Photo by John Hayes

Seminar summary: Life history trade-offs and raptor monitoring - what can we learn from log term programs

Mahi Puri, Joshua Ringer, and Allison Smith

Natura 2000 has been carrying out long term species monitoring across Europe, in over 27000 sites, of which 18% are on land and 6% in sea. It follows a bottom up process, with periodic monitoring on the conservation status, trends and dynamics of the target species.

Specifically, Dr. Zabala's work focused on a 30 year dataset for 5 raptors, namely Barn owl, Tawny owl, Bonelli's eagle, Montagu's harrier and Peregrine falcon. The parameters monitored included breeding success, i.e. the number of fledglings per nest, and other life history parameters such as years of individual survival. Monitoring effort varied across time periods and number of nests. The data was filtered to include (1) only those individuals which were monitored throughout their lifespans and (2) those whose age was known.

In order to understand the trade-offs between life history strategies the following questions were addressed in the study:

  1. Is offspring produced a reliable indicator of individual/territorial quality?
  2. Is more offspring produced in a given year better? How does number of offspring produced relate to number of offspring recruited?
  3. Which is the best short-term (single generation or shorter) proxy of fitness, and what is the capability of detecting changes in it with different monitoring designs?

The trade-off investigated was: do individuals with higher proportional breeding output breed less often or have shorter lives, and did breeding early in life trade-off with lifespan? The variables that mediated trade-offs were environmental conditions that affected average fledgling production per year. These included natal conditions (resource availability) and adverse environmental periods.

In the study, very limited evidence was found to support any kind of trade-off. Unless there were prolonged harsh conditions, there were no trade-offs found between the number of offspring produced and survival/lifespan. Additionally, offspring produced in a given year were related to the number of recruits. Thus, offspring was regarded as a safe proxy to assess individual/territorial quality.

With respect to factors affecting recruitment, both extrinsic (environment conditions and resource availability) as well as intrinsic factors (body size, clutch size and early arrival to nest) had an impact on recruitment success among the different species of raptors.

Lifetime reproduction success (LRS) was found to be the best proxy for recruits produced. Differences in LRS in the area of 30% were found to be detectable with relatively short monitoring periods and moderate effort.

We found the flow of the presentation to be a little chaotic due to which it was slightly difficult to relate the results/inferences directly with the study objectives.

Overall, the study highlighted the utility of long term datasets to understand life history strategies that may be adopted by different species.