has obvious effects on bird migration. It influences the times when
birds can travel, the energy costs and risks of the journey, and the
visibility of any celestial or ground-based cues that birds might use
for navigation.” This is a quote from Newton (2008) – a great book about bird migration –
explaining why checking weather forecasts is daily routine for many
birdwatchers. One of the websites I keep coming back to is www.windy.com.
Here is some useful theory about predicting how much migration is likely to occur at different places on particular days, again cited from Newton (2008) and based mainly on Richardson (1990).
“To a large extent, it is possible in high latitude regions to predict the likelihood of strong bird migration on particular days within the migration season from examination of synoptic weather maps, showing fronts and pressure systems over wide areas. The atmosphere at such latitudes is organised into high and low presure systems which move approximately eastwards. In the northern hemisphere, winds blow clockwise around highs and counter-clockwise around lows. Fronts separate air masses in these systems: warm fronts occur where advancing warm air is replacing cold air, usually east or southeast of an approaching low; and cold fronts occur where cold air is replacing warmer air, usually south or southwest of a low. Precipitation and thick cloud occur most commonly near lows and fronts.”
“Given the importance of wind direction and clear skies, migration timing can be related to these large-scale atmospheric features, even if birds sense only their local manifestations. In the northern temperate region, peak southward migration tends to occur with cool northerly tailwinds as a ‘low’ moves away to the east, or a ‘high’ approaches from the west, or both. Conversely, peak northward migration tends to occur with warm southerly tailwinds, as a ‘high’ moves away to the east, or a ‘low’ approaches from the west, or both, At both seasons, much migration also occurs with light winds near the centre of a high.”
“When birds are concentrated by coasts, ridges or valleys, the numbers passing a given point may be quite different from what the above weather patterns would suggest. Also, the association between synoptic weather patterns and migration volume is likely to decline as birds get increasingly far from their departure points.”
Have a great autumn!
|Published 14 August 2018|