Asynchronous changes in phenology of migrating Broad-tailed Hummingbirds and their early-season nectar resources

Background/Question/Methods:

Interactions involving species that migrate seasonally across latitudes may be especially prone to altered synchrony of resources, particularly at the poleward limits of migratory routes, where phenological advancements in the spring are progressing more rapidly than at lower latitudes.  Consequently, species migrating from lower latitudes are likely to arrive at northern breeding grounds after the first appearance of seasonal resources, with likely implications for reproduction. To investigate the possibility of asynchronous phenological changes for a migratory species and its food resources along its migration route, we compared the dates of first arrival of Broad-tailed Hummingbirds (Selasphorus platycercus) to the dates of flowering of nectar plants they visit near the southern limit and the northern limit of its migratory range.  

Results/Conclusions:

Near the southern limit of its breeding range in Arizona, neither hummingbird arrival nor first flowering dates have changed.  At a nearby migration stopover site, first flowering of a major food plant has advanced, but peak flowering has not.  Near the northern limit of its breeding range in Colorado, arrival of Broad-tailed Hummingbirds has advanced less rapidly than has first flowering and peak flowering of early season floral resources.  These findings are consistent with the expectation that hummingbird arrival to northern breeding grounds is constrained by slower shifts in flowering resource phenology further south along the migration route.  The consequence is a shrinking interval of time between arrival of the first hummingbirds and both first and peak flowering of two important early-season nectar plants at the northern site.  Given the reliance of this migratory species on these early-season floral resources, if phenological shifts continue at current rates, hummingbirds will eventually arrive at northern breeding grounds after flowering begins, which has the potential to reduce their reproductive success through the mistiming of nesting, reproduction, and peak flowering. The novel implication of this result is that the breeding range of some species will contract toward lower latitudes under continued climate change.