The USAF countered the MiG-15 by sending over three squadrons of its most capable fighter, the F-86 Sabre. These arrived in December 1950. The MiG was designed as a bomber interceptor. It had a very high service ceiling50,000 feet (15,000 m) and carried very heavy weaponry: one 37 mm cannon and two 23 mm cannons. They were fast enough to dive past the fighter escort of P-80 Shooting Stars and F9F Panthers and could reach and destroy the U.S. heavy bombers. B-29 losses could not be avoided, and the Air Force was forced to switch from a daylight bombing campaign to the necessarily less accurate nighttime bombing of targets. The MiGs were countered by the F-86 Sabres. They had a ceiling of 42,000 feet (13,000 m) and were armed with six .50 caliber (12.7 mm) machine guns, which were range adjusted by radar gunsights. If coming in at higher altitude the advantage of engaging or not went to the MiG. Once in a level flight dogfight, both swept-wing designs attained comparable maximum speeds of around 660 mph (1,100 km/h). The MiG climbed faster, but the Sabre turned and dived better.
To mitigate the loses of the B-29, what did the US Air Force do?
switch from a daylight bombing campaign to the necessarily less accurate nighttime bombing

Bird migration is the regular seasonal movement, often north and south along a flyway, between breeding and wintering grounds. Many species of bird migrate. Migration carries high costs in predation and mortality, including from hunting by humans, and is driven primarily by availability of food. It occurs mainly in the northern hemisphere, where birds are funnelled on to specific routes by natural barriers such as the Mediterranean Sea or the Caribbean Sea.
Why do birds primarily migrate?
availability of food

Based on the similar shifts underway the nation's economy after 1960, Philadelphia experienced a loss of manufacturing companies and jobs to lower taxed regions of the USA and often overseas. As a result, the economic base of Philadelphia, which had historically been manufacturing, declined significantly. In addition, consolidation in several American industries (retailing, financial services and health care in particular) reduced the number of companies headquartered in Philadelphia. The economic impact of these changes would reduce Philadelphia's tax base and the resources of local government. Philadelphia struggled through a long period of adjustment to these economic changes, coupled with significant demographic change as wealthier residents moved into the nearby suburbs and more immigrants moved into the city. The city in fact approached bankruptcy in the late 1980s. Revitalization began in the 1990s, with gentrification turning around many neighborhoods and reversing its decades-long trend of population loss.
Who replaced the wealthy?
immigrants