Date of Award


Publication Type

Master Thesis

Degree Name



Biological Sciences

First Advisor

Mennill, Daniel


Dear Enemy Effect; Fertility; Neighbour-stranger discrimination; Red-eyed Vireo; Song Sparrow; Testosterone




Many territorial animals respond more intensely to strangers than neighbours. This phenomenon is known as “the dear enemy effect”. This phenomenon occurs because strangers represent a threat to territory takeover and parentage whereas neighbours only represent a threat to parentage. Many studies have investigated whether diverse animals exhibit the dear enemy effect, but few have examined the underlying factors that mediate this phenomenon. I tested whether three factors – male repertoire size, female fertility status, and male testosterone levels – influence the dear enemy effect in male songbirds. I found that repertoire size had no effect on dear enemy effect expression; that female fertility status influenced flexibility in dear enemy effect expression over a breeding season; and that testosterone does not seem to be associated with dear enemy effect expression. Overall these results show that several factors influence the dear enemy effect and that the dear enemy effect is a dynamic phenomenon.