Editing Bird migration (section)

===Timing===
The primary physiological cue for migration is the changes in the day length. These changes are related to hormonal changes in the birds. In the period before migration, many birds display higher activity or [[Zugunruhe]] ({{langx |de|migratory restlessness}}), first described by [[Johann Friedrich Naumann]] in 1795, as well as physiological changes such as increased fat deposition. The occurrence of Zugunruhe even in cage-raised birds with no environmental cues (e.g. shortening of day and falling temperature) has pointed to the role of circannual [[endogenous]] programs in controlling bird migrations.<ref>{{cite journal |title=Stopover decision during migration: physiological conditions predict nocturnal restlessness in wild passerines |last=Fusani |first=L. |author2=Cardinale, L. |author3=Carere, C. |author4=Goymann, W. |journal=Biology Letters |year=2009 |doi=10.1098/rsbl.2008.0755 |pmid=19324648 |pmc=2679912 |volume=5 |issue=3 |pages=302–305}}</ref> Caged birds display a preferential flight direction that corresponds with the migratory direction they would take in nature, changing their preferential direction at roughly the same time their wild conspecifics change course.<ref>{{cite journal |url=http://jeb.biologists.org/content/202/16/2225.full.pdf |author1=Nievergelt, F. |author2=Liechti, F. |author3=Bruderer, B. |title=Migratory directions of free-flying birds versus orientation in registration cages |journal=Journal of Experimental Biology |year=1999 |volume=202 |pages=2225–2231 |issue=16 |doi=10.1242/jeb.202.16.2225 |pmid=10409493|bibcode=1999JExpB.202.2225N }}</ref>

[[Satellite]] tracking of 48 individual Asian houbaras (''[[MacQueen's bustard|Chlamydotis macqueenii]]'') across multiple migrations showed that this species uses the local temperature to time their spring migration departure. Notably, departure responses to temperature varied between individuals but were individually repeatable (when tracked over multiple years). This suggests that individual use of temperature is a cue that allows for population-level [[climate change adaptation|adaptation to climate change]]. In other words, in a warming world, many migratory birds are predicted to depart earlier in the year for their summer or winter destination.<ref>{{Cite journal|last1=Burnside|first1=Robert J.|last2=Salliss|first2=Daniel|last3=Collar|first3=Nigel J.|last4=Dolman|first4=Paul M.|date=2021-07-13|title=Birds use individually consistent temperature cues to time their migration departure|journal=Proceedings of the National Academy of Sciences|language=en|volume=118|issue=28|pages=e2026378118|doi=10.1073/pnas.2026378118|issn=0027-8424|pmid=34260383|pmc=8285904|bibcode=2021PNAS..11826378B|doi-access=free}}</ref>

In [[polygyny in animals|polygynous]] species with considerable [[sexual dimorphism]], males tend to return earlier to the breeding sites than their females. This is termed protandry.<ref>{{cite journal |author1=Diego Rubolini |author2=Fernando Spina |author3=Nicola Saino |year=2004 |title=Protandry and sexual dimorphism in trans-Saharan migratory birds |journal=Behavioral Ecology |volume=15 |issue=4 |pages=592–601 |doi=10.1093/beheco/arh048 |citeseerx=10.1.1.498.7541}}</ref><ref>{{cite journal |author1=Edwards, Darryl B. |author2=Forbes, Mark R. |year=2007 |title=Absence of protandry in the spring migration of a population of Song Sparrows Melospiza melodia |journal=Ibis |volume=149 |issue=4 |pages=715–720 |doi=10.1111/j.1474-919X.2007.00692.x}}</ref>