Seasonal forcing

Migration behaviours in birds predominantly revolve around seasonal changes to the world sphere; these changes affect the hours of daylight, temperature, vegetation presence, food abundances. Seasonal forcing occurs due to the eccentricity, obliquity and precession of Earths facing and orbital position around the Sun. These processes are known as the Milankovitć cycles, discovered by a Serbian astrophysicist, Milutin Milanković, in the 1920's. 


Eccentricity:

Figure 1. Showing one of the Milankovic cycles called eccentricity. Typical eccentricity cycles change approximately every 100,000 years. Illustrating how the elliptical shape of the Earth's orbit can have an effect on total irradiation reaching the surface of the Earth. Changes to this ellipse alter variation in seasonal irradiation levels; currently in 2017 the Earth is closer to the sun in January/February than it is in June/July, so Southern Hemisphere summers are exposed to greater irradiation than Northern Hemisphere summers. 

Obliquity: 

Figure 2. The axis of the Earth alters approximately every 41,000 years. Obliquity is the alteration to the angle of tilt exhibited by the Earth's axis. Over the long history of the Earth this has had a large effect on the presence of Ice-caps on the Earth's poles and the cascading effects associated. 

Precession:

Figure 3. Showing the cycling of procession where the Earth's 'wobbles' on its axis approximately every 26,000 years. This changes the apparent 'North Star' seen in Northern Hemisphere and the equivalent in the south. This also has a profound effect on exposure to solar insolation around the Earth. 


References:

Figure 1. http://www.ice-age-ahead-iaa.ca/large/eccentricity.jpg

Figure 2. http://planetfacts.org/wp-content/uploads/2011/03/Obliquity.jpg

Figure 3. https://media1.britannica.com/eb-media/98/94898-004-ADAA7A2D.jpg