- Declines in wild salmonid (Oncorhynchus spp.) populations in the Columbia River basin have resulted in managers identifying that avian predation on juvenile salmonids is an important limiting factor for salmonid recovery. Caspian Terns (Hydroprogne caspia), particularly those nesting in the Columbia River estuary, were identified as key avian predators that reduce the survival of juvenile salmonids in the Columbia River basin. To reduce the numbers of juvenile salmonids consumed by Caspian Terns in the Columbia River estuary, the amount of available nesting habitat for Caspian Terns on East Sand Island (ESI) was reduced from 2 ha in 2008 to 0.64 ha in 2012, and then was further reduced to 0.4 ha in 2015. The objective of this management was to reduce the size of the Caspian Tern breeding colony on ESI to about a third of its former size.
Caspian Terns are facultative colonial nesters and generally nest in ephemeral habitats. Caspian Terns nesting at ESI, however, have demonstrated very high colony site fidelity due to the consistent maintenance of nesting habitat, as well as the proximity to an abundant food supply and the paucity of terrestrial predators. Reproductive success for the ESI Caspian Tern colony has, on average, declined since 2001, and in both 2011 and 2017 no young were raised at the colony. The objective of my study was to understand variation in reproductive success of Caspian Terns at the ESI colony by investigating potential factors associated with nesting success at the scale of the colony and the individual. I investigated both top-down and bottom-up factors that may have affected the average annual reproductive success at the Caspian Tern colony on ESI during 2001-2017, as well as the relative importance of several intrinsic factors that may have affected reproductive success of individual Caspian Terns over two consecutive breeding seasons, 2015 and 2016.
I found that study year and the rate of kleptoparasitism on Caspian Tern bill-load fish by gulls (Larus spp.) during the chick-rearing period best explained the inter-annual variation in average annual reproductive success at the ESI colony during 2001-2017. My results suggest that year was acting as a surrogate variable for other factors that were changing in a gradual, consistent manner at or near the tern colony during the study period, such as nesting habitat availability, nest density, Bald Eagle (Haliaeetus leucocephalus) disturbance rates, and gull predation rates on tern eggs and chicks. The impact of average Columbia River discharge in May/June as a driving factor for Caspian Tern reproductive success was particularly evident in 2011 and 2017, the two years when river discharge was the highest recorded during the study period, and the only two years when no young were produced at the colony. My results support the hypothesis that both bottom-up factors (e.g., food availability) and top-down factors (e.g., gull kleptoparasitism rates) are drivers of reproductive success at the East Sand Island Caspian Tern colony. The bottom-up factor of average Columbia River discharge in May/June apparently affected the top-down factor of gull kleptoparasitism rates by altering the food supply of nesting gulls, thus prompting them to switch to stealing Caspian Tern bill-loads.
The second study in my thesis sought to gain a better understanding of which factors may influence reproductive success of individual Caspian Terns, based on data collected at the ESI colony in 2015 and 2016. I investigated the relative importance of (1) age, (2) previous breeding experience, (3) timing of breeding, (4) density of nearby conspecific nests, and (5) nest location relative to the colony edge for explaining variation in individual reproductive success. The date when an individual tern initiated its nesting attempt was ranked highest in relative importance among these explanatory variables, with nest success decreasing as the date of nest initiation increased. The density of conspecific nests within 1 meter of an individual’s nest was strongly and positively associated with the odds that the nest was successful. Nests located further from the colony edge were more likely to be successful, but only early in the breeding season; later in the season, nest location had no effect on individual reproductive success. Individuals that initiated nests earlier in the breeding season had more time to lay replacement clutches in the event that their earlier nesting attempts failed, and laying more than one clutch per breeding season was common at the ESI colony during the study period. Some individuals attempted to nest as many as four times in a single breeding season.
My study provides a better understanding of those extrinsic and intrinsic factors that are associated with reproductive success of Caspian Terns on two different scales – the breeding colony at ESI as a whole, and the individual Caspian Terns nesting at ESI. Understanding factors influencing reproductive success at each scale increases our knowledge of Caspian Tern breeding ecology at East Sand Island, and can inform managers about which factors likely regulate the size and productivity of the largest Caspian Tern breeding colony in North America. This study can also provide insight into factors that affect other seabird species and organisms that live in seasonal environments, as well as long-lived organisms that may experience considerable variation in overall reproductive success.