How robust are risk-taking associations in incubating birds? A test and a review
Flushing distance (FD, the horizontal distance between a parent bird when it leaves its nest and an approaching predator) is one measure of nest defense and of risk-taking; parents that stay too long risk being killed, whereas those that flush too early risk at the very least impairing development of their young, and at the very worst leaving them unprotected against predators. Thus, FD should be under strong natural selection. A general prediction is that incubating birds will remain on a nest being approached by a predator until risks of staying reach a threshold that outweighs costs of fleeing. This threshold is predicted to vary depending on a brood’s value, parental characteristics, environmental conditions, and learning that repeated visits pose a limited or no threat. We evaluated FD in a nest box population of tree swallows (Tachycineta bicolor) relative to each of these. We obtained 246 FDs from 66 different nests over 2 years. We found some evidence that FD increased with clutch size (tendency), female age, and air temperature, and decreased with greater overhead vegetation density; six additional associations were not significant. Given the lackluster support for the predictions we tested, we did a review of the literature and similarly found limited support for most of the associations we tested despite the entrenched view that these relationships are commonplace. We submit that further insights are needed into understanding predictors of FD in incubating birds.
Parent birds sitting on eggs are proverbial sitting ducks, although they do have the option of fleeing predators. However, parents may be willing to sacrifice themselves to protect their eggs if the latter become sufficiently valuable. One assumption is that eggs increase in value closer to hatch, and one prediction is that parents will be more reluctant to fly from their nests later in incubation. We tested this and other predictions in a population of nest-box-using tree swallows in eastern North America. The relatively weak support we obtained for our predictions suggests that we need to reevaluate our assumptions in this area of research.
KeywordsBrood value Flush distance Nest defense Parental investment
This manuscript was part of KG’s BSc (Hons) degree. We thank the many people inside and outside of Team Shutler that helped collect data in the field and that offered input, David Westneat and Brian Wisenden for valuable suggestions to improve our analyses, and Mark Stanback for encouragement. We thank Mark Stanback and two anonymous reviewers for helping substantially to improve the final product.
This work was supported by the Natural Sciences and Engineering Research Council of Canada via a University Research Award to KG and a Discovery Grant (RGPIN-2015-05617) to DS, a Nova Scotia Habitat Conservation Fund (Hunters and Trappers) grant to DS, Raddall Fund grants to DS, and Acadia University grants to DS.
Compliance with ethical standards
Our tree swallows occupy an anthropogenically modified landscape where interactions with humans and vehicles are commonplace, and we would have approached nests for routine monitoring even had we not measured flush initiation distances. When handling birds for sexing and banding, we worked as quickly and as quietly as possible to minimize stress. All procedures were approved by the Acadia University Animal Care Committee (Protocol 03-16R#1). All applicable guidelines for the care and use of animals were followed.
Conflict of interest
The authors declare that they have no conflict of interest.
- Caro T (2005) Antipredator defenses in birds and mammals. University of Chicago Press, ChicagoGoogle Scholar
- Clutton-Brock TH (1991) The evolution of parental care. Princeton University Press, PrincetonGoogle Scholar
- Environment Canada (2018) Historical climate data. Government of Canada. http://climate.weather.gc.ca/historical_data/search_historic_data_e.html. Accessed 1 Aug 2018.
- Graham KF (2018) Risk-taking by Tree Swallows (Tachycineta bicolor) during nest defense. BSc (Hons) thesis, Acadia University, Wolfville, Nova Scotia, Canada.Google Scholar
- Knight RL, Temple SA (1986) Why does intensity of avian nest defense increase during the nesting cycle? Auk 103:318–327Google Scholar
- Mitchell K (2010) Quantitative analysis by the point-centered quarter method. arXiv:1010:3303Google Scholar
- Obomsawin A (2017) Features associated with nest-site use in Tree Swallows (Tachycineta bicolor). BSc (Hons) thesis, Acadia University, Wolfville, Nova Scotia, CanadaGoogle Scholar
- Shutler D, Hussell DJT, Norris DR et al (2012) Spatio-temporal patterns across North America in nest box occupancy by Tree Swallows. Avian Conserv Ecol 7:3Google Scholar
- Strickler GS (1959) Use of the densiometer to estimate density of forest canopy on permanent sample plots. PNW Old Series Res Notes 180:1–5Google Scholar
- Trivers R (1972) Parental investment and sexual selection. In: Campbell BG (ed) Sexual selection and the descent of man. Aldine de Gruyter, New York, pp 136–179Google Scholar
- Westneat DF (1989) Intensity of nest defense in indigo buntings increases with stage and not number of visits. Auk 106:747–749Google Scholar
- Winkler DW, Ringelman KM, Dunn PO, Whittingham L, Hussell DJT, Clark RG, Dawson RD, Johnson LS, Rose A, Austin SH, Robinson WD, Lombardo MP, Thorpe PA, Shutler D, Robertson RJ, Stager M, Leonard M, Horn AG, Dickinson J, Ferretti V, Massoni V, Bulit F, Reboreda JC, Liljesthröm M, Quiroga M, Rakhimberdiev E, Ardia DR (2014) The interaction between clutch size and lay date changes across the Americas in Tachycineta swallows: what are the roles for direct selection, demography and organismal biology? Ecography 37:670–678CrossRefGoogle Scholar