Some of the other birders there that day were far more knowledgeable than I - about Reedlings, certainly, and perhaps about birds in general. From them I learned that there is a small resident population of Reedlings in the extensive reed beds near the old Oslo airport, but that they die out some years if temperatures stay in the -20s for a couple of weeks, as is not all that unusual. When that happens, it can be a few years before a good breeding season and good winter survival numbers in Southern Sweden push some of their Reedlings northward in search of a new territory. From that wave of migrants, the local population rebuilds in fits and starts, depending on food supply. Oslo is about as far north as they can go, since they feed by day and they need at least as many hours of that as they get in Oslo in order to eat enough reed seeds to fuel them through the winter nights. Survival for them comes down to staying above certain theshold combinations of seed availability and quality, daylight hours, and temperatures. Too cold, too few or too nutrition-meager seeds, and/or too few hours to eat in ... and the Reedlings will die. While they look quite fat to us, staying warm through a Nordic winter night burns up pretty much all the body fat they manage to put on during the day. They have only a wee bit in reserve to get through a day or two or three when the weather prevents them from foraging. I hope they made it this year, or course for their own sake, but also because I'd like to visit them again next winter. :)
Now I need to have dinner and a quick nap after 16.5 km (about 10 miles) of survey hike today, then I'll settle in to enjoy a good bird photo browse.
