Competition with the Locals
Migratory birds must compete for resources (such as food) with resident birds throughout their lives. Resident birds would seem to have all the advantages because on a daily basis they can minutely inspect their territories to find food, shelter, and water. How can migratory birds compete? Perhaps a superior intellect is their best weapon.
North America is the destination for neotropical migratory birds during the breeding season. From the vast boreal forests of Canada to the extensive deciduous forests of the eastern United States, from the grasslands of the Midwest to the arid scrub of the southwest migrants must quickly establish territories that provide enough extra food to raise a family. How do they know where the best areas are? Most adult birds simply return to the same place as the previous year. Their long-term memory is excellent and they can quickly find their old home, locate available food resources, and defend the territory.
Young birds, those that hatched the previous summer, are at a disadvantage. If their parents survived, they have no home to return to and searching for a new territory can be very time-consuming. These young birds also use their exceptional memories to find a good habitat. After fledging the previous summer, young birds spend time wandering around their home territory and explore looking for good areas to visit the next summer. They cue in on areas with lots of resident and migratory birds because these areas have abundant food and low numbers of predators. They will remember these places the following summer.
Resident birds must intimately know their territories in order to find food, water and shelter throughout the year. For many residents, winter is the most challenging time of year. While neotropical migratory birds enjoy warm weather and plentiful food in the tropics, resident birds must struggle to cope with a limited food supply.
Insects, the primary food source in warm weather, are much less abundant and difficult to find. Some residents, such as woodpeckers, find insects in dead wood, while others glean insect eggs from bark crevices. But to sustain themselves, residents must partake of other food sources such as seeds and fruit. Therefore, they have adaptations to allow them to take advantage of winter food sources, for example a conical bill that can open tough seed shells, or a chiseled beak that can chip away dead wood to expose insect grubs.
But despite these adaptations, mortality is quite high among resident birds during winter--there just isn't enough food to support large numbers of them, and they are more vulnerable to predators and extreme weather conditions. When returning in the spring, migratory birds have relatively few resident birds to compete with and abundant insect prey on which to feed. Many have thin beaks that are better at gleaning insects from foliage than the residents' winter-food-adapted beaks.
It is interesting to note that the very successful resident woodpeckers of the northern forests are able to find insect prey in winter by digging them out of wood. The migratory woodpeckers, sapsuckers and flickers, are relegated to specializing on foods not available in winter. Sapsuckers eat sap and the insects stuck to it, and flickers feed on ants by probing their nests with long tongues.
The fact that migratory birds leave their breeding grounds and avoid the high costs of trying to survive winter is not without risk. Migration is physically demanding and this is the most perilous time for them. They must quickly locate food on their migratory route to refuel for their long flights. Even so, birds spend a lot more time at stopover sites refueling than actually flying to their wintering grounds.
In addition, they must be able to find food despite competition from resident birds that intimately know the area and where the best food sources are. Migratory birds appear to have a different way of thinking about the areas than residents. A study of a migratory bird, the Garden Warbler (Sylvia borin), and the closely related but resident Sardinian Warbler (Sylvia melanocephala momus), demonstrated the differences between migratory and resident birds.
Individual birds could explore a new room connected to their familiar cage. Food was hidden in the room and Garden Warblers found the food twice as fast as the resident Sardinian Warblers. They did this by more rapidly exploring the new room than the resident. The ability to quickly assess a new area is of paramount importance to a migratory bird whereas a resident bird may be more concerned with intimately learning all the details of a new area in preparation for the upcoming season.
In another experiment, involving the same two bird species, the birds were allowed to explore two rooms adjancent to their cage, one with food and one without. After not being allowed to enter the rooms for up to 12 months, the migratory birds remembered which room had the food but the residents did not. Adults migratory birds can remember where the best stopover locations are from previous migrations. Also, migratory birds will often search for resident and other migratory birds when first arriving at a stopover location in order to follow them to the places that currently have the most food.
On the wintering grounds in the tropics, migratory birds must compete with resident birds that not only are intimately aware of the immediate area, but are adapted to feeding on insects, a food source also coveted by migratory birds. Because of the benign weather and abundant food, resident birds will often breed at any time during the year when there is enough extra food around to raise a family.
The most desired insects are often quite large, such as orthopterans (for example katydids, crickets, and grasshoppers), which are most abundant in the centers of large tracts of forest. Residents push migratory birds to the edges of the forests and into disturbed (regenerating) areas. Here there is an abundance of smaller insect prey, but, since the migratory birds are not raising families at this time of year, it is more than sufficient for their needs. Migratory birds remember the best wintering areas and return to them year after year.
While local resident birds would seem to have an advantage over migratory birds at all times of the year, the actual ability to migrate, which requires a good memory, excellent spatial recognition, and the ability to adapt to novel foods and experiences, gives the migratory birds the skills with which to compete across a vast geographic range.