All things considered, it is difficult to underestimate the importance of survival
A review of survival estimates for raptors and owls. Newton, I., McGrady, M.J. & Oli, M.K. 2016. IBIS. DOI: 10.1111/ibi.12355. VIEW
Survival is often the most important life history trait affecting fitness and population growth rate because all other things, including reproduction, depend on it. Bluntly put, an organism cannot reproduce or interact with other organisms or its environment unless it survives. Because of this, accurate estimates of survival rates and factors influencing these rates (gender, age or environmental factors) are critical to understanding species’ behaviour, ecology, and conservation… almost everything.
Figure 1 Ringing has been the most widely applied method to aid in estimating survival in birds. Alpha-numeric and PIT (Passive Integrated Transponder) ringing were the basis for making estimates of survival of Sooty Falcon (Falco concolor, pictured) © Mike McGrady
Despite its importance in species’ ecology and conservation, it is not always easy to accurately estimate survival. Amongst wild vertebrates survival is probably best studied in birds, and ringing has been the main tool used to collect data that permit estimation age or sex specific survival rates of avian species. In recent decades colour marking, microchip tagging, conventional and satellite-based radio telemetry and other methods have been added to the tool box used in the collection of data useful for estimating survival. These different data collection tools have their advantages and disadvantages, which can affect, amongst other things, the accuracy of the estimate, the spatial and temporal scale of the study and the cost. To some extent constraints imposed by data type, cost, quality, etc are being overcome by the development of new analytical tools that make the best use of rare (and perhaps expensive) data of different types.
Figure 2 Colour ringing and other colour marking can enable individual identification from a distance, in some cases allowing data to be collected by other observers, including citizen scientists. It can also preclude the need to recapture or recover the bird © Mike McGrady
Our paper reviews the literature on survival estimates for different species of raptors and owls, examines the methodology used to obtain the estimates, and draws out some general patterns arising. Because some information on survival, particularly of poorly studies species is hidden in lower profile journals and conference proceedings this effort also makes that information more easily available to researchers, and the patterns that emerge may suggest new, insightful research.
Published survival estimates were found for three species of Cathartidae, one of Pandionidae, 29 Accipitridae, 12 Falconidae, one Tytonidae and nine Strigidae, almost all from northern hemisphere, extra-tropical species. The main patterns to emerge included:
- a significant tendency for annual adult survival to increase with body mass, smaller species having annual survival rates of 60-70%, medium sized species mainly in the range 70-90% and the largest of more than 90%, in the absence of obvious human-caused losses;
- juveniles and pre-breeders had lower survival than older birds;
- a lack of obvious or consistent differences in survival between the sexes, where these could be distinguished; and
- in a few species in which enough data were available, a decline in annual survival rates in the later years of life.
Figure 3 Telemetry, including in recent years satellite radiotelemetry, can provide important information on mortality unattainable by ringing and other marking methods. For example this juvenile Egyptian vulture, Neophron percnopterus, with a solar-powered GPS-GSM receiver/transmitter © Mike McGrady
Most of these patterns are not very surprising; however, the lack of differences between the sexes and the dearth of data for senescing individuals points to areas where important details may remain hidden, and where more research would be valuable. The other fact highlighted by our review was that for most species of conservation concern, information on survival is generally not available or estimates are based on small sample sizes (and decreasing sample sizes as populations collapse). This potentially undermines conservation efforts. An exception to this general observation is the Spotted Owl (Strix occidentalis), which has been the subject of long-term studies using a variety of data collection tools, and modern analytical methods. As such, Spotted Owl conservation and management has not only benefited from some of the best research, it also illustrates what can be achieved when the best available methods and significant resources are used to tackle the question of survival rate (see Anthony et al. 2006).
Now that this blog post is done, perhaps I can get that cursed Gloria Gaynor song out of my head. Otherwise, I might not survive.
Anthony, R.G., Forsman, E.D., Franklin, A.B., Anderson, D.R., Burnham, K.P., White, G.C., Schwarz, C J., Nichols, J.D., Hines, J.E., Olson, G.S., Ackers, S.H., Andrews, L.S., Biswell, B.L., Carlson, P.C., Diller, L.V., Dugger, K.M., Fehring, K.E., Fleming, T.L., Gerhardt, R.P., Gremel, S.A., Gutiérrez, R.J., Happe, P.J., Herter, D.R., Higley, J.M., Horn, R.B., Irwin, L.L., Loschl, P.J., Reid, J.A. & Sovern, S.G. 2006. Status and trends in demography of Northern Spotted Owls, 1985–2003. Wildlife Monographs 163: 1–48. VIEW
Forsman, E.D., Anthony, R.G., Dugger, K.M., Glenn, E.M., Franklin, A.B., White, G.C., Schwarz, C.J., Burnham, K.P., Anderson, D.R., Nichols, J.D., Hines, J.E., Lint, J.B., Davis, R.J., Ackers, S.H., Andrews, L.S., Biswell, B.L., Carlson, P.C., Diller, L.V., Gremel, S.A., Herter, D.R., Higley, J.M., Horn, R.B., Reid, J.A., Rockweit, J., Schaberel, J., Snetsinger, T.J. & Sovern, S.G. 2011. Population demography of Northern Spotted Owls. Studies in Avian Biology 40: 1–106. VIEW
McGrady, M.J., Al Fazari, W., Al Jahdhami, M.H. & Oli, M.K. 2016. Survival of sooty falcons (Falco concolor) in Oman. Journal of Ornithology 157: 427–437. VIEW
Newton, I., Marquiss, M. and Rothery, P. 1983. Age structure and survival in a sparrowhawk population. Journal of Animal Ecology 52: 591–602. VIEW
Smith, G.D., Murillo-Garcia, O.E., Hostetler, J. A., Mearns, R., Rollie, C., Newton, I., McGrady, M. J. & Oli, M. K. 2015. Demography of population recovery: survival and fidelity of peregrine falcons at various stages of population recovery. <Em<>Oecologia 178:391–401. VIEW
Smyth, B. & Nebel, S. 2013. Passive integrated transponder (PIT) tags in the study of animal movement. Nature Education Knowledge 4(3): 3. VIEW
Featured image: Juvenile peregrine falcon (Falco peregrinus) © Steve Kane
Author portrait © Madan Oli
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