Endocrine Bases of Spatial and Temporal Opportunism in Arctic-Breeding Birds

The primary objectives of this paper are to define, explorethe environmental factors favoring, and discuss hypotheses concerningthe endocrine bases of two important arctic breeding strategiesthat we call spatial and temporal opportunism. We identify severalspecies that display spatial opportunism in the Arctic, andone that displays temporal opportunism. In spatial opportunism,breeding may be highly seasonal but the locality where individualsbreed may change from year to year as a result of unpredictablespatial distribution of food, nest site availability, or otherfactors such as predator abundance. We suggest that flexibilityof the transition from migration to settlement distinguishesspatial opportunists from site-faithful migrants. Thus far,data are available for only two hypotheses regarding the endocrinebasis of this flexibility. Circulating patterns of testosterone(associated with territory establishment) and corticosterone(associated with migratory activity) appear not to be involvedin the regulation of spatial opportunism in white-crowned sparrows(Zonotrichia leucophrys gambelii), but more detailed study isnecessary. In temporal opportunism (that may occur simultaneouslywith spatial opportunism), temporally variable food availabilityapparently selects for the capacity to initiate reproductionacross a wide portion of the year. Tonic activity of the hypothalamo-pituitary-gonad(HPG) axis could provide a mechanism to minimize the delay betweendiscovery of abundant food and onset of nesting in any season.However, the evidence for one arctic temporal opportunist, thewhite-winged crossbill (Loxia leucoptera), indicates that theHPG axis is not tonically active, but probably switches off(i.e., becomes photorefractory) in autumn, as for other birdsbreeding at these latitudes. Opportunistic breeding very earlyin the year (e.g., March near Fairbanks, Alaska) is associatedwith increased luteinizing hormone secretion, probably in responseto a combination of abundant food and social stimuli, afterrefractoriness dissipates. We have taken the first step of identifyingspatial and temporal opportunism as important phenomena in theArctic, and discussing hypotheses related to endocrine mechanisms.Future research should identify specific environmental cuesinvolved, and elucidate the neuroendocrine and endocrine mechanismsunderlying these two reproductive strategies.