Here is a very interesting article that talks about all the issues of racing pigeons. He stresses three very important points.
First that in his opinion pigeons navigate very badly if on release the sun is not shining. He thinks that they need the sun to navigate by if possible.
Second the wind direction is very important too. A Sami talked to me about navigating in bad conditions. He did it by feeling the direction of the wind and keeping it at the same bearing. In the article it describes what happens with there is low pressure system on the route that gyrates the wind direction.
Lastly racing pigeons are trained on a line North/South for instance. Changing the race direction causes problems.
Please note that we think that pigeons change their strategy at different stages of the route. They need a general sense of direction on release, then they use remembered geographic way points, and near their loft they use hippocampus mediated neighbourhood mapping.<<<<<