- The large-scale, seasonal, annual movement of all or some of a population between breeding (in our case summer) and non-breeding (wintering) grounds.
- Why? Birds migrate primarily to gain access to the best food sources.
- Risks: Migration takes a lot of energy and presents birds with many obstacles, both natural and human-caused including adverse weather, collisions, exhaustion, predators in an unfamiliar place and habitat loss since they last traveled the route.
- Not all birds migrate and some birds only migrate certain years (irruptive species).
- There are pros and cons to migration, and selection pressures (survival versus reproduction) that must be balanced.
- How do they navigate? Land based cues, stars, UV light, and magnetic fields. Timing affects their strategy. Raptors migrate during the day and thus do not use stars, but they do use land cues.
- How do they know when to go? Primarily because of the length of day. Lesser cues include temperature, food supplies, and genetic programming.
Stay or Go?
- Some species disperser more than other.s
- Many young of the year disperse widely probably to avoid inbreeding and keep the population size appropriate.
- Some species have high hatch year returns, such as colonial seabirds (gulls and terns) usually return to their natal island colonies. Penguins can use smell to recognize kin and avoid inbreeding. Also helps adults find their chicks when they return to the colony to feed.
- Some species, such as the White-throated Sparrow have high fidelity to their breeding grounds and wintering grounds.
- Some species, like the Blue Jay, don’t migrate, but are site faithful to their location year round.
- Some species are nomadic and wander in search of food (crossbills) and will breed or winter depending on what they find.
Climate Change- Are birds adapting and if so, how?
- No: Some birds have not adapted to the immediate effects of climate change. Seabird colonies are already facing habitat loss and loss of nesting and breeding grounds.
- Yes: Some species are arriving in the spring a few days earlier when compared to 50 years ago. This presents a problem because the insects that the birds rely on as their main food source, hatch and reproduce based on local weather, not on the birds’ arrival. Birds arriving from South America have no idea what our weather is like here when they leave Brazil. If they arrive in the North too late for the insect hatches it’s basically defeated the purpose of their having migrated here. Long distance migrants have shown less change (like the Great-crested Flycatcher), short distant migrants have adapted more.
- Yes: Birds that live at specific elevations are moving to higher elevations as their current habitats have changed.
- Yes: Range expansion of species (like the Northern Cardinal, Carolina Wren, Tufted Titmouse, and Northern Mockingbird) are moving farther north in the U.S. than they used to be.
- Eventually, habitat loss and change will become too great and birds, as well as wildlife, will run out of places to go.
- Migration is affected by weather. Strong head winds drain a bird’s energy and so birds will stall trips to wait for favorable winds. Diurnal migrants use hot air thermals (formed by sunny days heating the air) to conserve energy by riding the air currents. Birds also can sense approaching storms and change their course to avoid them.
- Birds can survive changing weather conditions, by adjusting their feathers to trop all the hot air generated by their body underneath.
- Feathers that are properly in place shed water and rain like shingles on a roof. Penguins, like other birds that live in a cold climate, have adaptations to avoid losing too much heat and to preserve a central body temperature of about 40 °C.
- The feet pose particular problems since they cannot be covered with insulation in the form of feathers or blubber, yet have a big surface area (similar considerations apply to cold-climate mammals such as polar bears). Two mechanisms are at work. First, the penguin can control the rate of blood flow to the feet by varying the diameter of arterial vessels supplying the blood. In cold conditions the flow is reduced, when it is warm the flow increases. Humans can do this too, which is why our hands and feet become white when we are cold and pink when warm. Control is very sophisticated and involves the hypothalamus and various nervous and hormonal systems. However, penguins also have ‘countercurrent heat exchangers’ at the top of the legs. Arteries supplying warm blood to the feet break up into many small vessels that are closely allied to similar numbers of venous vessels bringing cold blood back from the feet. Heat flows from the warm blood to the cold blood, so little of it is carried down the feet. In the winter, penguin feet are held a degree or two above freezing to minimize heat loss, whilst avoiding frostbite. Ducks and geese have similar arrangements in their feet, but if they are held indoors for weeks in warm conditions, and then released onto snow and ice, their feet may freeze to the ground, because their physiology has adapted to the warmth and this causes the blood flow to feet to be virtually cut off and their foot temperature falls below freezing.
- Birds can radiate off extra heat by spreading their wings wide to create more surface area. They can pant by ruffing up their feathers to allow more airflow between them.