Migration is a particularly vulnerable time of year for songbirds. Birds are facing novel habitats, exhausting exercise, and are encountering more predators and human modified environments. Why do some bird species take this risk? Birds are in search of predictably abundant resources to eat through the winter, and on their breeding grounds food becomes scarce and highly coveted as temperatures decrease . Birds also spend a large chunk of their annual cycle migrating (see below for a Blackpoll Warbler’s annual cycle to see what I mean). So studying how birds are making these journeys, what they are eating on the way, where they go, when, and in what direction is important and can lead to better decisions about how to help them on their way.
Understanding how physiological condition (how much fat and/or antioxidant capacity a bird is able to store) influences a bird’s behavior can help us to better understand their needs during this susceptible time. Further, physiological and behavioral actions during spring and fall migration can have consequences that spill over into the winter or breeding seasons (called carry-over effects). We are in the midst of a manipulative field experiment that is trying to tease apart whether fat stores and antioxidant stores are important drivers of decisions birds make during their travels.
But before I start talking just about birds and how cool they are, I want you to take a second and think about an animal that embodies athleticism to you. Did you think of the sprinting cheetah, or the fast swimming sailfish, or maybe the remarkable dive of a hunting peregrine falcon? Well, of course you would be right! All those species are incredibly fast athletes.
However, when I think of any sort of endurance athleticism in the animal world, I tend to think of animals that are migrating – especially migratory birds. Every year thousands of birds make long migrations around the globe, moving from areas of declining resources to areas of abundant resources. And, these birds fly hundreds to thousands of miles in the process, which is a crazy feat of endurance exercise. Among migrating birds, there are definitely some rock stars. The Arctic Tern is pretty famous (to us bird people) since it flies almost 60,000 total miles during migration. As is the Bar-tailed Godwit that migrates nonstop from Alaska to New Zealand or Australia, covering more than 6,000 miles in about 8 days of continuous flying (which is exhausting for me even to contemplate). However, although some birds are able to migrate in one flight, birds have many different strategies to help them travel these enormous distances and there is a lot of variation in how these journeys are made.
First, there could be variation in the routes a bird may take - in North America a songbird breeding in the arctic and wintering in South America may take a completely overland and direct route to get there, or they could fly east and then down the coast and across the gulf or straight across the ocean.
Second, most migrations are not non-stop flights, and therefore, during migration, short periods of endurance flight are traded off with periods of feeding and rest at stopover sites. There may be many stopovers on a bird’s migration from their breeding location their wintering location.
Migratory stopover sites are crucial for birds to rebuild energy stores, and the time a bird spends on a stopover can influence the timing and success of its overall migration. Additionally, during flight, birds have an elevated metabolism leading to an increase in the production of a byproduct we call reactive species. Reactive species can cause damage to cells, tissues, or DNA if not balanced by antioxidants**. Luckily for the birds, during the fall, there are a ton of seasonally abundant fruits around that are full of dietary antioxidants and fats. Birds that normally eat insects during the rest of the year generally switch to eating these fruits during migratory stopovers. However, the quality of stopover sites and the amount of fruit available to birds varies among sites and across a migratory season. We were curious about whether birds that have more fat and/or antioxidants in their diet can spend less time on a stopover site, and whether they are more likely to depart in a seasonally appropriate (southerly) direction.
To examine these differences, we headed out to Block Island, Rhode Island, an offshore stopover site that is popular for migrating birds and performed a field experiment. We caught four species of birds (Blackpoll Warblers, Hermit Thrushes, Red-Eyed Vireos, and Myrtle Warblers) that varied in their migration patterns, and manipulated their physiological condition.
Looking at multiple species of birds will allow us to compare how important condition is for birds with different migration strategies (land-based vs. over the sea) and migration distances (short vs long). Myrtle Warblers (Setophaga coronata coronata), migrate shorter distances than many of the other species passing through Block Island in the fall, and winter farther north than any other wood warblers. Hermit Thrushes (Catharus guttatus) are medium distance migrants that travel from Block Island to winter in the southern United States and Central America. Red-Eyed Vireos (Vireo olivaceus) are long-distance migrants that regularly stopover on Block Island during the fall. After leaving Block Island they are more likely to migrate overland until they reach the Gulf of Mexico, which means they will probably stop many more times as they travel south. In contrast, after leaving Block Island in the fall, Blackpoll Warblers (Setophaga striata) make an insane journey out across the open ocean for 3-5 days of non-stop flight before reaching a wintering destination in the Caribbean or South America.
We used mist nets to capture these four species and and then kept them in an outdoor aviary for a couple of days to manipulate their physiological condition. Once caught, we either gave the birds a diet rich in fat and/or antioxidants (we called this the ad lib diet) or a diet without extra fat and/or antioxidants (we called this the maintenance diet since birds, well, maintained the weight that we caught them in). We wanted to fatten the birds up (ad lib diet) and give them a lot of dietary antioxidants so that they were in better condition to simulate birds on a stopover site that would be abundant with fruits. We contrasted that diet treatment with the maintenance diet to simulate birds that wouldn’t have as much access to fruits on stopover. We predicted that birds that were able to stuff themselves with fat and antioxidants would be in better condition and would be more likely to migrate sooner and, potentially, reach their wintering grounds sooner than birds that were unable to do so.
We also took blood samples to look at their antioxidant capacity. A bird’s blood can tell us all sorts of levels of circulating antioxidants or other metabolites, kind of like when you go get checked for your cholesterol at the doctor. We predicted that birds given dietary antioxidants would have increase their circulating levels of antioxidants during captivity. After several days in captivity, birds on the fat rich diet had gained a lot of fat, where birds on the maintenance diet were still in the condition that we had caught them in. We then attached small radio-transmitters called Nanotags to these birds. Each nanotag sends out a unique signal every 10 seconds that can be picked up passively by receiving stations in the MOTUS network. We built one of those receiving stations on Block Island, but it was a part of the network of towers that extends from Canada down into South America. If any of our tagged birds fly along the Atlantic flyway then there is a good chance we’ll know about it.
Station on Block Island
This 40ft tower is a part of the MOTUS network and will passively pick up the signals from any Nanotag within it’s range
Nanotags will help us determine whether birds on an ad lib diet or one that was given dietary antioxidants can leave Block Island sooner and in a more seasonally appropriate direction than those on the maintenance diet or ones not given dietary antioxidants.
Linking behavioral decisions and physiological condition of songbirds together can help us to understand the types of habitats and food resources different bird species need on stopover sites. In turn, that could help to determine how we can best conserve those areas, or how we can restore them by planting native fruits and berries (see this helpful guide on what to plant!) to help these incredible athletes on their way.
**Antioxidant Definition: Animals have a multifaceted antioxidant system made up of endogenous antioxidants, micromolecular sacrificial molecules and dietary antioxidants that work synergistically to protect against oxidative damage (from those pesky reactive species). For birds in migration, the relationship between reactive species production, antioxidant protection and oxidative damage is not straightforward, and various aspects of the antioxidant system may respond differently depending on the type of damage, the duration of flight or the physiological state a bird. Dietary Antioxidants: Antioxidants produced by plants and consumed by animals in their diets. The two broad classes of dietary antioxidants include lipophilic antioxidants (vitamin E or carotenoids) and hydrophilic antioxidants (vitamin C or polyphenols). In this study we specifically examined polyphenols.
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