Migration

Question 1

Compare bird mortality during migration and non-migration seasons as documented by Sillett and Holmes (2002)

Answer 1

Mortality during non-migrations (summer and winter): low, 99% survival in New Hampshire and 93% in Jamaica
Mortality during migration (fall and spring): higher, ~70% survival for both seasons. Calculated using annual survivorship and the product rule.

Question 2

What factors determine migratory behaviour in facultative migrants?

Answer 2

Are they competitive/high-status? Individuals that are dominant can defend a territory during the winter can reside their and avoid the stress of migrating, those that can’t are forced to migrate due to reduced resources.

Question 3

Explain why factors associated with high mortality are not always the ones that limit population size

Answer 3

There might be a carrying capacity in a season with lower mortality, or other macroecological reasons. Limits set last season/year may carry over

Question 4

Define the terms “winter range compression” and “relative winter range size”

Answer 4

Winter range compression is bird species migrate from a large breeding area in North America to a much smaller area in Central/South America during the winter.

Relative winter range size is the size of the winter range as a percentage of the breeding range. Rwrs is a measure of population compression, and the compression is greatest at about 17oN (Jamaica, Belize)

Question 5

Explain why winter range compression is prevalent in migratory birds that breed in North America

Answer 5

Only species that can breed further north or south from the equator can be large enough to support larger breeding ranges.

Question 6

Explain how parts of a bird’s breeding range can become a macroecological sink, with reference to the four features of migratory systems that can contribute to macroecological sink formation:

• Philopatry
• Geographic fecundity gradients
• Winter limitation
• Winter commingling of populations