PHILLMORE*, L.S.; MACDOUGALL-SHACKLETON, S.A.; University of Western Ontario; University of Western Ontario: Photoperiodism in Black-capped Chickadees

Most seasonally-breeding birds terminate reproduction when they enter a state of photorefractoriness. At this time, the length of daylight that was sufficiently long to stimulate reproduction in the spring is no longer stimulatory, and birds regress their gonads and enter moult. In many species, photorefractory birds are completely unresponsive to even 24L (i.e. absolutely refractory). In other species, stimulation with very long days may lead to birds breaking refractoriness and resuming reproduction (i.e. relatively photorefractory). We explored the responses to photoperiod by black-capped chickadees (Poecile atricapilla) by measuring gonad size in birds held under controlled photoperiods. Chickadees are year-round residents and their annual behavioural cycle (breeding, flock formation, food caching) is well-studied. However, the mechanisms underlying this annual cycle are relatively unstudied. We housed captive birds on short days for several months prior to the experiment. In this condition they developed relatively large, but smaller than breeding size, gonads. We then transferred birds to 15L: 9D. Birds quickly recrudesced their gonads to breeding condition, then eventually (8-10 weeks) all birds regressed gonads and began moult. At this time we maintained some birds on 15L: 9D and transferred others to 24L: 0D. Birds transferred to continuous light continued moult and maintained regressed gonads. These results indicate that chickadees respond to changes in photoperiod similar to other well studied seasonally-breeding species such as European starlings (Sturnus vulgaris). Thus their annual changes in behaviour are likely mediated by a changing sensitivity to photostimulation resulting in a yearly cycle through photosensitivity, photostimulation and photorefractoriness.