Where do they go?

Each year the continent of North America is the stage for an incredible dance. Driven by seasonal changes and instinct, migrating species each begin their individual journeys from their breeding grounds to their winter ranges. Carried by the wind and their amazing ability of flight, these birds demonstrate the stunning distances that animals can cover in the course of their migrations.

From our earthbound perspective, we humans observe these epic migrations within a static frame of reference, catching glimpses through windows, glances skyward, or, for the enthusiast, through eager binoculars. These brief moments often punctuate the seasonal changes. In the northern hemisphere, robins signal the arrival of spring and geese heading south indicates the return of winter. Since we can't fly up and join them, our experience of their migrations is typically momentary and leaves much unanswered for the curious; where do they go, and what do they do beyond those fleeting moments when they’re perched outside our window?

Science and technology haven’t quite been able give us the ability of graceful flight so that we might fly along, but we can join them in other ways. Scientists have been able to attach miniature GPS recording devices to individual birds. These devices grant us the ability to measure many metrics, including logging where birds go and when. Exploring these digital tracks, we can gain a glimpse into some of their incredible feats of migration.

So, consider this an invitation to explore bird population and migrations using data as your guide. Together we’ll explore a handful of North American bird species learn about migration behavior along the way. Hopefully, these incredible migratory journeys will inspire you to continue exploring the amazing feat of migration and learn about the challenges species face.

Larus argentatus

Herring Gulls are common across the continent and are often identified as the classic seagull. However, the Herring Gull is just one of some two dozen gull species that call North America home. As shoreline birds, Herring Gulls tend to stick near large water bodies where they’re close to their primary food source of marine invertebrates, fish, and insects. Being opportunistic predators, these birds also scavenge fish, carrion, trash, and of course, boldly snatch the occasional abandoned French fry.

If you reside in the coastal United States or Canada, or near the Great Lakes then you’ll likely recognize these birds as the ever-present source of squabbling above parking lots and beaches. Those birds who prefer to reside year-round in this part of the continent make up just a portion of the total North American population. Other populations spend their summers inland across their breeding range throughout northern Canada and into Alaska. These gulls then migrate south, spending the winter along the coasts of North America and even into the southern United States and Gulf of Mexico.

During this migratory period throughout late-June and July, scientists and citizen scientist volunteers tirelessly record species observation data as part of the North American Breeding Bird Survey (BBS). The BBS is a methodical data collection effort that has been documenting population trends for continental species in North America since 1966.

Today this survey is performed at over 4,500 locations across the continent. The quantitative data collected through the survey is invaluable to analyze population trends and inform management practices and conservation policies.

Exploring the BBS survey data using ArcGIS Insights provides numerous spatial and temporal visualizations that can reveal trends in the data. Through analyzing the species observations, we can visualize where Herring Gulls are prevalent during their breeding season and how observations have changed over time.

By mapping and charting various aspects of the survey observations we can verify what we found in our initial research: the average observations recorded in the northeastern United States and Canada far exceeds any observations recorded further inland.

Digging deeper, the BBS indicates that Herring Gull populations are trending downwards. Between 1966 and 2015 the BBS has recorded a decline of over 3.5% a year. This decline accumulates to an 83% decrease in observations during that time. Competition with fisheries, depleted fish stocks, and disturbances to their natural coastal habitat have influenced this decline. These impacts to their habitat and food sources may be effecting their behavior and ultimately can change their migration patterns, even driving these gulls to a more urban habitat.

The Breeding Bird Survey provides valuable data for understanding the baseline trends for the species and providing us with contextual information. However, to answer the question “where do they go?” we’ll need more granular data to fully experience the journeys of individuals within the species. For this we’ll turn to GPS tracking datasets from Movebank. These datasets are gathered as part of scientific studies investigating movement behavior and, in our case, allow us to travel with birds on their incredible migrations.

but where exactly are they going?...

It takes four years for Herring Gulls to reach adult maturity. During this time not only does their plumage change, from mottled brown to gray and white, but so does their migration behavior. Younger, non-breeding individuals, tend to migrate farther south for the winter where adults tend to stay near the breeding grounds.

To further explore the differences between individuals and their destinations, we can analyze movement tracking data from Anderson et al. (2020) that studied differences in migration behavior of a variety of both short and long-distance migrating gulls. Studying movement patterns in migratory species adds to our knowledge of how environmental changes can impact a species, especially during the challenging process of migration when birds are expanding significant energy to travel vast distances.

The GPS track data illustrates the incredible journeys of individual birds. The migratory routes become clearly mapped across the continent and differences between the routes and distances used by the various colony populations show the variability within the species.

Tracks don’t tell the whole story. The data demonstrates where the birds were moving and certainly answers where they traveled. However, as we’ve learned, Herring Gulls are flexible migrants: some travel longer distances than others and they aren’t about to set any non-stop endurance records. Rather, as with many other species, they need to stop and refuel along the way. Essential habitat and resources along the way dictate much of the Herring Gull journey and can greatly influence the success of their migration.

To visualize how these birds move across the continent, we can analyze the GPS data using stop detection and the Insights scripting environment. This analysis uncovers where gulls stop along their journeys and quantifies the time spent at these locations.

The results show where the activity of GPS data was found to exist within the same general location and serves to pinpoint important habitat for Herring Gulls throughout their migration cycle. Even a casual examination of the quantified stop detection yields interesting insights into where Herring Gulls spend time throughout the year. Those gulls who migrated south in this study wintered in several wildlife refuges including the Aransas and Brazoria National Refuges in Texas. In comparison, others spent their time within Fire Island National Seashore on Long Island and a railyard in Philadelphia.

Regardless of whether the gulls were short or long-distance migrants, studying the movement of individuals provides clues to understanding environmental factors impacting the species and can highlight the need to protect both permanent habitat and stopover habitat essential to the success of the species.

Cathartes aura

The soaring silhouettes of Turkey Vultures punctuate the sky across North America. Floating on thermals above the countryside, these birds are often mistaken for other raptors, but can be distinguished by the outstretched “fingers” of their wingtips. These birds are the most common large carnivorous bird on the continent; in terms of their size, they are only exceeded by their relative, the California Condor, and the Bald Eagle.

Turkey Vultures have an expansive range across much of North America and are most noticeable while using their excellent sense of smell, which is uncommon in birds, to search for carrion, patrolling roadsides and farmland, and frequenting landfills scavenging their next meal.

The Turkey Vulture is a permanent resident in the southern United States, preferring the open country or shrublands, deserts, and foothills of the continent. It shares much of this southern range with the Black Vulture. In these areas where the species ranges overlap, the two vultures associate and can be observed flocking together and swarming carcasses. Turkey Vultures have a much more proficient sense of smell than the Black Vulture and can detect decaying flesh from a great distance. To simplify their search for food, Black Vultures can often be observed following the nose of their Turkey Vulture counterparts to locate an easy meal.

Currently, Turkey Vulture numbers are increasing in North America and overall their population is stable and of overall their population is stable and of low concern. However, this was not always the case. Turkey Vultures have few natural predators, and human influence has the greatest impact on the species. Like other large birds, the use of DDT pesticide threatened the species before it was banned.

Today, human perception and activities remain a primary factor in their population. Despite benefiting the ecosystem by cleaning up carrion and preventing disease, their scavenging nature has often led to Turkey Vultures and their Black Vulture relatives being incorrectly accused of spreading disease.

Migratory bird legislation covers Turkey Vultures and aims to safeguard the species against direct harm, but indirect human activities are an ongoing threat to the species. Their scavenging behavior frequently causes them to become victims of poisoning through lead shot in hunting carcasses or other poisons in dead animals. This is a significant concern and impacts other members of the Cathartidae family including the critically endangered California Condor.

Biologists recognize six subspecies of Turkey Vulture. Two of these subspecies, septentrionalis (eastern North America) and meridionalis (western North America, primarily reside in North America. The GPS track dataset of these vultures clearly delineates the different subspecies within their respective regions of the continent.

how far do they migrate?...

The migration behavior varies between the subspecies, with the western subspecies migrating much greater distances from all along the west coast of North America, and even central Canada, down into South America during the breeding season. For example, Ozzie migrates from Central Canada to Colombia, and Grayland migrates along the west coast of North America from Canada to southern Mexico. Their eastern counterparts have shorter migrations between the northeast United States and Canada (extending further north with climate change) south to the Carolinas down through Louisiana. For example, Disney and Butterball both migrate between Pennsylvania and Florida in the eastern United States.

As they travel south, Turkey Vultures gather in large groups numbering in the thousands. This gathering can be seen in the GPS data where the western and central migration tracks converge together and funnel through western Central America. Some of these birds then continue into South America including Colombia and Venezuela, with some experts believing that they may also reach as far south as Brazil and Argentina.

Yet again, the tracks alone don’t tell the full story. The migratory journeys are impressive in their own right, with some birds travelling a stunning 7,000 kilometers (4,300 miles) in a one-way trip to their winter range before returning home. Additional movement analysis allows us to identify stopover locations and quantify the time spent along the way.

Performing stop detection on this study of 80 different tagged vultures reveals another aspect of their journey and the use of habitat in between their primary ranges. The first thing that becomes apparent is the clustering exhibited by the eastern vultures and that they make much shorter trips while migrating, if at all, and gradually move up and down the eastern United States.

In comparison, the stops of the western vultures, apart from an intense clustering in Arizona, are much more distributed between longer traveled stretches. Examining this clustering more closely reveals that numerous birds spent a great deal of time around the White Tank Mountains west of Phoenix. In addition to the birds that stopped there, several non-migratory vultures remained there throughout the year.

Turkey Vultures are an extremely successful and flexible migrant. Through analyzing their movement and migration patterns, we can begin to understand how they excel at these incredible journeys and further understand how changing environment and human influence may impacts the species.

Bubo scandiacus

The elusive Snowy Owl is an exciting sight for birders and non-birders alike; their pale plumage, striking yellow eyes, and regal appearance capture the attention of even the most common of observers. These distinctive owls are the heaviest member of the owl family and certainly one of the largest. This cements the Snowy Owl as the largest avian predator in the high arctic. They’re well suited to the conditions of the remote frozen tundra of the north with insulating feathers and specialized hunting tactics. They survive on hunting small mammals, such as lemmings, but can also adapt to other prey depending on available population.

In the breeding season, Snowy Owls take up residence north of the Arctic Circle and all the way up to the northernmost points of Canada. Here breeding pairs will lay claim to their expansive nesting territory of one to two square kilometers.

The remoteness, Arctic conditions, and sparse density of their breeding territories make these magnificent owls elusive subjects to observe and study. The BBS has very few observations recorded for these birds. Rather, opportunities to observe and study them are much greater in the winter when they migrate to more southern latitudes closer to denser human populations in southern Canada and the northern portion of the United States. During this time, they’re much more visible and recorded in other studies such as the Christmas Bird Count.

Despite making these migratory movements, the extent of their wintering range is difficult to delineate. Their movements are wide ranging, unpredictable, and disperse far from their birthplace. Some intrepid owls have been recorded traveling as far as eastern Russia from their birthplace in the Canadian Arctic.

Furthermore, their distance and destination are not the only variables. As an irruptive species, that occasionally appear much farther south and in much greater numbers than usual, Snowy Owl movements can also vary from year to year. During normal years they appear regularly in southern Canada and some of the northern United States. In other irruptive years they may appear much farther south or west, towards the Pacific coast, and in very rare extremes some young owls have been recorded visiting California or Texas. Inversely, some individuals may not even leave their breeding range in the winter, opting to instead overwinter within the north of the Arctic.

Exploring the GPS tracks of Snowy Owls from this Movebank dataset sheds some light on the movements of these owls and illustrates the variability in their movement behavior. These tracks demonstrate just how ‘wild’ nature is, and create a stark contrast between the generalized behavioral descriptions aggregated from vast amounts of research and the actual raw movements of wild owls in nature.

Performing some movement analysis on this GPS dataset again yields some interesting results. Movement behavior differs greatly between those birds that migrated south versus those that remained in the Arctic. Compared to other species, the owls appear to have many more stops, where GPS data was found to be stationary for extended periods.

where do they spend their time?...

The abundance of stops could potentially be explained by the hunting strategies of the owl. Snowy Owls prefer to sit and hunt prey from a perch. Surveying the landscape from a building, hill, or fencepost, Snowy Owls use their excellent binocular vision to spot movement from a distance before swooping down to snatch their prey.

Looking at the location of stops, we can see just how far these owls can spread. Many of the studied owls remained within the Arctic or traveled to Greenland, but those who migrated further south in the winter spent time in Minnesota, the Dakotas, and parts of Montana. Other migrants flew east through northern Quebec, where Snowy Owls are the official bird of the province, and onward to Newfoundland.

Studying the movement ecology of these birds is vital to protecting them. Being so well suited to the Arctic conditions, the implications of climate change in the north may impact their breeding range making them vulnerable.

Delving into these datasets, we’ve come to learn just a little more than we might have ever uncovered by simply watching from our windows. Our journey of discovery here has allowed us to follow along on these birds’ incredible and challenging migrations. Perhaps, you’ve even come to know a little bit more about their behavior including where they go, where they stop, valuable habitat, and threats to their survival.

In addition to providing us with a brief glimpse into the world of birds, these observations and digital tracks equip scientists with valuable data to further research ultimately help inform important efforts to protect them.

If this has opened your eyes to the astonishing travels of birds or migration is general, you can dig in and learn more, perhaps about a particular species of interest or even how you might get involved in citizen science or local conservation efforts.