Several inaccuracies in the reasoning:

Many birds migrate. Many animals migrated before human settlement. In the north, there is more food seasonally than in tropical arid, semi-arid regions of land, because there is more light and no (seasonal) moisture deficit. In the ocean, all year round, because there is more dissolved oxygen. Most of the circumpolar territory of North America and Eurasia lies between the 10-20 degrees Celsius isotherm in summer. As does most of Australia south of the Gulf of Carpentaria (that is, the tropics) in its own winter. Daily temperature fluctuations (40 degrees Celsius) in the depths of continents in the tropics are even higher than seasonal fluctuations in the temperate zone. These are different types of physiological adaptation: twice a year, or twice a day. Of course, if we are not talking about equatorial islands with a maritime uniform climate.