Lecture: Avian Ecology and Declining
bird populations
Patterns of Species Distribution:
Global:
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more species in tropics than temperate regions
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possible reasons:
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historical factors: longer time since disturbance (ice age), more speciation
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current ecological factors: more structural diversity, more opportunity
to specialize, avoid competition
Local:
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most species in edge habitat
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has species from each habitat on either side of the edge and species adapted
to the edge
Habitat quality:
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edge has most species, but not necessarily good habitat -- many species
have lower nest success in edges
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species require different habitat types
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Northern Spotted Owl -- requires undisturbed old-growth forest
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several woodpeckers -- require recently burned areas
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can not manage habitat to have one habitat good for all
Population Decline
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best documented: about 75% of eastern forest neotropical migrants show
decline between 1978 & 1987
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also reported: decline in a number of grassland species, including ground
nesting waterfowl, bobwhite, ground nesting passerines
Possible causes of decline:
Destruction of winter habitat: tropical deforestation
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primarily a problem for winter habitat specialists (most are generalists)
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example: Townsend's Warbler, in west, one of few species that is a winter
habitat specialist, may be showing more decline than winter habitat generalists
Fragmentation of breeding habitat
Inference from studies of breeding habitat
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example: black-throated blue warblers: correlation between young produced
one year and breeding adults next year suggests breeding, not wintering
effects determine population size
Comparison of large vs. small areas of breeding habitat shows small areas
have:
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increased competition from other bird species
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increases in numbers of nest predators in fragmented habitat
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increases in numbers of brood parasites in fragmented habitat
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among foliage-gleaning insectivores, birds with high risk of nest predation
or parasitism because of nest location/ structure show strongest decline
Other human activities that may increase predator populations
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decline in large predators (ex wolves) that are not nest predators may
allow increase in smaller predators (ex. fox, coyotes) that are nest predators,
especially on waterfowl
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farm buildings, sheds provide dens for skunks, etc. that are nest predators
on waterfowl
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farm areas, some winter bird feeders increase populations of blue jays,
crows (nest predators)
Habitat modification:
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logging
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fire supression
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loss of native grasses; introduced grasses with habitat structure less
conducive to feeding