Global Bird Diversity Is Facing Increasing Threats

Human activities in the modern world continue to drive the loss and degradation of natural habitats such as forests, grasslands and wetlands, placing global bird diversity under unprecedented pressure. Measures aimed at mitigating these threats—including the establishment of nature reserves and national parks, restrictions on hunting, stronger wildlife protection laws and policies to reduce carbon emissions—have so far proved insufficient to halt species extinctions and the erosion of ecological functions. By integrating global information on conservation threats with bird trait data, researchers constructed predictive models to explore how different conservation strategies might influence species persistence and functional diversity over the coming century. The trait data used in this work were drawn from the AVONET database, which compiles morphological measurements for more than 11,000 bird species worldwide, including attributes such as body mass, beak length and wing morphology. These traits effectively describe the ecological niches occupied by different bird species and are frequently used to estimate functional diversity—the diversity of ecological roles performed by organisms within ecosystems. Differences in body size, flight style, pollination behavior, trophic level, diet type, burrowing behavior and nesting strategies all contribute to this functional variation.

The results indicate that even if threat-reduction measures were implemented for nearly all bird species globally, only about half of the projected bird extinctions and losses of functional diversity could be avoided. Relying solely on threat mitigation is therefore insufficient. More targeted recovery programs are required if bird diversity and ecological functions are to be effectively preserved.

The study analyzed 9,873 extant bird species and incorporated threat information from the International Union for Conservation of Nature (IUCN) Red List. By combining morphological traits with evolutionary relationships among birds, the researchers constructed a Bayesian mathematical framework based on Markov chain Monte Carlo (MCMC) methods to simulate extinction scenarios over the next century. Under a baseline scenario in which current human pressures and environmental threats continue unchanged, approximately 5.2% of bird species—around 517 species—are predicted to disappear within the next 100 years. This figure is more than three times the number of bird extinctions documented since the year 1500. Such losses are expected to reduce global avian functional diversity by roughly 3.2%, indicating a substantial contraction in the range of morphological forms and ecological roles represented by birds.

The researchers then modeled three levels of threat-reduction strategies based on direct drivers of extinction. These direct drivers are identifiable factors that directly cause declines or extinctions of bird populations. According to IUCN classifications and the definitions used in the study, they fall into six primary categories:

1. Habitat loss and degradation

2. Hunting and collection

3. Climate change and severe weather

4. Invasive species and disease

5. Disturbance and accidental mortality caused by human activity

6. Pollution

In the most optimistic scenario—complete threat abatement—every direct driver of extinction would be removed across the entire geographic range of every species. Even under such ideal conditions, the model predicts that only about half of the projected species extinctions and functional diversity losses would be prevented. Approximately 254 bird species would still be expected to go extinct. This finding highlights that even if all identifiable threats were eliminated, some species might remain unable to recover due to severe historical damage, extremely small populations or constraints imposed by their evolutionary history and geographic distribution.

A moderate strategy termed partial abatement assumes that all extinction drivers are eliminated across at least half of a species' geographic range. Under this scenario, roughly 26% of species extinctions and associated functional diversity losses could be prevented. A minimal abatement strategy, in which threats are removed from only a small portion of each species' range, produces far smaller conservation benefits.

The impact of different threats also varies markedly. Eliminating habitat loss and degradation produces the largest conservation benefit, preventing the extinction of approximately 141 bird species and avoiding about 0.9% of functional diversity loss. In contrast, threats such as hunting and collection or disturbance and accidental mortality affect fewer species overall, yet they play a disproportionately important role in preserving functional diversity. These threats often target species with particularly distinctive ecological roles.

Bird species with different morphological traits also show very different levels of vulnerability to threats. Large-bodied birds such as ostriches and albatrosses tend to face particularly low survival probabilities under pressures from climate change or hunting. Species with broad wings are especially sensitive to habitat fragmentation, whereas birds with large beaks and relatively short tails are more vulnerable to extreme climatic events. Because extinction risks correlate with morphology, conservation strategies that remove threats without considering the distribution of functional traits may inadvertently shift or reduce the diversity of ecological roles performed by birds, weakening ecosystem stability and resilience.

The findings also challenge a key assumption underlying some conservation metrics, such as the Species Threat Abatement and Restoration (STAR) framework. These indicators often assume that once threats are eliminated, extinction risk will largely disappear. The study suggests that this assumption is overly optimistic. Even after threats have been removed, many species may continue to decline due to past damage. Small population sizes can lead to inbreeding, reduced genetic diversity and diminished ability to adapt to environmental pressures. Consequently, threat elimination alone cannot achieve conservation goals; it must be combined with additional proactive measures.

The researchers therefore examined the potential of targeted recovery programs. By analyzing functional uniqueness—the degree to which a species contributes uniquely to global functional diversity—they assessed how protecting particular species might influence overall ecological diversity. The results indicate that if conservation efforts focused on the 100 most functionally unique threatened bird species worldwide, approximately 68% of the projected loss in functional diversity could be avoided. From a practical standpoint, this approach could be feasible if sufficient financial resources and conservation capacity are available.

These highly distinctive birds occur across many continents and ecosystems. Examples include the Sulu hornbill (Anthracoceros montani) of the Philippines, the Ascension frigatebird (Fregata aquila) of the Atlantic Ocean, the Andean condor (Vultur gryphus) of South America, the Madagascar serpent-eagle (Eutriorchis astur), and Stresemann's bristlefront (Merulaxis stresemanni), a species restricted to a single region. Such birds are not only evolutionarily unusual but also occupy unique ecological roles. Their disappearance could disrupt ecological interactions and destabilize ecosystem structures in ways that are difficult to predict.

Anthracoceros montani（Image source：John Gerrard Keulemans，CC0 1.0 ）

Fregata aquila（Image source：Drew Avery， CC BY 2.0 ）

Vultur gryphus（Image source：Pedro Szekely， CC BY-SA 2.0 ）

Eutriorchis astur（Image source：JG Keulemans，CC0 1.0 ）

Protecting bird diversity and its associated ecological functions over the next century therefore requires more than simply reducing threats. Targeted recovery actions must also be implemented simultaneously. Effective conservation strategies should incorporate trait-based information and functional value assessments rather than focusing solely on species counts. While prioritizing unique species can provide strategic benefits, practical implementation must also consider ethical concerns and potential biases, ensuring that other species and their ecological contributions are not overlooked.

Author: Shui-Ye You

References:

1. Stewart K et al. (2025). Threat reduction must be coupled with targeted recovery programmes to conserve global bird diversity. Nat Ecol Evol.

2. Tobias JA et al. (2022). AVONET: morphological, ecological and geographical data for all birds. Ecology Letters.

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