I wrote a few weeks ago about how some bird species are more closely tied to particular elevation bands than others. With global warming, species seeking to avoid rising temperatures may rise in altitude or move towards the North (or South) pole. But species that already live on mountaintops, like black finches, have fewer options. There is no more “up” available, although they can move north…for now.
This is just a simple example of how species might react to climate change. It’s possible to take a big step back, look at all types of bird species, and ask what sort of characteristics of a species make it better able to cope with environmental change over time? This type of analysis is called a vulnerability assessment.
The international bird conservation partnership that I coordinated for 15 years, Partners in Flight (PIF), uses a six-factor system. Species with large breeding (factor 1) and non-breeding (factor 2) ranges should fare better in the future than species with small ranges. The mere fact that they are widespread shows that they occupy a wide variety of habitats, altitudes, latitudes, etc. Think of the American robin.
Second, species with large population sizes (factor 3) will fare better than those with small population sizes. It should be obvious. And, no, there is no perfect correlation between range size and population size. Consider the osprey. They have a huge (global) range, but they are not found very far from large lakes, rivers and sea coasts. Thus, their population size is smaller than their range size would suggest.
Species with decreasing populations are worse off than species with stable or increasing populations (factor 4). You might think, duh. But vulnerability assessments are about trying to predict the future. Species whose populations are currently in decline may or may not continue to decline when things change. As with the stock market, past performance does not guarantee future returns.
Finally, birds with high current threats during the breeding season (factor 5) and the non-breeding season (factor 6) are likely to be worse off than those with lower threats. It is difficult to judge threats, especially for migratory birds. They move around a lot and we don’t know much about the status of a species at any given place and time. Moreover, it is about the future. So it’s even trickier. Fly with a Swainson’s Falcon from Idaho to Argentina and back each year. How many places, events and threats does a single bird face in those 365 days?
There are other species attributes that PIF does not consider directly but are used by BirdLife International. One of them is the clutch size. Species that lay a lot of eggs will be better off than those with smaller clutches. For example, on the high end, our California quail lays 12-16 eggs in each nest. Bald eagles, on the other hand, lay 1-3 eggs per nest. If all else were equal, we would be swimming in quails. Obviously, clutch size isn’t the only thing that matters.
One factor that generally works in the opposite direction of clutch size is lifespan. Species like quail are not long-lived (3-5 years) while species like bald eagle are long-lived (20-30 years). They are two ends of a continuum — having many children but not living long or having few children and living long. Both strategies were selected by evolution.
One factor that we don’t normally think about is “age at first breeding”. How old must an individual be before it can produce offspring? Again, we can look at quail, which can breed within a year of being born. But bald eagles can only lay eggs when they are six years old. If conditions change quickly, you must think that a species that reproduces quickly will be more successful.
There are three motion measurements. The first is what BirdLife calls the movement pattern. Species that live in groups all year round are more vulnerable than more solitary species. Imagine a thunderbolt. A flock could all be killed at once while with a solitary species only one bird would perish.
Another measure is the scatter distance. Species that disperse very far are safer than species that do not. The latter are less flexible in their exploration, discovery and selection of breeding sites. So, as the climate changes, they lack the innate ability to get out and find new opportunities to keep going. Dispersal distance is difficult to assess because we do not have good data for most species on how far the young venture before deciding where to establish a territory and try to nest.
The last movement factor is simpler: migration distance. Species that migrate over long distances are more vulnerable than others. Again, we can think of Swainson’s hawk as a long-distance migrant and, at the other end of the spectrum, the house finch, which is a resident.
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House finches do not go anywhere, which allows them to familiarize themselves with the area in which they live. They can become very knowledgeable about food sources and danger zones as Swainson’s hawks fly over tens of thousands of miles of unknown or little-known territory.
A vulnerability assessment looks at each of these factors and usually provides a total score for a given species by adding the scores for each factor. Then we can get an idea of how we should care about osprey, bald eagles, house finches or any other species.
I did a little analysis of 658 North American bird species just to see how this might pan out. Which species, or groups of species, might be most vulnerable to climate change or any other major disturbance affecting them?
Looking at four major groups of birds, I found shorebirds to be the most vulnerable, but only slightly more so than waterfowl. These include herons, gulls, albatrosses and a host of other species that use the water, but are not “waterfowl”.
Land birds are slightly less vulnerable – warblers, sparrows, flycatchers, cuckoos, nightjars and other groups that are found mainly in upland woods and forests.
Waterfowl were the least vulnerable: ducks, geese and swans. I found this particularly interesting because waterfowl have benefited the most from the action of conservationists since the formation of the North American Waterfowl Management Plan in 1986. These species have already received a real thumb from humans. And based on the characteristics described above, they should be in relatively good shape in the near future.
Digging a little deeper, we can look at taxonomic orders. Parrots (Psittaciformes) are the most vulnerable. But we only have three species in the United States, and they’re not better preserved here. We need to look at their situation in Mexico and in the south.
Next is the order that includes albatrosses, shearwaters and petrels. These oceanic species are unfamiliar to landlocked states like Idaho, so I’ll leave them out for now.
The third most vulnerable order is the shorebirds (Charadriiformes). Here are 111 species facing a multitude of problems. They are mostly long-distance migrants, so they face the usual gauntlet of threats as they fly north and south each year. But because they depend on coasts, wetlands and shorelines, they are also affected by issues that affect water, such as decreasing quantity and increasing pollution. Additionally, ocean coastlines are increasingly being ruined by human encroachment. And realize that sea level rise is reducing and even eliminating coastal habitats that shorebirds need.
What surprised me was that the most vulnerable order among land birds, after parrots, were hummingbirds and swifts (Apodiformes). Without trying to analyze this in too much detail for my purposes here, I can jump straight to the question, “what can we do?” I bet conserving albatrosses and shorebirds seems out of reach for most Idahoans. Yes, we can donate money to organizations that keep these species, but most of us can have little direct impact.
Swifts are a little tricky too, but hummingbirds are different. I wrote about the flying birds of Idaho a few weeks ago. The fact is that these species migrate into our gardens in spring and fall. We can plant flowering plants, reduce or eliminate the use of pesticides and herbicides, prevent collisions with windows, and keep cats indoors on our quarter-acre grounds to help these species. This is where you and I can intervene directly on climate change and help. Join me and do it.