Notes on Migration
Seasonal migrations are widespread life-history and
evolutionary strategies adopted by a majority of bird species across the world.
Species are considered migratory if they use and travel between distinct areas
at different life stages or times. Most commonly, birds will migrate between
breeding and wintering grounds, often along a north-south latitudinal flyway. Many
species make incredible, long-distance journeys spanning continents or
hemispheres. This is such a prevalent
phenomenon that the word “migration” to many people is synonymous with birds,
but considering how prevalent it is scientists still know relatively little
about it. Migration is incredibly complex, affecting every species and every
individual differently, and the evolutionary drivers behind migration are
debated by scientists still. Bird migration as it pertains to Novasolan endemic
species is no less interesting or recondite a topic.
Novasola has at numerous times in prehistory been isolated
from the mainland and connected to it, cycling between island and peninsula as
sea levels fell and rose. During the Pleistocene epoch, sea levels rose and
fell periodically and the area that is now the Bering Sea would intermittently
become a land bridge connecting Eurasia, North America, and Novasola. During
the Last Glacial Maximum, between 30,000 and 11,000 years ago, Novasola was
connected to Alaska and Russia via the Bering land bridge before sea levels
rose to their current levels, separating Novasola for the final time. Birds,
which had already begun to speciate on Novasola from previous periods of
isolation, were then once again completely disconnected from mainland
populations. While most endemic bird species on the island had already evolved
or began to evolve by this time, Novasolan and mainland populations were
connected and allowed to share genetic material as recently as 11,000 years
ago. As such, most endemic species are relatively young and still closely
related to mainland ancestral species and populations, which explains why most
endemics are still quite similar in appearance, behavior, and ecology to those
mainland species. Many endemic species originally evolved from migratory
populations, but because Novasola is an island and far from other landmasses,
every species had to adapt their migratory behaviors. It is in their
differences in migratory strategy that we can usually see some of the greatest
differences between Novasolan and mainland species.
Most terrestrial bird species endemic to Novasola are either
entirely non-migratory or migrate short distances without leaving the island. It
seems most species have lost their migratory instincts and evolved away from long-distance
migration as an adaptation to the island isolation. However, there are still a
number of species that retain their ancestral migratory conditions and do
migrate off the island, mostly to areas of western North America. What follows
is a summary of the major migratory strategies.
Long-distance migration
Far and away the least common migration strategy exhibited
by endemic terrestrial birds on Novasola, ‘long-distance migration’ refers to
seasonal movements away from Novasola and onto the mainland. Because Novasola
is an island, isolated from the mainland by hundreds of miles of ocean, few
species make this sort of migration. Of
those that do, all of them migrate seasonally to parts of North America; while
there are many species that frequently travel to areas of Asia like Siberia and
Korea, these are considered vagrants and not true migrations at a
species-scale. To be considered endemic to Novasola, a species must breed on
Novasola, so this excludes species that breed on the mainland but migrate to
spend winters on Novasola, of which there are a few.
Long-distance migration is variable and just how a bird
migrates off-island may be dependent on species, population, or individual
choice. In general, long-distance migrants spend the spring or summer breeding
season on Novasola and winter in parts of North or Central America. A few
species travel as far south as South America. The bulk of species travel along
the Pacific coast in what’s called the Pacific Flyway. All long-distance migrants
must cross the Pacific Ocean during their journeys, which is extremely arduous;
there are three major routes endemic birds take to reach the mainland, each
with energetic benefits and drawbacks.
The most common route, called the “Alaska Crossing”, sees birds migrate north from Novasola across the Ricci Strait to Alaska and then on to the wintering grounds. This strategy is adopted by most birds because it involves the shortest distance travelled over open ocean, but at 255 miles to the nearest Aleutian island and over 450 miles to the mainland proper, this is still an extreme distance to fly. It also means birds are travelling north and east for some time before they can turn south. As such, migrations along this route may take significantly longer than those using other routes. This, in part, explains the shortened breeding seasons exhibited by many of Novasola’s long-distance migrants.
A second route, called the “BC Crossing”, sees birds fly
across the Gulf of Alaska between Novasola and British Columbia before reaching
the wintering grounds. This route bypasses Alaska and allows birds to shorten
the length of travel needed to go south, but it requires a significantly more
costly oceanic flight. To cross the gulf and reach Canada’s west coast, birds
from Novasola must fly over 1,000 miles over the Pacific. Only a small portion
of birds choose to migrate using the BC Crossing.
A third route, called the “California Crossing”,
unsurprisingly sees birds fly across the Pacific Ocean between southern
Novasola and California and Oregon. This is the least commonly used route, as
very few birds make the difficult 1,200+ mile journey over open ocean. Those
that do often find themselves swept off-course by storms and wind currents and
can end up anywhere along the Pacific coast or, more likely, deeper out at sea,
and it’s thought that this migration route has played a large part in
explaining the occasional Novasolan vagrants that appear in Hawaii, nearly 1,400
miles to the south.
Which crossing a bird chooses is dependent on a number of
factors and is still poorly understood. Some species stick to one route, while
other species may be seen using any or all three routes. Furthermore, birds may
use one crossing in the fall and another in the spring. Many birds that use the
BC or California crossings in the fall may return using the Alaska crossing, avoiding
the need to fly against the direction of major currents thus easing the burden
of the crossing somewhat. Regardless of route, long-distance migrants depart
and arrive on Novasola along the northeast and southeast coasts, which makes these
areas, especially the north coast, extremely important to migratory endemics
and a great place for birders to observe huge numbers of migrants in dense
numbers. Some places, like the foothills of the Stellar Range, show the highest
seasonal densities of endemic birds of anywhere on the island and scientists continue
to monitor migratory populations at numerous locations throughout the region,
most notably the Ricci Strait Bird Observatory, an extremely remote science
station along the north coast which famously records huge numbers of migrants
each year with some estimates suggesting as much as 70 percent of all
long-distance migrants go through the area.
Most endemic birds which can be considered migratory never
actually leave Novasola. These birds are considered “short-distance migrants”,
and the forms their migrations may take are varied. Some species may migrate
from north to south, mimicking a long-distance migration on a smaller scale,
while others may migrate between areas of high and low elevation, others migrate
between distinct seasonal habitats, and often it is some sort of combination. While
short-distance migrants may avoid the dangers and costs of cross-Pacific
long-distance migrations, they must instead cope with drastically different and
changing conditions like climate, weather, and resource availability, and these
intra-island latitudinal and altitudinal movements can help birds survive and
withstand Novasola’s extreme seasons, from winter ice storms to summer
wildfires.
Many short-distance migrants move along a north-south
gradient, typical of seasonal migrations, to take advantage of or avoid
changing weather patterns and temperatures. Using this strategy, birds that breed
farther north will travel south to spend the winters along Novasola’s southern
coasts. Because a warm equatorial current flows just to the south of Novasola,
the island’s southern parts are kept warm year-round and have a temperate
climate. However, a colder arctic current flows southward along the west coast
and the waters in the Ricci Strait are similarly cold, so northern parts of the
island are much cooler year-round but especially during the winter, when frequent
and heavy precipitation result in lots of snow and ice. For comparison, Cape George
has a similar climate to San Francisco while Vodograd is more akin to Anchorage.
Birds may winter in the south without suffering major winter weather or temperatures,
but many move back north in the summers to take advantage of bountiful
resources and less competition for them. Black Cranes are a famous example of
this. In some species these migrations may be erratic, meaning they don’t
reliably occur every year but instead only occasionally in response to specific
catalysts, namely food availability.
A similar phenomenon can be seen along Novasola’s slopes.
Many birds breed at high elevations in Novasola’s mountains, but when winter
comes these peaks are inundated with snow and ice and become inhospitable to
all but the hardiest birds. In these cases, birds will usually move downslope
to more moderate climatic zones where conditions are better and resources more available
but move back upslope when the weather warms. Like latitudinal migrations,
these too may be erratic or irruptive, as some birds will only migrate down or
upslope in particularly harsh winters or hot summers. White-throated Juncos and
Grand Emeraldines are well-studied examples.
Birds may also migrate between distinct or disjunct habitats or home ranges throughout the year which are unrelated to latitude or elevation. In many cases, birds migrate between areas to take advantage of different resources. For example, Gold Hummingbirds will breed throughout the prairie but will avoid any grasslands or shrublands during the winter. This strategy is like nomadism, but follows a clear and repeated pattern, and similarly evolved as an adaptation to the island’s severe seasonality. Novasolan seasons cycle not only between hot and cold, but also wet and dry. Summers on Novasola can be quite hot and dry, especially so in some places like the interior prairie, which impacts everything from water availability, vegetation and insect communities, and wildfire risk. Conversely, winters are cold, in many places freezing, and wet. Much of Novasola west of the Paramounts is considered temperate rainforest, with the bulk of the precipitation occurring in the fall and winter. In general, west of the divide is harsher, experiencing more dramatic shifts between seasons with colder, wetter winters and hotter dryer summers. East of the divide is generally milder, and the changes are less extreme. Buffered by the Paramounts and the moderate Gulf of Alaska, the east side has a more constant climate, though still experiences the same seasonal patterns. As a result, Novasola is a diverse set of ecosystems with ever-changing conditions throughout the island, and most endemic birds have evolved strategies to best take advantage of that.
In most cases, short-distance migrant species are actually
only partially migratory (more below), because some individuals may migrate to
lower elevations or southern shores, but others may already be there and
persist there year-round.
Residents
Perhaps the most obvious migratory strategy is not to
migrate at all. A number of endemic species on Novasola are non-migratory,
meaning they make no seasonal movements between distinct ranges. These birds
may be found within their greater range at any time of year. There may be many
different causes and strategies within this grouping, however, and even
non-migratory birds each showcase unique habits. Most non-migratory birds on
the island originally evolved from migratory ancestors and have lost that
instinct as an adaptation to island life, but many retain the instinct somewhat,
which often manifests in nomadism. Some endemic species are nomadic, meaning
they move throughout their range and don’t defend specific territories, or at
least not territories they return to in following years. Feasting Pigeons, one of the most common birds on Novasola, are nomadic: their flocks are constantly moving, following seed masts and insect swarms, and even during nesting flocks may not return to reuse sites. Nomadism, while still a
major movement, is not a seasonal migration as it may not follow any discrete seasons and birds will not travel between specific regions but instead consistently
move across the landscape following resources. Novaspiza sparrows are another clear example of this.
Other birds are non-migratory and remain in the same home
range throughout their lives. These birds, called residents, are well adapted
to both summer and winter conditions. In some cases their home range may be
quite small, like the Burrowing Woodpecker, while in others the home range
could be enormous, like the Novasola Condor. In the latter case, birds may
appear to be migratory because they travel vast distances, but they are in fact
residents of their territory year-round.
Partial Migration
While many endemic birds may be considered non-migratory by
field guides and the like, few species on Novasola are truly non-migratory, as
there are usually individual birds, if not entire populations, that will
undergo short-distance seasonal migrations, if not long-distance ones. This
phenomenon, where there are both migratory and non-migratory individuals
represented in a population, is called “partial migration”, has been documented
in numerous species across taxa including invertebrates, fish, birds, and
mammals. The drivers behind partial migration as an evolutionary strategy are
not fully understood, though many ecological factors may contribute to an
individual animal’s decision to migrate, including density dependence, inter-
or intraspecific competition, phenotypic conditions like size, sex, or age, or
individual personality. Whatever the cause, individuals exhibiting different
migration strategies will face varied conditions like weather and climate,
habitat quality, resource availability, and energetic costs, and will thus
likely face different fitness consequences, including survival, especially
during the migration period. Partial migration is an understudied topic, but as
researchers begin to focus more on migration, especially in regard to Novasolan
endemics, they are revealing more and more species or populations which exhibit
it. In fact, most species that exhibit short-distance migrations are actually
partially migratory; some birds may migrate while others remain in the
wintering grounds to breed. This is exceptionally common, so much so that
scientists now believe partial migration may be the most common strategy employed
by endemic birds.