Ornithology Lecture 18

Bird Population Changes Across Biomes

  • Biome-Specific Losses: Bird population losses are broken down by biome to show where the most significant declines are occurring.

  • Absolute Loss Estimates: Figure represents absolute estimates of bird population loss in millions.

    • Example: Grassland birds show a 60-70% decline by 2019 relative to 1970 estimates.

  • Significant Losses: Observed across various biomes, including:

    • Coastal birds

    • Arid bloom birds

    • Boreal tundra (forests in Northern US and Canada)

    • Grassland bird species

  • Proportion of Declining Species: 75% of grassland species are facing declines.

  • Other Declining Species: Around half of boreal species and approximately 65% of Western forest species (Rocky Mountain, Intermountain range habitat) are also in decline.

  • Eastern Forest: Eastern forest species are experiencing significant losses as well.

  • Wetland Species: Wetland species, particularly water pellets, are doing better, largely due to the Clean Water Act.

Clean Water Act and Wetland Conservation

  • Clean Water Act Impact: Enacted around 1970, along with the Clean Air Act.

  • Wetland Definition: The Clean Water Act indirectly protected wetlands by requiring states to define them, leading to conservation efforts.

    • Note: The protection status of wetlands might be changing.

  • Wetland Recovery: Efforts to protect freshwater sources have benefited wetlands, which were previously in poor condition due to eradication in many states (e.g., Ohio).

Declines and Recoveries Across Bird Families

  • Net Loss: Overall, there's a significant net loss of birds.

    • 3,900,000,0003,900,000,000 birds lost

    • 250,000,000250,000,000 birds gained in some families

    • Net loss of 2,900,000,0002,900,000,000 birds

  • Sparrows: American and Old World sparrows have been heavily affected, contributing to the decline in grassland species.

  • Wood Warblers: Decline is significant, linked to deforestation.

  • Blackbirds: Most blackbird species have declined, except for grackles due to range expansion from Mexico into the US.

Non-Native Species Decline

  • Non-Native Declines: Non-native species like the house sparrow and European starling are also declining in North America since 1970.

  • Implications: The decline in non-native and generalist species suggests the factors causing bird losses are severe, with no niche replacement occurring.

  • Grassland Bird Decline: Attributed to land conversion, pesticide use, and declines in insect diversity, impacting insectivorous birds like blackbirds, sparrows, finches, and flycatchers.

  • Raptors: Raptors are generally doing better due to the banning of DDT around 1970.

Gains and Losses by Type

  • Decline Definition: Species with at least a 50% decline are defined as declining.

  • Gain Definition: Species with at least a 15% increase since 1970 are defined as gaining.

Causes of Bird Population Decline

  • Human Impact: Human activities are the primary cause of bird declines.

  • Population Growth: Bird species evolved when the human population was around 400,000,000400,000,000, but the Industrial Revolution led to exponential population growth.

  • Resource Use: Increased human population leads to greater landscape use, impacting other species, including birds.

  • Common Species Decline: The decline of common bird species is particularly concerning because these species are crucial in food webs.

  • Underestimation of Loss: The loss of common species is often underestimated, potentially bringing us closer to ecological tipping points than we realize. Statistical bias may overestimate commonality and underestimate rarity.