Summary
The spatial structure of populations determines the relative importance of reproduction, survival and movement on population dynamics. However, the mechanisms by which local individuals and immigrants interact and the subsequent effects of immigrants on productivity are poorly known. We developed an integrated population model (IPM) to study the extent and consequences of immigration on the dynamics of a neotropical migrant (American redstart, Setophaga ruticilla) over an 11‐year period in Ontario, Canada. New immigrants represented the majority of the study population each year with higher immigration rates for males than females and for first‐year breeders than breeders in their second year or older. Immigration was negatively density dependent, with immigrants replacing previously established breeders in a compensatory manner following their death or emigration. Because of the tradeoff between immigration and apparent survival, neither had a strong influence on population growth and reproductive output was most strongly correlated with a change in abundance between years. However, if immigration ceased, the study population would become locally extinct within 7 years and thus immigrants were essential for local population persistence. We found no evidence for reduced breeding success when immigrants represented a higher proportion of the study population. Our research highlights the importance of movement in the stability of open populations and the strong correlation between the fates of local breeders and the number of immigrants entering the population. We recommend the use of IPMs to address the spatial scale over which immigration occurs and how different scales influence its contribution to population dynamics.
Methodology
Study area and sampling methods
We conducted field work for this project at the Queen's University Biological Station (QUBS), near Chaffey's Lock, ON, Canada (44°34′N, 76°19′W) from May–July 2001–2011. American redstarts were monitored in a mixed‐deciduous forest consisting primarily of sugar maple (Acer accharum) and Eastern hop hornbeam (Ostrya virginiana). The approximately 100 ha study area consists of a 25 ha campground and a largely undisturbed 75 ha forest separated by a two lane country road (for a detailed description see McKellar et al. 2015). The study site is nested within a broader area of heterogeneous habitat consisting of forest, wetlands, lakes, agriculture and urban areas. Hereafter we refer to the individuals in the 100 ha study area as the ‘study population’. American redstarts exhibit protandry, with males arriving on the breeding grounds several days before females. Upon arrival, males begin singing and establish territory boundaries. From May 1–31, we surveyed the study population from 0600 to 1200, mapping territory boundaries of males, recording whether females were present on territories and identifying any previously banded individuals that returned. The abundance of males and females provided our annual count estimates that were used in the IPM. We also recorded pairing dates and the location of the nest, which we monitored to record laying date, number of eggs laid, hatching success, and fledging success (see McKellar et al. 2014).
Birds were captured as soon as possible upon arrival using mist nets and simulated territorial intrusions consisting of song playback and a decoy or fledgling distress calls. All captured individuals were banded with a single Canadian Wildlife Service‐issued aluminum leg band and a unique combination of 2–3 colour bands. Nestlings were banded with a single aluminum band, generally between days 5 and 8 after hatching. Age classes for males were determined based on plumage coloration. Males exhibit delayed plumage maturation, with second‐year (SY) males exhibiting female‐like grey and yellow plumage and after‐second‐year (ASY) males exhibiting black plumage with orange patches on the wings, tail, and flanks, and a white underside. Female age can only be reliably determined in the hand and was based on retrix coloration and wear, and molt limits when individuals were captured (Pyle 1997).