8 dec. 2009

Not Flying South

Backyard feeders plus a strange sense of direction may have begun to split one bird species into two.
In southern Germany, some 10 percent of blackcaps (Sylvia atricapilla) fly not south, toward warmth, but rather northwest for the winter, says evolutionary biologist H. Martin Schaefer of the University of Freiburg in Germany. This novel journey, on record since the 1960s, probably became survivable thanks to the rise of backyard bird feeding in Britain, he says. Enthusiasts setting out seed and suet have kept the birds from starving until it’s time to wing back to Germany to nest.
The returnees from Britain nest in the same German forests as the more conventional migrators that fly to Spain. Yet the two groups now show subtle but distinct genetic and visible physical differences, Schaefer and his colleagues report online December 3 in Current Biology.
“It’s a good example of how humans can influence evolutionary trajectories,” Schaefer says.“The really cool thing here is that it seems to be driven by migration,” says behavioral ecologist Jeff Podos of the University of Massachusetts in Amherst. Other researchers have looked for these kinds of genetic differences between populations with different migration destinations, but have not found any, he says.
One or just a few genes are thought to control the migration direction for blackcaps, Schaefer notes. A classic study showed that offspring of parents that flew in divergent directions for winter grew up to migrate in an intermediate direction.
Migratory genes would, of course, differ between differently migrating populations, but Schaefer and his colleagues wanted to look for other genetic differences as well. Northwesters and southwesters varied slightly in parts of their genome that don’t involve migration. Small as that difference was, it was greater than the difference between southwest migrators hailing from far-flung parts of Germany, says Schaefer.
Northwester birds also had slightly rounder wings than the southwesters, the researchers said. Rounder wings improve flight maneuverability but don’t perform as well on long hauls. The northwester birds fly only two-thirds the distance the others do, so the slight shift in wing shape might be the beginning of adaptation to their new route.
Likewise, beaks on Anglophile blackcaps tended to take on a different shape, one more like the narrow beaks on generalist feeders. Schaefer speculates that winter feasting on olives and other big, fruity mouthfuls encourages wider beaks in the Spanish migrators. These blackcaps are more likely to choose mates that have similar winter tastes, thus preserving migratory differences in their offspring. The two bird groups aren’t new species yet, Schaefer says. But they’ve taken a few flights in that direction. More than 50 species that have recently rerouted their migrations may be experiencing similar changes, Schaefer says.

Sediment dynamics in the Rhine catchment :

Quantification of fluvial response to climate change and human impact
Publisher: www.knag.nl
Author: Erkens, Gilles

Pointer's birth place
Fluvial systems are strongly respon- sive to changes in climate and land use — but take their time to show it. Accurate prediction of the timing and degree of future fluvial response requires comprehensive understanding of fluvial response in the past. This PhD-thesis studied the response of the river Rhine over the last 20,000 years, as recorded in the morphology, composition and volumes of its sediment. Borehole data and various dating techniques were combined to time-slice cross-sections and to construct a series of palaeogeographic maps. A method to calculate amounts of sediment carried both in suspension and as bed load during millennia-long time slices was developed. This research shows that only climate changes during the glacial-interglacial transition and the strong rise of human land use during the late Holocene were large enough to trigger response and overwhelm ongoing autogenic behaviour of the river and the buffering capacities of the Rhine catchment. The climatic changes at the glacial-interglacial transition triggered incision and abandonment of the Pleniglacial braidplain terrace along the entire Rhine valley. This was accompanied by a shift from a braided to a meandering fluvial style, but it took several millennia to complete the transition to a single meandering channel. As a result, the Rhine was a multi-channel river during the Late Glacial and most of early Holocene. The last phase of fluvial response to the glacial-interglacial transition consists of increased delivery of fine sediment during the first half of the Holocene (until ~ 6000 years ago). This sediment had been stored in the upstream tributary valleys since the full glacial, and was released by incision of the Rhine and its tributaries when they adapted to the interglacial climate conditions.

Overall, this study shows that sizable fluvial catchments adapted slowly to large scale climatic changes, in both spatially and temporally complex ways. During the Holocene other external factors than climate change became important for the development of the Rhine system. In the Rhine delta, the combination of eustatic sea level rise and subsidence provided in large accommodation space for sediment delivered from upstream. Release of bed load by the meandering and incising Rhine in the trunk valley forms the source for bed load delivery to the delta. Both the Rhine trunk valley and delta are sinks for suspended sediment, and the amount of stored sediment in these areas was used to reconstruct fine sediment transfer through the lower Rhine. Following the aforementioned increased delivery of fine sediment during the first part of the Holocene, sediment transfer stabilised during the middle Holocene. During the last 2000 years the delivery of fine sediments to the sinks is up to 150 % higher compared to the directly preceding millennia. This can only be explained as the result of progressive deforestations for agricultural land use since the late-Neolithic (6300 years ago). The timing of the increased delivery of fine sediments suggests that pre-Roman land use was already extensive. Hence, prehistoric land use should be regarded as a drainage basin-wide forcing factor for fluvial systems.