Herring or Ring-billed Gulls have also made dramatic "reverse" migrations, but are not readily detectable among local breeders?). And what about other families of birds?
It is well known that other families (i.e., blackbirds) will fly south in the spring in response to severe weather (i.e., heavy snow), but I've started noticing a potential pattern of this occurring throughout the spring (i.e., not just when it snows). I had noticed on several occasions (often at the tip of Point Pelee in May) that particularly cold days were ideal conditions to find late or lingering species (i.e., Dark-eyed Junco or Winter Wren). I had always assumed that their "desire" to migrate north was strong, given that they were late, and had crossed Lake Erie from the south despite the cool weather. But now I am starting to wonder if these birds had actually flown *south* WITH the passing cold front???
On Jun 11, 2020 I was monitoring a "morning flight" along the Lake Erie shoreline. My focus was on potentially displaced songbirds following several severe thunderstorms the previous night, which were also associated with the passage of a moderate cold front. While not a "morning flight" bird, I was kept company by a Palm Warbler for most of the morning... Which is very *late* for the species to be migrating... But was it late? Or had it simply decided to fly south the previous night, to avoid poor weather conditions, and was then prepared to go back north in short order? The most anomalous cold temperatures anywhere in the northern hemisphere were occuring in northern Ontario at the same time...
I can't help but wonder how often a bird is willing to do this? Does it happen every year? Or is it possible that a spring migration can happen without any "reverse" movement? Or is it standard to "cycle" their way north? I assume that MOTUS/Sat tagging birds is the only way to really get an answer?
I also believe that quite a few late (or even early) migrants are undertaking behaviours that we don't really understand - and aren't actually reflective of the "record late" or "record early" labels we give them.