- [Wiki]
http://en.wikipedia.org/wiki/Empidonax_difficilis.
- [AnAge]
http://genomics.senescence.info/species/entry.php?species=Empidonax_difficilis.
- [avibase]
https://avibase.bsc-eoc.org/species.jsp?lang=EN&avibaseid=44A2028364A252A6&sec=lifehistory.
- [DaviFisl1963]
Davis, J., Fisler, G. F., and Davis, B. S. (1963).
The breeding biology of the western flycatcher.
The Condor, 65(5):337--382.
- [Kooy2010]
Kooijman, S. (2010).
Dynamic Energy Budget theory for metabolic organisation.
Cambridge Univ. Press, Cambridge.
- [PrinPres1991]
Prinzinger, R., A.Preßmar, and Schleucher, E. (1991).
Body temperature in birds.
Comp. Biochem. Physiol., 99A(4):499--506. doi:10.1016/0300-9629(91)90122-S.