Weather wonders
A fine sunny day means good migration weather for big birds such as storks and eagles. The sun heats up the land, causing warm air currents called thermals to spiral upwards. The birds use these thermals to gain height without using up precious energy by flapping. Once they are up high, they glide towards their destination on the wind, just like hang-gliders. Without thermals, many large birds could never get across seas and deserts. Flying speed depends on wind. In still conditions, small birds fly at an average speed of 30–35 kph and medium-sized ones at 45–55 kph. But with the wind behind them, birds can easily cover 1,000 km (625 miles) in just 24 hours. Wind allows some birds to perform amazing flying feats. Bar-tailed godwits have been recorded migrating from Alaska to New Zealand – a distance of almost 11,000 km (6,800 miles) – in just six days. Sooner or later the urge to move will overcome any barrier the weather may create. |
