Its fall, so its time to chat about songbird migration in New England!

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.

A Blackpoll Warbler’s annual cycle: notice how much time each year it will spend traveling between breeding and wintering locations

A Blackpoll Warbler’s annual cycle: notice how much time each year it will spend traveling between breeding and wintering locations

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.

Possible migratory routes birds can take on their way south.

Possible migratory routes birds can take on their way south.

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.

Migration = short endurance flights punctuated with longer periods of rest and refueling with native berries and fruits at stopover sites along the way

Migration = short endurance flights punctuated with longer periods of rest and refueling with native berries and fruits at stopover sites along the way

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.

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Automatic Receiving

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.


 
About the author:  Clara Cooper-Mullin is a PhD student studying the impacts of diet and body condition on songbird migration the  Scott McWilliams lab  at the University of Rhode Island

About the author:
Clara Cooper-Mullin is a PhD student studying the impacts of diet and body condition on songbird migration the Scott McWilliams lab at the University of Rhode Island

 

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Bring on Spring! Animal physiology is as transformative as our seasons

Physiology is the study of how animals work and perform everyday functions like breathing, walking, and maintaining a normal body temperature. In many species, animal physiology responds to environmental conditions including the amount of available water, the temperature, and the time of year. A general rule is that when the environment changes then animal physiology responds!

It is important to note that changes in physiology and behavior often occur together. For example, a bird readying itself for migration will eat a larger amount of food in order to double the size of their fat stores that are crucial for providing fuel during long-distance flight. This behavior would be impossible if their physiology was inflexible, but luckily they remodel their digestive system to cope with processing all of this food!

An alternative tactic that animals use to fit into their environment is that they change only their behavior so that they experience a constant set of environmental conditions, and as a result they avoid changes in their physiology. Aquatic turtles use this strategy to keep their body temperatures fairly constant by switching between basking themselves in the hot sun and plunging themselves in the cooler water.

Whether or not animals change their physiology, their behavior, or their physiology and behavior depends on the species and on their environmental situation. For this post I'll summarize the types of changes in physiology that this blog will focus on!

An unfrozen wood frog ( Rana sylvatica ) found by a biology undergraduate student in Massachusetts, September 2017,  Photo credit: Kristen DeMoranville

An unfrozen wood frog (Rana sylvatica) found by a biology undergraduate student in Massachusetts, September 2017, Photo credit: Kristen DeMoranville

Physiology can change on a short-term timescale soon after their environments have changed, and these changes are reversible. This happens to us! Standing outside on a cold winter day in thin mittens, it only takes about 30 minutes to notice our hands are cold and beginning to hurt. In this situation, our blood flow has been rerouted to our center to keep our important organs warm. During the winter in North America a specific species of frog, the wood frog (Rana sylvatica), buries themselves beneath the soil in preparation for freezing temperatures. These frogs are unable to keep their body temperatures high enough so that their organ systems could properly function. These frogs embrace that shortcoming and allow their bodies to freeze nearly solid immediately as ice forms around them in order to survive the cold. Special physiological adaptations protect their organs during this freeze and help them to completely recover as soon as the temperatures warm and ice disappears!  The Dr. Richard Lee & Dr. Jon Constanzo lab investigates the physiology of these amazing frogs. An overview of Dr. Lee & Constanzo's research can be found here.
Spring is here and wood frogs are emerging from their frogsicle forms NOW! If you are in Canada or eastern North America then keep your eyes peeled and ears open.

A banded male North American cardinal ( Cardinalis cardinalis ) in Rhode Island, April 2018,  Photo credit: Steve Brenner

A banded male North American cardinal (Cardinalis cardinalis) in Rhode Island, April 2018, Photo credit: Steve Brenner

Animal physiology can change on a long-term scale either days, weeks, or months after their environments have changed, and these changes are reversible. We can relate to this too! Taking a long walk on the first scorching day of the summer we feel drained. After experiencing this heat day after day it seems easier to take this same walk in mid-summer. Our bodies adjust to the heat with repeated exposure, and as a result we can better endure hot weather! Songbirds that stick around for cold winters rather than migrate to tropical regions have to change their physiology so that they can stay warm enough to properly function and survive. Winter resident birds like the Northern Cardinal (Cardinalis cardinalis) keep warm by enhancing their ability to generate heat through a tactic similar to mammalian shivering, but without the muscle trembling that we experience. They also increase their fat and feather layers in the winter to improve insulation which is crucial for keeping that extra heat that they produce inside their bodies. These physiological adjustments do not happen after the first freeze or even with the first snowfall, rather it takes weeks at cold temperatures for birds to transform their bodies into fat, fluffy, heat generating machines. The Audubon society covers how birds stay warm in more detail here and Dr. David Swanson's lab focuses much of their research on cold hardiness.
Spring is here, and despite the recent snow, cold hardy birds like the Northern Cardinal and Black-capped Chickadee (Poecile atricapillus) are losing fat and shedding fluffy feathers in preparation for the breeding season!

 
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A hibernating 13-lined ground squirrel (Ictidomys tridecemlineatus) in Dr. Jim Staples lab at the University of Western Ontario, December 2015, Photo credit: Kristen DeMoranville

Animal physiology changes in repeating patterns either daily, monthly, or yearly under the control of an animal's internal biological clock (defined as the bodily components that keep track of time within an animal). Whether you are a night owl or an early bird, our nightly sleep habits are controlled by our biological clocks. Read more about how our biological clocks control sleep here
Mammals like the 13-lined ground squirrel (Ictidomys tridecemlineatus) are adapted to hibernate during harsh winters when temperatures are below freezing, days are short, and food is unavailable. The internal clocks of these mammals filter through the environmental cues (e.g., temperature, light levels, and food availability) to control physiology and make hibernation possible. Ground squirrels lower their body temperature (and actually feel cold to the touch! See above picture) and slow their metabolism* (defined below) so that they are barely using or producing any energy. This means that their heart rates drop from 200 beats per minute to 20 beats per minute. Their biological clocks use similar environmental cues to stimulate their metabolism and rouse them from hibernation. National Geographic gives more overview about hibernation here, and Dr. Jim Staples investigates the metabolism of hibernating ground squirrels.
The onset of Spring is inciting hearts to flutter and, as Owl would explain to Bambi and Thumper, squirrels are twiterpated! If you live central North America then go searching, and don't forget to count their lines!

Animal physiology responds to environmental changes either on a short-term scale, long-term scale, or periodically in a repeating pattern. These changes in physiology can be difficult to observe firsthand. Although, behavioral changes can often act as a flag that alerts us to unnoticeable changes within an organism. Next time that you observe a behavioral change in one of your favorite critters I challenge you to think about the types of physiological adjustments that your animal might require to make that behavioral change possible. I'll review this post's comments in one week to see how well you all did with the challenge!

*Metabolism is a concept we will continue to revisit! This is how I like to think of the concept: All organisms require energy to perform their daily activities, and this so called fire for life is the product of chemical processes collectively referred to as an organism's metabolism.

 

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About the author:
Kristen J. DeMoranville is a Ph.D. student researching the effects of diet and long-distance flight on a migratory songbird in Scott McWilliams lab at the University of Rhode Island

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