Natural selection shaped the rise and fall of passenger pigeon genomic diversity.

The extinct passenger pigeon was once the most abundant bird in North America, and possibly the world. Although theory predicts that large populations will be more genetically diverse, passenger pigeon genetic diversity was surprisingly low. To investigate this disconnect, we analyzed 41 mitochondrial and 4 nuclear genomes from passenger pigeons and 2 genomes from band-tailed pigeons, which are passenger pigeons' closest living relatives. Passenger pigeons' large population size appears to have allowed for faster adaptive evolution and removal of harmful mutations, driving a huge loss in their neutral genetic diversity. These results demonstrate the effect that selection can have on a vertebrate genome and contradict results that suggested that population instability contributed to this species's surprisingly rapid extinction.

Metal Levels in Shorebird Feathers and Blood During Migration Through Delaware Bay.

We investigated levels of arsenic mercury, lead, cadmium, and chromium in Red Knot (Calidris canutus), Semipalmated Sandpipers (Calidris pusilla), and Sanderling (Calidris alba) migrating through Delaware Bay, New Jersey to determine if contaminant levels are likely to be causing negative effects on the populations of these shorebirds, to compare among species, and to explore differences among individuals collected early and late during their migration stopover. We analyzed blood and feathers, both nonlethal ways of exploring contaminants in birds. Blood contaminant analysis provides a direct measure of recent dietary exposure, whereas feathers reflect body burden at the time of feather molt. We found some differences among species and between early and late samples. Levels of Hg and Pb were higher in Sanderling blood collected early (36.52 ± 8.45 and 145.00 ± 12.56 ng/g ww respectively) compared with later (16.21 ± 6.03 and 33.60 ± 4.05 ng/g ww respectively) during the migration stopover. Blood Pb levels of Sanderling in the early period were higher than those of the other two species (75.38 ± 15.52 ng/g ww in Red Knot and 42.39 ± 8.42 ng/g ww in Semipalmated Sandpipers). Semipalmated Sandpipers had lower blood As levels than the other two species (254.33 ± 40.15 and 512.00 ± 66.79 ng/g ww early and late respectively) but higher feather levels (914.01 ± 167.29 and 770.00 ± 116.21 ng/g dw early and late respectively), and their blood As was higher in the later sampling period compared with the early sampling period. Arsenic levels in shorebird tissues were relatively high and may reflect levels in horseshoe crab eggs, their primary diet item in Delaware Bay. In Red Knot, blood Cr levels were elevated in the later samples (572.17 ± 62.82 ng/g ww) compared to the early samples (382.81 ± 95.35 ng/g ww) and to the other species. The mean values of the metals analyzed were mostly below effect levels-the level that has a measurable negative impact-although relatively high As levels in Semipalmated Sandpiper feathers and some high levels of Pb need to be further explored.

Rapid population decline in red knots: fitness consequences of decreased refuelling rates and late arrival in Delaware Bay.

Most populations of migrant shorebirds around the world are in serious decline, suggesting that vital condition-dependent rates such as fecundity and annual survival are being affected globally. A striking example is the red knot (Calidris canutus rufa) population wintering in Tierra del Fuego, which undertakes marathon 30,000 km hemispheric migrations annually. In spring, migrant birds forage voraciously on horseshoe crab eggs in Delaware Bay in the eastern USA before departing to breed in Arctic polar deserts. From 1997 to 2002 an increasing proportion of knots failed to reach threshold departure masses of 180-200 g, possibly because of later arrival in the Bay and food shortage from concurrent over-harvesting of crabs. Reduced nutrient storage, especially in late-arriving birds, possibly combined with reduced sizes of intestine and liver during refuelling, had severe fitness consequences for adult survival and recruitment of young in 2000-2002. From 1997 to 2002 known survivors in Delaware Bay were heavier at initial capture than birds never seen again, annual survival of adults decreased by 37% between May 2000 and May 2001, and the number of second-year birds in wintering flocks declined by 47%. Population size in Tierra del Fuego declined alarmingly from 51,000 to 27,000 in 2000-2002, seriously threatening the viability of this subspecies. Demographic modelling predicts imminent endangerment and an increased risk of extinction of the subspecies without urgent risk-averse management.