Summary
Bird species around the world are threatened with extinction. In North America, aerial insectivores are experiencing particularly severe population declines. To conserve these species, we need to know which life stages have the largest influence on population growth. We monitored a box-nesting population of Tree Swallows (Tachycineta bicolor) from 1975 to 2017. From this long-term dataset, we derived estimates of 9 vital rates: clutch size, reproductive attempts, and overwinter return for 2 age classes of adult females, and hatching, fledging, and juvenile recruitment rates. We conducted a life-stage simulation analysis on this population based on a 3-stage, female-based population projection matrix to determine which of these vital rates had the greatest influence on overall population growth rate. We determined each vital rate's sensitivity (i.e. the effect of a small change in each vital rate on population growth), elasticity (i.e. the effect of a proportional change in each vital rate on population growth), and ability to explain variation in population growth rate. Juvenile recruitment, female return for both age classes, and fledging success determine population growth because they have high sensitivity, elasticity, and explained large amounts of variation in population growth rate. Contrary to expectations, the number of nesting attempts, clutch size, and hatch rate have little impact on population growth rate. To stem Tree Swallow decline, and potentially the declines we see across the aerial insectivores, fledging success or overwinter survival must increase.
Methodology
Study System
We used data from a box-nesting population at the Queen’s University Biological Station in Chaffey’s Lock, Ontario (44.5212°N, 76.3854°W), from 1975 to 2017. Nest boxes were placed in grids in large open areas and along roadsides. The grids mimicked the distribution of natural cavities (Robertson and Rendell 1990).
Starting between late April and early May in 1975–2017 we monitored nest boxes to track the outcome of each nesting attempt. We recorded clutch size, number of eggs that hatched, and number of nestlings that fledged. We banded all nestlings with a uniquely numbered Canadian Wildlife Service band. Throughout the breeding season, we caught adult Tree Swallows to identify returning birds and banded new immigrants. Adults were sexed according to the presence of a cloacal protuberance (male) or brood patch (female). Females, but not males, in this species show delayed plumage maturation, allowing us to estimate the age of females as one year old (second year, SY) or older (after second year, ASY) (Hussell 1983). Because female capture records were more complete and females could be aged more precisely, we calculated female-based vital rates. As a socially monogamous species requiring biparental care (Robertson et al. 1992), Tree Swallow population dynamics are unlikely to be influenced by sex-specific differences (Caswell 2001). From 1975 to 2017, we monitored 5,506 nests and 3,740 unique females.
Estimating Vital Rates
We estimated annual averages for each of the vital rates relating to fecundity. We estimated mean annual clutch size for SY and ASY females as the total number of eggs laid across all nests in the population for a given year divided by the total number of nests with eggs in the same year. Although Tree Swallows in our population were single-brooded, if her first nest fails early in the year, a female may attempt a second nest. We calculated the yearly average number of nests per female as the total number of nests of known female parentage divided by the total number of known parental females for SY and ASY females separately. Similarly, we calculated yearly hatch rates as the number of eggs that hatched divided by the total number of eggs in the entire population and fledge rates as the total number of nestlings that fledged divided by the total hatched.