Long-term Monitoring: Determining population trends of Neotropical migrants and permanent residents in key habitats

In the Dominican Republic, the National Aviary’s ornithologist has maintained one of the longest-running bird monitoring programs in all of the Western Hemisphere. These data have become instrumental in understanding how and why populations of permanent resident birds – as well as migratory birds from North America – are changing.

The conservation and management of terrestrial landbirds requires the collection of habitat-specific data on bird populations because a vast majority of conservation and management activities are related to the population size of a species.

In the Caribbean and the Neotropics, despite some prominent efforts, there has been insufficient monitoring to fully assess the status of vulnerable bird populations and their habitats, or to guide conservation strategies. Some highly threatened species with very small populations are monitored regularly, and citizen-based counts with less rigorous protocols and reduced utility have been inaugurated, but long-term, locally intensive efforts using mist nets to intercept and record birds are badly needed. 

A model long-term monitoring study at Guánica, Puerto Rico, supported for many years by the International Institute of Tropical Forestry (IITF), has unveiled dramatic declines in many species of birds, especially over-wintering Neotropical migrants. The Guánica study has been invaluable for detecting population changes, but similar efforts are needed in diverse localities as patterns uncovered elsewhere will help determine if the decline in numbers of migrants at Guánica is general or specific to that site.

Since 1996 we have used constant-effort mist netting as a long-term monitoring program in the Sierra de Bahoruco, Dominican Republic. Our monitoring involves as many as 9 sites in 3 habitats, including coastal thorn scrub, mid-elevation classic dry forest, and high elevation endemic pine forest. With nets deployed for several days during mid-winter at each site, we catch, band, and assess condition of hundreds of birds each year.

Our goal is to continue mist netting annually to determine whether dramatic declines of overwintering migratory birds seen in Puerto Rico are also occurring in the Dominican Republic, and to determine population trends of permanent resident species as well. In addition, because our study sites have been relatively unaffected by overt habitat change, the data will prove valuable in determining the impacts of a variety of human-caused drivers of declines in bird populations, including habitat degradation and climate change. 

Initial analyses indicate many bird species with relatively stable population sizes. But as human encroachment of habitats on Hispaniola and across North America increase, our bird population database will serve as a critically important benchmark to measure future changes and to guide conservation efforts.


Birds in Early-Successional Habitats: How birds use agricultural lands and scrub can guide reforestation efforts

Birds throughout the world are threatened by habitat change. Agricultural lands, scrub, and early-successional (or young) forests represent a large majority of available habitats in many regions. Yet, the question of how birds respond to these early-successional habitats, and the impact of habitat degradation on species, is seldom studied by avian conservationists.

Most studies of the effects of anthropogenic change on tropical birds have focused on the impact of forest fragmentation. Bird communities and population trends in early-successional habitats are less frequently studied, even though degraded habitats are likely to be a dominant feature of tropical landscapes now and for the foreseeable future. The question of how bird populations – including over-wintering migrants as well as Caribbean residents - respond to the prevalence of these early-successional habitats is of critical importance to conservation biologists.

Previous studies of over-wintering migrants and permanent residents in the Dominican Republic have focused on demographics, site fidelity, and survival in presumably optimal habitats. However, considerable amounts of native habitats have been converted to agriculture; for example, on Hispaniola 72% of land cover is now agricultural crops, early-successional scrub, or pasture. Large portions of some national parks and their buffer zones have been converted to agricultural use through slash-and-burn practices, and other forested sites are selectively cut and the understory burned to promote forage for cattle grazing. The effects of these practices, and the use of these early- and mid-successional habitats by birds, have never been studied. Thus, for management planning, there is a need to know how this most common form of habitat conversion affects bird demographics and survival, and how birds respond to regeneration of these habitats.

Our overall project goals are to compile a comprehensive database of the demographics and non-breeding ecology of winter-resident bird species in regenerating broadleaf habitat, to measure site fidelity and survival among these birds, and to provide a basis for sound management and conservation activities of birds. Demographic data, including sex ratios and age distributions, and survivorship will be combined with habitat relationship data to determine factors or events that may be regulating populations in these habitats.

Results will be used to guide land management or reforestation efforts, with the goal of creating conditions favorable to over-wintering migratory bird species, and permanent resident species in the Caribbean basin.

 Avian Malaria: Understanding interactions among birds, parasites and their vectors, and in applying these concepts to avian conservation

Conserving birds requires an understanding of a species’ ecology and habitat needs, but may also require an understanding of how that species interacts with a diverse community of parasites and pathogens. At the National Aviary we are at the forefront in developing new understandings of the complex interactions among birds, parasites and their vectors, and in applying these concepts to avian conservation.

Many species of birds can be infected by haemosporidian parasites (often referred to as malaria) which require a vector like a mosquito in order to reach the avian host. Recent advances in our ability to sequence DNA have led to the discovery of dozens of different lineages or species of avian haemosporidian parasites, and these can vary geographically, across habitats, or among species of birds which serve as hosts.

Collaborating with Dr. Robert Ricklefs and his students at the University of Missouri-St. Louis, we have collected and utilized a very large data set to better understand the distribution of these parasite lineages across the Caribbean Basin, their impacts on birds and bird populations, and how species form in malaria parasites. 

We have shown through a relatively high degree of parasite lineage-sharing between migrants and residents that long-distance migrants connect communities of avian haemosporidian parasites in breeding and wintering areas, even though disparate avifaunas and different vector communities occur in each region. 

But at the individual level, longitudinal data suggest that while many hosts might harbor the same parasite lineage for more than a year, other hosts appear to clear infections from their circulating blood, while others acquire new infections of a different parasite lineage. At the level of the community, our results suggest a strong influence of some avian haemosporidian parasites on host populations, sometimes resulting in indirect interactions through apparent competition.

But haemosporidian parasites are not stable; over time, such as at intervals approaching a decade, the avian communities of some islands were seen to experience an apparent loss or gain of lineages, indicating the potential for relatively frequent lineage turnover. We have found that host shifting, often across host genera and families, is common. The geographic distribution of individual parasite lineages in diverse hosts suggests that species formation predominantly involves the expansion of the range of the host species followed later by local shifting of parasite lineages to new host species. 

Continuing studies across all major Caribbean islands will contribute to a growing understanding of the role of parasites in shaping bird communities, and fundamentally reshaping our understanding of community ecology, and even conservation. In addition, we are planning to now utilize our understanding of the avian malaria system to apply to the Zika virus and its impact on human populations.