Date: 7/10/19 4:29 pm From: DJ Lauten and KACastelein <deweysage...> Subject: [obol] Re: shorebird question
On 7/10/2019 7:36 AM, <t4c1x...> wrote: > > Does anybody know why the heaviest movement of migrating shorebirds in > spring seems to coincide with a strong headwind from the north? > Darrel
I would say we have some ideas of what is happening, but I am uncertain any of us truly "knows" exactly what is up. But here is a few cents......
First, we need to clarify a bit. There are two related things happening: one is the normal northward movement of shorebirds heading to major spring stopover locations on their way to the arctic nesting grounds. The second is the weather conditions that coincide with the time span that the birds are traveling north based on daylight length, and therefore, hormonal control of migration timing. So in other words, shorebirds are going to migrate north in a pretty tight spring window span no matter the weather conditions. But they will utilize weather conditions to make their trip efficient. It so happens that we human birders observe this behavior, and in some years along the outer coast the birds are really concentrated and numerous. In other years, not so much. We see birds, but not as numerous nor concentrated. It also so happens that when we see the former event, it is almost always in high pressure, stiff northwest windy conditions. When it is not like this, we don't see as many birds, but they are still somewhere, migrating north - either further out over water, further inland over the mountains, or higher in altitude. When there is high pressure and resulting northwest winds, we see amazing numbers of birds. My assessment of the situation is that the high pressure does two things, it pushes, or keeps the birds at much lower altitudes. But more importantly, we may think that flying into the wind is difficult, maybe costly, but it is exactly what birds do. When a bird opens it's wing, it unfolds an airfoil, which creates lift, exactly like an airplane does. The lift is created by a difference in airspeed over and under the wing, resulting in a mini low pressure effect that causes lift. When a bird closes it's wing, gravity instantly takes effect and pulls the bird down. This uplift followed by a down pull creates momentum, which translate into forward motion. All a bird has to do is open it's wing, and close it, and it's moving forward. It's as simple as humans walking, they, and we, just do it. So while it may not make sense to us, it makes perfect sense to a bird. Ask any pilot which direction they take off into - into the wind or with it? And land for that matter. I suspect when we do not see these large movements, the birds are finding the right altitude and location to do the same thing - so they get up much higher into those winds and just cruise along, flipping open their wings occasionally to stay afloat. But if the high pressure is so strong that it is counterproductive to fly up high, you simply stay near the ground and head north. And we silly humans see them and are totally amazed, and ponder why or how they are doing it! It is very amazing, and when it happens we should all watch in awe, but it is happening every year, just mostly above our heads and/or out of sight.