Linking frequencies of acorn masting in temperate forests to long-term population growth rates in a songbird: the veery (Catharus fuscescens)

Many terrestrial ecosystems are characterized by intermittent production of abundant resources for consumers, termed pulsed resources. The impact of resource pulses on populations downwind of the initial pulse are only beginning to be characterized, while the relationship between the frequencies of pulses and the long-term growth rate of affected species is unknown. I monitored the reproductive success of veeries (Catharus fuscescens) breeding in oak-dominated forest in southeastern New York State from 1998 to 2002. During this time veeries experienced high interannual variability in growth rates as a consequence of trophic cascades stemming from pulsed production of acorns. Rodent populations that benefited from acorns also depreciated veery nests, while raptors that increased in response to rodent outbreaks are major predators on adult and juvenile birds. Veeries may recoup losses following low to moderate acorn crops that lead to rodent population declines. Thus, veeries fluctuate between years of positive and negative growth rate, however, long-term population trends, and thus true source-sink designation, cannot be made until the frequency of various year types is characterized. I simulated long-term growth rates using reproductive parameters estimated from field studies and survivorship data from the literature. Simulations suggest that variability in the frequency of masting events in oaks can lead to similar to 10% fluctuation in long-term growth rates in veeries. These studies suggest that temporal variability in masting dynamics has the potential to substantially influence songbird population trends. Furthermore, spatial variability in masting characteristics (e.g. the frequency of masting events and/or the size of seed crops) may greatly contribute to regional differences in songbird population trends. Because even less is known about the relationship between sizes of acorn crops and songbird populations, the influence of pulses in seed production on songbird population dynamics is likely to be underestimated.