Biology Reference
In-Depth Information
insect life of those areas, increasing bird populations of species that associate with in-
creasing plants or insects and decreasing bird populations of those that don't.
Droughtscanhaveunexpectedimpactsonbirds.Forexample,whenwaterlevelsdrop,
feeding swans often pick up grit at the bottom of shallow waters that are normally too
deep for them to reach. Most North American lakes have large amounts of lead shot sit-
tingatthebottom like tinytime bombs.Effortstophaseoutlead shotbeganinthe1970s,
but a nationwide U.S. ban on lead shot for all waterfowl hunting was not implemented
until 1991. Canada instituted a complete ban on the use of lead shot in 1999, after ban-
ning its use near bodies of water and on national wildlife areas earlier. Even though lead
shot is no longer raining down on lakes, rivers, and streams, the lead shot already there
continues to sit until a swan, loon, or other bird picks it up as grit. The lead is ground up
in the gizzard, dissolves into the bloodstream, and usually leads to the bird's death.
Rising Temperatures
Within certain ranges, insects are more active as temperatures rise, so birds depending on
them may benefit from a warming trend. Considering how many birds are found in trop-
ical areas compared to temperate ones (tiny Costa Rica, about the size of West Virginia,
has more bird species than all of North America north of Mexico!), it's possible that in
the long run, in some places, birds may benefit from rising temperatures.
But harmful insects would also grow more active. In Hawaii, where native birds were
decimated by malaria after mosquitoes were introduced, most remaining populations of
several endangered species now exist only at elevations too cool for mosquitoes. As tem-
peratures rise, mosquitoes and the diseases they carry will move higher in the mountains,
shrinking and even eliminating remaining habitat for these vulnerable birds.
Rising water temperatures also encourage eutrophication , marked by excessive aquat-
ic plant growth. While the plants are actively growing, they produce oxygen, but as they
die, even as other plants are growing, decomposition of dead plants absorbs oxygen and
releases carbon dioxide. Mosquito larvae can thrive in oxygen-depleted water because
they take in their oxygen at the surface through a breathing tube.
Mayfly nymphs feed on mosquito and other larvae in their aquatic stage, and provide
enormous quantities of food for migrating and nesting birds when they emerge as flying
insects intheir adult stage. But mayflies require highly oxygenated water.When eutroph-
ication makes lakes and ponds inhospitable for them, we lose a natural control over mos-
quito numbers and an important supply of food for such beloved and already-declining
birds as Purple Martins and Common Nighthawks.
Aswarmertemperaturesarriveearlierinspring,amismatchbetweentimingofnesting
and availability of food seems to be arising. For example, many plants and insects are
emerging earlier. Migratory songbirds that spend the winter in the southern and central
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