Prologue
- Most everything is tied to driving forces of the need to feed and the need to breed
- 4000 species of birds migrate (of 11k total species)
- Largest concentration of birds occurs in the tropics around equatorial belt
Neotropical migration
Spring: Birds from central/south america tropics migrate to Northern America for breeding
Fall: Birds leave to their original places to escape north america winters.
Early migrants advantates
Birds that return first in the spring claim the best territories with the best sources of food. These are most successful at reproducing. Male migrants seem to arrive early than female. All migrants are half their size due to their long travel. With ample food, so male migrants produce healthy looking feathers to advertise their prowess and attract the best females.
Peace in Winter (non-breeding season):
These migratory birds dont breed during winter, so theres no reason to claim territories and attract mates. Unlike the breeding season, when fighting for territory is the norm, birds generally coexist very well during the winter months, which is at southern places where they come from during spring.
Types of migration:
Complete migration: Generally all the birds travel long distances(hundreds to thousands of miles) under predictable times each year. (ex: Warblers, Tanagers).
Partial migration: Not all birds migrate, and not all migrate at same time. Few stay back etc. And most migrations here are not super long distances. They migrate when food is short in the area (ex: american robin, red tailed hawk)
Irruptive migration: No predictability here. migrations dont follow any rule/logic. The birds migrate for scarcity of food
Non-migration: There are birds who dont migrate, but their reason for success in surviving is very interesting. Few examples: "Black-capped Chikadee" - has 1500 feathers in summer, 2500 in winter."White breasted Nuthatch"- birds huddle in a shelter to generate heat. Some birds have different way to thermal regulation. Some birds enter a state of 'Torpor'. It happens when an animal slows its body functions to conserve energy and heat. The bird's heart rate will slow; so does its breathing and body temperature.
Fueling for the trip
Migrant birds increase their food consumption by 25-30% before migration. They go to fruit diet, since fruits to fat very faster, compared to nuts/insects. Birds have to aquire addition fat to burn, but cannot be overweight since they have to travel hundreds/thousands of miles.
Day migrants/Night migrants
Few birds prefer day migration and few birds prefer to migrate in the night.
Perks of Day migration: Having more sun in the day heats up the earth and creates warm air currents/thermals. Once birds ascend to the top of the column, they glide effortlessly. Few birds like Swallows eat flying insects during the day while migrating, Some birds stop quickly to feed (increases migration time though).
Perks of night migration: Most birds which migrate in the night dont migrate in flocks, they migrate individually. In the night, there is less air turbulence and hence airflows are easy to manage for birds. Also temperatures are cooler in the night, hence dehydration might not be as problematic. Also less chances of overheating of bodies, hence enabling birds to fly farther without stopping. Also few/no predators in the night. Its possible that navigation through stars might be a plus. Birds who migrate in the night are supposedly more successful.
Birds have navigation chip (genetic memory):
When birds migrate to north in spring, and breed. The juvinile birds even though only 1-2months old can migrate back to south, and how/who trains it? no one. In an experiment, some birds are caught and flown to various places in the country and released, and all of them went back to their seasonal home, easily (even though its not their usual migratory highway)
Exceptions: We always have exceptions. In this case, "Whooping crane" learns migratory path from their parents.
Theories for bird sense of navigation
- Olfactory map: Birds ability to smell odors that people cant detect
- Magnetic map: using earth's magnetic field
- Stars orientation: Especially the birds which migrate over night
4 Flyways
https://abcbirds.org/blog/north-american-bird-flyways/
Elevations
Most songbirds/smaller birds travel over night and fly at lower elevations. Larger birds fly at higher elevations. Songbirds fly below 2000 ft. 90% of night travelers fly below 6000 ft. High flyers like Mallards fly above 21000 ft. Eagles and vultures fly even higher - 29000 ft (height of boeing/airbus).
Birds migrate super fast in spring, almost like a competition for territory and breeding. Same birds come slowly over fall migration to south, taking breaks a day/two along the way, almost like no hurry
Flying Formations
Few birds fly in formations, few in straight lines, few species in v shape. All these formations have incredible advantages, it modulates airflow making it easy for flyers, ultimately saving energy
Birds can sense storms
Birds while migrating will deviate 300 miles at times, since they sense the storm long before we humans can
Some facts:
- 1 billion birds die hitting glass windows every year
- 7 million die from hitting cell phone towers/wind turbines
- Millions are killed by domestic cats
