why does the northward migration take two months longer than the southward migration?

I guess this refers to grey whales again.

Look at what they are doing during the migration

http://www.learner.org/jnorth/tm/gwhale/MigrationRoute_Map2008.html

The northward migration typically takes two months longer than the southward migration due to a few factors:

1. Breeding Patterns: Many bird species migrate to more northern regions during the summer to breed and raise their young. This process requires finding suitable nesting sites, establishing territories, and engaging in courtship rituals. These activities take time and thus, extend the duration of the northward migration.

2. Availability of Food: During the spring (northward migration), birds rely heavily on the abundance of food sources that become available as the weather warms up in the northern regions. They take advantage of the increased insect activity, blooming plants, and newly hatched insects, which provide ample food resources for them to refuel after their long journey.

3. Weather Conditions: The timing of the northward migration is influenced by the changing weather conditions. As winter transitions into spring, the northern areas experience milder temperatures, longer daylight hours, and favorable wind patterns. These factors support the growth of vegetation and the return of insects, creating optimal conditions for birds to find food and establish suitable habitats.

To specifically determine the exact reasons for the two-month difference between the northward and southward migrations, you can refer to scientific studies and observations that track migratory patterns of different bird species. Researchers often use techniques like banding and satellite tracking to gather data on individual bird movements and timing, helping to understand the specific factors affecting their migration durations.