One of the most important tasks during the field season is getting resights of individual birds. It’s a never ending, daily task that is just as important at the beginning, middle, and end of the season. Remember that most (and hopefully almost all) the godwits in my study area are individually and uniquely marked with engraved flags and color bands. They are identifiable by their unique alpha-numeric flag codes from a distance. We can read their code with binoculars or spotting scopes while we work in the bog or at the mudflats. This allows us to identify them each time we see them.
I can use these resights in many different ways to answer a lot of different questions. I can use information from our mudflats scans to get arrival dates for individuals based on when they were first seen. Most importantly, they provide information on who might be breeding on plot that we haven’t found a nest for yet (and where their territory is). This gives us a clue for where to search for their nest. Another important use is to derive hatching and breeding success estimates based on when and for how long we see them after most nests have hatched. We can tell based on their behavior if they have a brood of chicks with them - which is useful if we never found the nest for a pair. If they’re present in the bog and acting defensive for a long enough period, then we can usually assume (or hopefully confirm visually) that at least one chick survived until it could fly. With this same information and more scans on the mudflats, I can derive departure dates based on our last sighting of an individual. You can see that resightings can be used to provide a lot more information than meets the eye.
Potentially most importantly of all, sightings of individuals between years can be used in demographic analyses. Demography is the study of population trends. With sightings of individuals between years, we can build a dataset that will allow us to estimate survival rates, average breeding success, and ultimately population trends. More on that in a second.
Before I explain what all that means, I’ll attempt to explain why anyone would care. Hudsonian Godwits are a species of conservation concern. They are not listed under the Endangered Species Act, but they are recognized by federal agencies, non-profits, and panels of scientists as being at risk of listing because there are viable threats to the species. For HUGOs, one of the main threats has to do with their distinct breeding and wintering populations. Since all the birds that breed in Alaska winter on Chiloé Island in Chile, if a volcano erupted that killed half the birds on Chiloé there wouldn’t be individuals to buffer that loss to the AK breeding population. The species underwent severe declines at the turn of the century due to hunting pressures, and total numbers have never again increased. Threats due to global climate change, energy development, and human disturbance are additional factors leading to their designation as a high priority species of conservation concern. But all of those rankings look at the species level, not at the breeding population level. Are HUGOs in Alaska doing well? Or poorly? One of the main goals of our project is to answer this question.
So back to population trends. What does that mean? It means determining if the breeding population that we study is increasing, stable, or declining. For a species that is presumed to be long-lived, it’s a difficult question to answer. We don’t know the average lifespan of HUGOs because there haven’t been enough of them banded for a long enough time period. We assume that they live for about 18 years based on their well-studied European cousin the Black-tailed Godwit. So it’s hard to say if the population is increasing or stable if the number of adults is the same from year to year. Why is that? Because seeing that an individual is still alive doesn’t mean that it has bred successfully.
And that is the crux of population trends.
Let’s think about this in a way you’re most familiar with - human population growth. For the population to be stable (stay the same), a couple would need to ‘replace’ themselves in the next generation, so they need two children to survive and reproduce. An individual that doesn’t reproduce isn’t contributing to the population and doesn’t 'count’ towards the 'effective population size’. If the population is growing, then a couple would have more than two kids survive and reproduce. If the population is declining, than there is not replacement - less than two kids survive. This of course is averaged across couples at the population level. It’s tricky to get a handle on growth trends because it takes a long time for generations to pass with humans. Just because I was alive last year and I’m still alive this year doesn’t provide any details on my reproductive success. In humans, we estimate growth trends with census counts - every ten years we count numbers and estimate survival and reproductive rates. From that data, scientists can estimate our population trends.
We have a similar problem with godwits. Since they’re long-lived and we’ve only been individually marking them for seven years, it’s hard to know if a breeding pair has had a chick survive, and if they will breed (godwits delay breeding for three years much like humans take at least 14 years to mature).
So, in order to understand growth rates, we need to know reproductive success, but we also need to know survival for adults, chicks, and juveniles and how it might differ for the different sexes. Getting too confusing yet?
There are a lot of complicating factors for survival that biologists have to try to measure or account for in some way. Researchers on other species often find unequal survival rates for different sexes. Even in humans, it’s common knowledge that women tend to live longer than men. That affects the number of possible breeding attempts an individual has to breed successfully. In order to understand population trends and survival estimates, you have to know how many breeding pairs are even possible, and the total number of individuals isn’t very enlightening. For instance, if there is 1 female and 5 males, the population size is effectively 2 not 6 (as was the case for the very famous Chatham Island Blue Robin in New Zealand). There can also be sex ratio bias in broods. Some bird species can alter the ratio of male to female offspring depending on conditions they see. For instance, if a female had trouble finding a mate because there aren’t very many males in the population, she might preferentially lay male eggs. When those young grow up, they’ll have a better opportunity to mate, increasing the mother’s indirect fitness. Survival rates can also be complicated by mate and site fidelity - how often a pair breeds together versus splitting up, how reliably they are found on their previous territory or within a study area. All of that affects your detection rates - how often you see an individual - which are used to help estimate survival between years.
Understating how sex influences survival is only one complication. Age is equally important. Do older birds gentrify? Or once they survive to breeding age are they equally likely to die from one year to the next? How do mortality rates for chicks versus juveniles influence survival to breeding age?
You can see that it quickly gets complicated trying to understand population trends, but it’s extremely important to do so to make informed conservation and management decisions. Understanding if this population of godwits is stable, increasing, or decreasing will shed some light on other populations. It will aid in future designations or appeals for listing.
Demographic studies, especially on long-lived species, take time - years in fact. We’re slowly working our way to having enough data to make these kinds of informed decisions. Part of my doctorate research aims to answer a few facets of the complicating factors. In a few more years after that, we’ll have a robust dataset to estimate population trends.
I can use these resights in many different ways to answer a lot of different questions. I can use information from our mudflats scans to get arrival dates for individuals based on when they were first seen. Most importantly, they provide information on who might be breeding on plot that we haven’t found a nest for yet (and where their territory is). This gives us a clue for where to search for their nest. Another important use is to derive hatching and breeding success estimates based on when and for how long we see them after most nests have hatched. We can tell based on their behavior if they have a brood of chicks with them - which is useful if we never found the nest for a pair. If they’re present in the bog and acting defensive for a long enough period, then we can usually assume (or hopefully confirm visually) that at least one chick survived until it could fly. With this same information and more scans on the mudflats, I can derive departure dates based on our last sighting of an individual. You can see that resightings can be used to provide a lot more information than meets the eye.
Potentially most importantly of all, sightings of individuals between years can be used in demographic analyses. Demography is the study of population trends. With sightings of individuals between years, we can build a dataset that will allow us to estimate survival rates, average breeding success, and ultimately population trends. More on that in a second.
Before I explain what all that means, I’ll attempt to explain why anyone would care. Hudsonian Godwits are a species of conservation concern. They are not listed under the Endangered Species Act, but they are recognized by federal agencies, non-profits, and panels of scientists as being at risk of listing because there are viable threats to the species. For HUGOs, one of the main threats has to do with their distinct breeding and wintering populations. Since all the birds that breed in Alaska winter on Chiloé Island in Chile, if a volcano erupted that killed half the birds on Chiloé there wouldn’t be individuals to buffer that loss to the AK breeding population. The species underwent severe declines at the turn of the century due to hunting pressures, and total numbers have never again increased. Threats due to global climate change, energy development, and human disturbance are additional factors leading to their designation as a high priority species of conservation concern. But all of those rankings look at the species level, not at the breeding population level. Are HUGOs in Alaska doing well? Or poorly? One of the main goals of our project is to answer this question.
So back to population trends. What does that mean? It means determining if the breeding population that we study is increasing, stable, or declining. For a species that is presumed to be long-lived, it’s a difficult question to answer. We don’t know the average lifespan of HUGOs because there haven’t been enough of them banded for a long enough time period. We assume that they live for about 18 years based on their well-studied European cousin the Black-tailed Godwit. So it’s hard to say if the population is increasing or stable if the number of adults is the same from year to year. Why is that? Because seeing that an individual is still alive doesn’t mean that it has bred successfully.
And that is the crux of population trends.
Let’s think about this in a way you’re most familiar with - human population growth. For the population to be stable (stay the same), a couple would need to ‘replace’ themselves in the next generation, so they need two children to survive and reproduce. An individual that doesn’t reproduce isn’t contributing to the population and doesn’t 'count’ towards the 'effective population size’. If the population is growing, then a couple would have more than two kids survive and reproduce. If the population is declining, than there is not replacement - less than two kids survive. This of course is averaged across couples at the population level. It’s tricky to get a handle on growth trends because it takes a long time for generations to pass with humans. Just because I was alive last year and I’m still alive this year doesn’t provide any details on my reproductive success. In humans, we estimate growth trends with census counts - every ten years we count numbers and estimate survival and reproductive rates. From that data, scientists can estimate our population trends.
We have a similar problem with godwits. Since they’re long-lived and we’ve only been individually marking them for seven years, it’s hard to know if a breeding pair has had a chick survive, and if they will breed (godwits delay breeding for three years much like humans take at least 14 years to mature).
So, in order to understand growth rates, we need to know reproductive success, but we also need to know survival for adults, chicks, and juveniles and how it might differ for the different sexes. Getting too confusing yet?
There are a lot of complicating factors for survival that biologists have to try to measure or account for in some way. Researchers on other species often find unequal survival rates for different sexes. Even in humans, it’s common knowledge that women tend to live longer than men. That affects the number of possible breeding attempts an individual has to breed successfully. In order to understand population trends and survival estimates, you have to know how many breeding pairs are even possible, and the total number of individuals isn’t very enlightening. For instance, if there is 1 female and 5 males, the population size is effectively 2 not 6 (as was the case for the very famous Chatham Island Blue Robin in New Zealand). There can also be sex ratio bias in broods. Some bird species can alter the ratio of male to female offspring depending on conditions they see. For instance, if a female had trouble finding a mate because there aren’t very many males in the population, she might preferentially lay male eggs. When those young grow up, they’ll have a better opportunity to mate, increasing the mother’s indirect fitness. Survival rates can also be complicated by mate and site fidelity - how often a pair breeds together versus splitting up, how reliably they are found on their previous territory or within a study area. All of that affects your detection rates - how often you see an individual - which are used to help estimate survival between years.
Understating how sex influences survival is only one complication. Age is equally important. Do older birds gentrify? Or once they survive to breeding age are they equally likely to die from one year to the next? How do mortality rates for chicks versus juveniles influence survival to breeding age?
You can see that it quickly gets complicated trying to understand population trends, but it’s extremely important to do so to make informed conservation and management decisions. Understanding if this population of godwits is stable, increasing, or decreasing will shed some light on other populations. It will aid in future designations or appeals for listing.
Demographic studies, especially on long-lived species, take time - years in fact. We’re slowly working our way to having enough data to make these kinds of informed decisions. Part of my doctorate research aims to answer a few facets of the complicating factors. In a few more years after that, we’ll have a robust dataset to estimate population trends.