The density dependence of demographic parameters and its implications for population regulation have long been recognized. Recent work has revealed potential effects of density on mating systems and sexual selection, but few studies concurrently assess the consequences of density on both demography and sexual selection. Such an approach is important because population processes and individual behaviors can interact to influence population growth and evolutionary trajectories. In this study, we tested the density dependence of breeding success, extra‐pair paternity, and the opportunity for sexual selection in a population of American redstarts Setophaga ruticilla using two different measures of density. To evaluate temporal patterns, we analyzed annual territory density, based on the total number of territories at our study site each year. To evaluate spatial patterns, we analyzed local territory density within years, based on the number of territories surrounding a focal territory. Greater annual density was associated with fewer offspring fledged per female, a reduced mean population rate of fledging success, and a lower relative contribution of extra‐pair paternity to male fitness. Greater local density was associated with fewer offspring fledged, reduced fledgling success, higher rates of nest loss, and higher rates of paternity loss on focal territories. Interestingly, greater local density was also associated with greater nestling mass on focal territories, which could imply that more densely‐packed territories contain superior resources. Overall, our results suggest that the effects of crowding via greater territory density reduce fecundity through increased nest predation, rather than reduced food availability, and increase rates of extra‐pair paternity. Thus, the selective pressures faced by individuals and their reproductive behaviors are likely to differ based on the annual and local density they experience, which may then feed back into population demography.
American redstarts are small (7–8 g) insectivorous migratory songbirds that breed throughout much of North America and spend the stationary portion of the non‐breeding season throughout the Caribbean, Mexico, Central and northern South America (Sherry and Holmes 1997). Redstarts are sexually dimorphic and show delayed plumage maturation, such that males in their first breeding season (second year or SY) resemble females and are easily distinguished from older males (after‐second year or ASY). After‐second year males are first to arrive to the breeding grounds and establish territories. They are followed by females approximately one week later and then by SY males. Females pair with males within a day or so of arriving and start nest‐building within several days (Sherry and Holmes 1997). American redstarts are an obligate single brooded species, but rates of nest loss are high, mostly due to predation by raptors, corvids, sciurids, and snakes (Sherry and Holmes 1997), and females may re‐nest up to six times in response to failed nesting attempts (McKellar unpubl.).
General field methods
We monitored a population of American redstarts on a 100 ha site (gridded at 10 m intervals) from May–July 2001–2011 at the Queen's University Biological Station, Chaffey's Lock, Ontario, Canada (44°34’N, 76°19’W). The annual number of territories held by pairs and un‐paired males ranged from 25–49 (mean = 38) over the course of the study. The study site was composed of mixed‐deciduous forest, primarily dominated by sugar maple Acer saccharum and eastern hop‐hornbeam Ostrya virginiana and spanned two contiguous plot types: continuous forest and fragmented forest. We monitored plots daily from 06:00 to 12:00 to detect the presence of males by listening for singing and subsequent visual identification. We captured males in mist nets upon their arrival by simulating a territorial intrusion with the use of a stuffed male decoy and song playback. We captured females in a similar fashion or with the use of fledgling distress playback during nestling or fledgling feeding. We banded males and females with a unique combination of color bands and a single Canadian Wildlife Service (CWS) aluminum band. We extracted 50 μl of blood for paternity analysis by puncturing the brachial vein. To determine territory boundaries and pairing, we recorded the position of color‐banded males on detailed maps of our study site for at least 20–30 min every day. Once males were paired to females, we monitored territories each day until we located nests, and we determined first egg date, clutch size, and hatch date with the use of an extendable mirror pole. If a nest failed, we monitored the territory daily until the re‐nest could be found. We did not always know the precise reason for nest failure, but most nest failures were assumed to be due to predation. We monitored active nests and territories of un‐paired males every other day throughout the season. We used an extendable ladder to retrieve nestlings to weigh them and band them with a single CWS band. From 2004 on, we retrieved 15–20 μl of blood for paternity analysis. Though we aimed to sample nestlings on day five after hatching, this was not always possible due to inclement weather or to nests being discovered after day five. When nests were inaccessible, we captured offspring on the day of fledging for blood sampling. Beginning in 2006, we recorded the GPS position of each nest and downloaded the coordinates onto our detailed study site maps to estimate territory centers.