Saturday, May 21, 2011

the drying, the drought

"...this is one of the effects of an extended drought that most persons don't realize or notice directly." - Dr. Keith Arnold

There's still a Green-tailed Towhee and a White-crowned Sparrow on 'the back forty.' They should have left a long time ago. They have the energy to stay, but not to leave.*

Common Nighthawk - one - returned to town on May 19th. The evenings are silent and empty without them. Lesser Nighthawk - one - returned on April 17th. A few seem to be around. Where are the rest?

Bats had a good, strong, dense show early on. Since then, maybe one or two each evening. Perhaps that was migration.

Barn Swallows sit on the ground; they have no mud for nest building nor energy to do so. They wait for insects to come to them.

Western Wood-Pewees perch uncharacteristically low, barely above the ground.

Audubon's Yellow-rumped Warbler did not wake up this morning. Second, perhaps third adult bird I've ever knowingly picked up that died of natural causes. The keel, breast bone, photographed above, is a jarring reminder that fat stores = life. Lost muscle mass, emaciation, a beautiful bird with no life inside, that is the story of a severe drought. Native predators are coping. Unnatural predators are wreaking havoc.

Scott's Oriole (below) did not wake up this morning either. Third or fourth adult bird that I've ever knowingly picked up that died of natural causes.

It is fairly well known that birds lose a significant amount of weight overnight. The question of how much will definitely vary by species and a considerable range of external factors. I highly suggest reading up on this as a way of understanding what pressures birds face without ANY human/predator/artificial pressures.

This sort of widespread bird die-off is more common among insectivores who migrated too early and were caught in a freeze that killed off their prey. But this is drought.

* Now for a bedtime story. Emphasis/paragraph interruption is mine.

Moisture as a determinant of habitat quality for a nonbreeding Neotropical migratory songbird

Smith et al

Identifying the determinants of habitat quality for a species is essential for
understanding how populations are limited and regulated. Spatiotemporal variation in moisture and its influence on food availability may drive patterns of habitat occupancy and demographic outcomes. Nonbreeding migratory birds in the neotropics occupy a range of habitat types that vary with respect to moisture. Using carbon isotopes and a satellite-derived measure of habitat moisture, we identified a moisture gradient across home ranges of radio-tracked Northern Waterthrush (Seiurus noveboracensis). We used this gradient to classify habitat types and to examine whether habitat moisture correlates with overwinter mass change and spring departure schedules of Northern Waterthrush over the late-winter dry season in the tropics. The two independent indicators of moisture revealed similar gradients that were directly proportional to body mass change as the dry season progressed.

Birds occupying drier habitats declined in body mass over the study period, while those occupying wetter habitats increased in body mass. Regardless of habitat, birds lost an average of 7.6% of their mass at night, and mass recovery during the day trended lower in dry compared with wet habitats. This suggests that daily incremental shortfalls in mass recovery can lead to considerable season-long declines in body mass. These patterns resulted in consequences for the premigratory period, with birds occupying drier habitats having a delayed rate of fat deposition compared with those in wet habitats.

Taken together with the finding that males, which are significantly larger than females, are also in better condition than females regardless of habitat suggests that high-quality habitats may be limited and that there may be competition for them. The habitat-linked variation in performance we observed suggests that habitat limitation could impact individual and population-level processes both during and in subsequent periods of the annual cycle.

The linkage between moisture and habitat quality for a migratory bird indicates that the availability of high-quality habitats is dynamic due to variation in precipitation among seasons and years. Understanding this link is critical for ascertaining the impact of future climate change, particularly in the Caribbean basin, where a much drier future is predicted.


So what does that all mean? It means, indirectly, that birds overwintering in dry places [here] may not have built up enough energy stores to migrate back north. Or if they have, by the time they get there they're not going to be able to compete as well as birds who wintered in areas with higher moisture. Or, perhaps, if birds who live in this area haven't had a good rain since September, there are few bugs to eat and a lot of competition. It means that 7.6% (this varies by species, I've seen 22% noted on winter studies) of their body mass goes away every night regardless of whether or not there's a bug to be eaten in the morning.

And in closing, this is a Western Tanager who was either roadkill or emaciated but who had been around long enough for me to not want to investigate further. One hopes that his kin have fared better.

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