Birds in Migration & Winter

Roughly 85% of New Hampshire’s breeding birds migrate to some degree. Some may only shift south within New England for a few months, while others make trans-hemispheric journeys to South America or the southern oceans and are gone for over half the year. At the same time, many other species that don’t breed in the state occur here in their non-breeding seasons. Some of the same threats already discussed continue to apply to “our” birds when they aren’t in New Hampshire, or to non-breeding birds that find themselves within our borders. This section will discuss the threats that affect bird populations during this critical time in their annual cycle: from when young leave the nest until birds return to nesting sites in the following breeding season.

Birds are generally assigned to categories based on how far they migrate. This chart shows the trends for four such groups in New Hampshire, as defined here:

Residents

Residents are species that don’t migrate at all, or at best make irregular local movements. This group includes most woodpeckers and owls, titmice, cardinals, grouse, and corvids (crows and jays).

Short-Distance Migrants

Short-distance migrants spend the winter primarily in the southern United States, with some also in northern Mexico or the Caribbean. A few even winter as far north as New England, especially those that breed in the arctic and boreal Canada. This group includes most water birds, many raptors, and all finches and sparrows. Most such migrants, as well as our residents, are increasing or stable.

Long-Distance Migrants

Long-distance migrants leave the United State entirely to winter in the Caribbean, Central America, and South America. In South America, the destinations of most songbirds are in the Andes or Amazon Basin, while shorebirds and seabirds are along the coasts.

The farther a species migrates, the greater the chance that its population is declining. More species are increasing or stable than declining among residents and short-distance migrants. The numbers flip for the longest migrants however, where more species are decreasing then increasing. No matter how far they travel, migratory birds face additional challenges.

Understanding the Annual Cycle is Important for Conservation

Imagine for a moment that you are a migratory bird. It doesn’t matter if you breed in New Hampshire and fly south for the winter, breed in the arctic and pass through, or breed elsewhere and spend part of your non-breeding season here, you will follow the same general annual cycle. Understanding this cycle and how the different components interact is critical to understanding the conservation of migratory birds.

The threats that operate during the breeding season have already been discussed, but much can still befall “our” breeding birds before they even leave New Hampshire. Between nesting and migration is a poorly understood time known as the post-breeding period, when adults and young alike wander away from nesting areas and prepare for their upcoming migratory journeys. They may not stray far, but they often use very different habitats. For instance, many of New Hampshire’s forest birds move into shrublands in late summer, where they feed on abundant insects and newly-ripe fruit in preparation for migration. Active birders know to seek out habitat edges at this time of year to maximize the number of birds they encounter.

Biologists are only just starting to study birds during the post-fledging period, but it is increasingly clear that any factors that reduce survival in late summer will be additive to the threats which operated while birds were still nesting. Even if your local swallows successfully produced large numbers of young, there is no guarantee that these young will even survive until the start of southbound migration. They may be more vulnerable to predators (including cats), more likely to collide with structures, or more susceptible to changes in food supply.

Diagram showing the annual cycles of three representative birds. The colored arcs represent generalized date ranges for each stage of the life cycle, with red lines indicating when that species predominately occurs in New Hampshire. The American Redstart is a long-distance migrant that winters in the Caribbean, and only occurs in New Hampshire from May through September. In contrast, Wood Ducks only migrate as far as the southern United States, and spend significantly longer here in the north: arriving in March and not leaving until November. The Least Sandpiper is an example of a species that doesn’t breed in New Hampshire. It nests in the arctic and winters in the tropics and only occurs in the state during a short period in spring and slightly longer period in fall.

And then there’s fall migration itself, a time of shifting weather patterns, increased physiological costs, and for young birds the simple fact that they have never done this before. Long-term studies of migratory birds generally tell us that the vast majority of annual mortality occurs during migration (both fall and spring). If a bird was less successful at finding food during the post-fledging period, it might not be in prime condition to migrate, and might die early in the journey. Whether short or long, this annual journey is a gauntlet of new and familiar threats. There are predators (including people), obstacles (e.g., buildings, towers), unpredictable weather (including the fall hurricane season!), and ongoing habitat loss and degradation. A migrating bird can be killed outright, or it can fly through the night only to find an important resting area converted to a subdivision, drained for agriculture, or otherwise rendered unsuitable. If the bird can’t rest or find food, it’s that much less likely to continue the next day. Over the course of weeks or months, these incremental stressors result in at least a third of annual mortality occurring during fall migration, and another third in spring.

Once a bird reaches its wintering area, many of the threats it faced during migration may remain. Habitat loss is still important, and for many long-distance migrants the loss of suitable habitat in the winter range has long been considered an important driver of declines. Depending on the habitat, more pervasive use of pesticides and other chemicals is an additional threat, although one still very poorly understood. Although mortality is relatively low during this stationary period, habitat quality can have important carry-over effects for the annual cycle. Poor quality winter habitat means birds are in poorer condition when the time comes to migrate north. Even if it doesn’t die along the way, it may arrive too late to find a mate and breed.

The American Redstart is one of the best studied migratory songbirds across its annual cycle, with research carried out on both the breeding grounds (red on map) and wintering areas (blue). New technologies are increasingly providing information on what happens during migration (yellow), including timing, routes, and linkages between breeding and wintering areas. Photo credit: Leo McKillop

Thus we see that conditions in late summer can impact fall migration, and conditions over the winter influence spring migration and even breeding success. If we want to conserve migratory birds, we cannot ignore these connections, for the most important threats might not be the ones we’re able to address here in New Hampshire.

With this in mind, what follows is a list of threats and how they operate outside of the breeding season.

Threats During Migration and Winter

Habitat Degradation & Loss

Birds still need food and shelter during migration and winter, and if the habitats that provide these are degraded there is a higher risk of mortality from predation or starvation. Areas where birds rest en route are called stopover sites, and these vary greatly in the quality of resources they provide. Something as small as a clump of trees in an urban neighborhood might be attractive to a migrating warbler in desperate need of a place to land, but the best stopover sites tend to be in more intact native habitats. The same applies once a bird has arrived at its winter destination. If the habitat there is intact the bird stands a good chance of surviving until spring migration, but if not it may be forced into poorer habitat where survival is compromised. As more suitable wintering habitat disappears, fewer birds survive the winter to return to northern breeding grounds. Even if they survive, birds from low-quality winter habitats may be in poor condition and less likely to reproduce successfully.

Stopover habitat loss occurs all along a species’ migration route and is especially detrimental along coastlines. Coastal plains are often the first or last landfall for migrants making long overwater crossings, and can be an important source of food. New Hampshire’s coastal shrub habitat provides berries and insects in the fall, a combination that allows birds to build up fat quickly to fuel their migration. In the US, and increasingly in Central and South America, development in the coastal plain eliminates this vital stopover habitat, fragmenting forests and degrading or filling in wetlands. Extensive loss of wintering habitat is most pervasive in Latin America and the Caribbean, where many people still rely on subsistence agriculture and continue to clear forests for this purpose. At the same time, even larger areas of habitat are being converted to commercial agriculture, including cattle farms, aquaculture, coffee plantations, soybean fields, avocado groves, and other uses. Agriculture doesn’t have to be a total loss in terms of habitat, as evidenced by studies of birds in shade vs. sun coffee plantations, and there are increasing efforts to develop more sustainable practices for other commodities such as chocolate and rice. Note also that winter habitat loss is not just a tropical issue. Wetlands and bottomland forest in the southern United States have long been removed or altered for agriculture, effecting species like wintering Rusty Blackbirds. The urban/residential footprint required to support growing human populations is an ongoing issue everywhere.

Chemical Use

In many cases, pesticides and other contaminants are less regulated outside the United States, where they have the potential to significantly impact non-breeding and resident birds alike. In 1996, for example, application of a pesticide to control grasshoppers resulted in the death of thousands of wintering Swainson’s Hawks in Argentina. DDT, long banned in the US and Canada, is still used for mosquito control in parts of South America, and the effects of other pesticides remain largely unknown. There is increasing concern that intensifying agriculture in parts of South America may be contributing toward declines in aerial insectivores such as swallows and nighthawks.

Collisions

Most mortality from collisions with buildings (windows) and towers likely occurs during migration, when birds are in unfamiliar surroundings and often migrating at night, when they can easily become confused by artificial light. Even if they are not killed outright, confused birds are easy prey for predators such as cats.

Human Disturbance

During migration, it is critical that birds build up sufficient fat reserves to fuel the next leg of their journey. When not actively foraging, they need to rest and conserve energy, and anything that disturbs them may compromise their migratory preparation. Most research on this topic relates to migrating shorebirds, which often roost on beaches that are also used by people. When these resting birds are flushed by beach-goers or their dogs, they expend valuable energy, and may end up delaying their departure as a result. The later they leave, the less likely they are to survive the entire journey. An extreme version of this threat is thankfully rare, and involves unregulated hunting of migrating or wintering shorebirds and terns. This practice is limited to a handful of countries in the eastern Caribbean and South America, but is believed to have contributed to declines in some species.

Climate Change

Climate change is likely to severely impact the availability and suitability of habitat. Sea level rise will inundate coastal areas used by water birds, and increased drought will likely reduce food supplies in wintering habitats. Increasingly volatile weather patterns may increase mortality risk during migration as a result of increased storm frequency or unseasonal temperature swings. Finally there is the issue of phenological mismatch, wherein the timing of seasonal events becomes out of synch. For example, many studies have documented increasingly early dates for leaf-out or insect emergence as temperatures warm. Migrant birds, however, may not advance their arrival or nest-initiation dates, especially if they are migrating in from far away. If they fail to time their arrival appropriately, they risk missing the peak in insect abundance they require to successfully feed their young. Studies on this issue have shown variable results, but in extreme cases birds may be significantly compromised.

Threats faced by migratory birds on their wintering grounds also affect resident species in tropical countries. Conservation actions implemented “south of the border” can thus benefit “our” birds as well as rare or declining species such as Blue-headed Quail-Doves (found only in Cuba) that use the same habitats. Everything is connected.