Moving to a new site for next brood? Good or bad? And why? These questions are answered in the Early View paper “Mechanisms and reproductive consequences of breeding dispersal in a specialist predator under temporally varying food conditions” by Julien Terraube and co-workers.
In this study, we explored the factors linked to variations in breeding dispersal behaviour and their consequences in terms of reproductive parameters in a raptor species. Which factors influence individual dispersal decisions? Are Eurasian kestrels Falco tinnunculus able to increase their own reproductive success after moving from one site to the other between two consecutive breeding seasons? Is this relationship mediated by environmental factors like food abundance or individual traits like gender or age? All these fascinating questions are hard to answer particularly in avian predators because of methodological limitations associated to size of the study area and even more in species like Eurasian kestrels breeding in boreal ecosystems, which have high breeding dispersal propensity and in which movements are driven by cyclic fluctuations in abundance of main foods (voles) (see Vasko et al. 2011).
In spring 1977, a long-term study of a local kestrel population breeding in western Finland (the Kauhava region) was initiated along with the monitoring of Tengmalm’s owl populations (see Korpimäki and Hakkarainen 2012). Hard work in the field has generated a fantastic long-term, large-scale dataset combining data from breeding success and individual traits of breeding kestrel parents captured at their nest sites over the last 25 years (1983-2013).
Given the increasing demand for long-term population studies in order to understand current impact of environmental changes, the authors would like to stress the importance of long-term studies on demographic parameters in long-lived vertebrate populations. In this study, the assessment of breeding dispersal distances was made possible through systematic capture of most kestrel parents breeding in the main study areas, ringing and recovery of previous rings. We would like to focus here on the capture procedure that allowed collecting breeding dispersal data and share the experience acquired during the hours spent in “Wild-West” of Finland when checking traps.
Capture occurs during the brood-rearing period when chicks are two-to-three weeks old, in order to avoid unnecessary disturbance of young nestlings during the most vulnerable phase. Virtually all the breeding population monitored breeds in nest-boxes that were set up on barns from early 1980s onwards. The total number of nest-boxes has varied from 350 to 450 throughout the study period in agricultural fields of the study area. We have used swing-door traps attached to the front of the nest box for trapping parents. The “trapping routine” starts by erecting the traps early in the morning from 5-6 am on a group of 5 to 10 breeding sites selected according to nestling age. Then trap-checking rounds are performed every two-to-three hours to check if any individual is trapped. The aim is to capture both female and male from each breeding site within 12 hours. Adults are ringed, measured and weighed near the breeding site and released as soon as possible. A capture day ends by giving newly-hatched rooster chickens to the kestrel nestlings to compensate for the decrease in prey delivery rates experienced during the trapping of their parents.
We have been lucky in the sense that voluntary birdwatchers and ringers have set up many large nest-box networks for kestrels in surrounding areas in western Finland. In addition, many voluntary ringers, particularly Erkki Rautiainen and Jussi Ryssy, have also made huge efforts to trap and ring kestrel parents and to ring fledglings at these nest-boxes.
A total of 2089 males and 2544 females were trapped at nests during 1985 to 2011 in our study areas. Trapping success remained relatively constant over the period: of all the nesting attempts on average 70% of the male and 80% of the female parents were successfully captured yearly. This large-scale trapping and ringing program allowed us to collect 631 dispersal events from 1985 to 2011 that were analysed in this study.
Overall, we found that females dispersed further than males and older individuals dispersed further than yearlings. A noteworthy aspect of this study involved the evidence of body-condition dependent dispersal strategies in kestrels as the individual body condition index was positively correlated to breeding dispersal distances, particularly in females. Strikingly, our results also evidenced complex patterns of non-linear relationship between previous breeding success and dispersal distances. Finally, longer dispersal distances were associated with reproductive costs in males under increasing vole abundance, whereas those females dispersing further increased their breeding success under all conditions of food abundance.
These results call for further research as clearly there is more to learn about the link between potential pre- and post-breeding prospecting movements, optimal dispersal decisions and population dynamics in avian predators inhabiting fast changing boreal ecosystems.
Korpimäki, E. & Hakkarainen, H. 2012. The boreal owl: Ecology, Behaviour and Conservation of a Forest-Dwelling Predator. – Cambridge University Press, Cambridge. 372 pages.
Vasko, V., Laaksonen, T., Valkama, J. & Korpimäki, E. 2011. Breeding dispersal of Eurasian kestrels (Falco tinnunculus) under temporally fluctuating food abundance. – Journal of Avian Biology 42: 552-563. (doi: 10.1111/j.1600-048X.2011.05351.x)