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Current and recent graduate students have studied the role of maternal steroids in early development in red-winged blackbirds, European starlings, and dark-eyed juncos; seasonal profiles of testosterone and prolactin in red-eyed vireos and solitary vireos; the influence of testosterone on the pre-basic molt in dark-eyed juncos, mate choice across subspecific boundaries in dark-eyed juncos; genetics of variation in sexually selected traits in orioles; transmission of passive immunity in Japanese quail and pied flycatchers, ageing and the stress response in common terns, correlational selection on body size and an attractive plumage trait in dark-eyed juncos, testosterone, aggression, nest defence, and immune function in female dark-eyed juncos, hormonal correlates of dominance in female dark-eyed juncos, variation among female dark-eyed juncos in extra-pair fertilizations.
 Prior to the advent of genetic techniques, it was assumed that the majority of monogamous birds remained faithful to their partners.  However, it is now known that extra-pair copulations (EPCs), extra-pair fertilizations (EPFs), and extra-pair paternity (the raising of another’s offspring, EPP) are actually quite common in a variety of avian orders and families.  Roughly 70% of birds that used to be considered genetically monogamous actually engage in EPCs and raise extra-pair young (reviewed by:).
Stutchbury, B. J. M. , T. E. Pitcher, D. R. Norris, E. M. Tuttle, and R. A. Gonsar. 2005. Does male extra-territory foray effort affect within and extra-pair fertilization success in Hooded Warblers. Journal of Avian Biology 36:471-477.
Again, there is no great mystery about why males often make themselves available for extra-pair copulations, or EPCs. The evolutionary payoff in fathering additional children can easily make up for the costs, assuming that some EPCs result in EPFs (extra-pair fertilizations), and the cuckolded male doesn’t find out and take violent revenge. Of course, an EPCing male runs other risks as well, such as possible desertion by his own mate, or the chance that while he is trying to seduce someone else’s female, that same someone else — or another — is making time with his own! But the bigger question, yet to be resolved, is why a female, especially one already mated, should seek EPCs.
Regarding extra-pair copulations, the study found that 19% of males and 31% of females engaged in these, yet genetic analysis of the offspring revealed no evidence of extra-pair fertilizations. Males generally performed these at their nests, whilst females tended to perform these away from their nests, usually also avoiding directly neighbouring burrows. No particular choice of location was evident.