4 September 2014 Science Briefs

SNAMP Pub #31: Life-history tradeoffs in Spotted Owls (STRIX OCCIDENTALIS): implications for assessment of territory quality

Article Title: Life-history tradeoffs in Spotted Owls (STRIX OCCIDENTALIS): implications for assessment of territory quality

Authors: M. Zachariah Peery and R. J. Gutiérrez

Research Highlights:


  • Evidence suggests that individual owls that fledged in pairs had a greater probability of surviving than individuals that fledged either as singletons or in triplets, an advantage that was evident in juvenile, sub-adult, and adult life stages.

  • Age of recruitment into the territorial population and the future reproductive output of offspring were not related to parental reproductive output in the year of birth.

  • Measures of territory quality based on parental reproductive output were correlated with measures of territory quality based on offspring fitness, and thus, ranking territories for conservation planning on the basis of parental reproductive output can be useful in territorial species.

Background:

We characterized the tradeoffs between parental reproductive output and offspring fitness in a closed population of California Spotted Owls using multistratum mark–recapture models to evaluate how potential tradeoffs influenced measures of territory quality. We tested the hypotheses that offspring produced in large broods are less likely to survive to adulthood, have delayed recruitment into the territorial population, and experience lower reproductive success than offspring produced in small broods. We then determined whether measures of territory quality based on parental reproductive output are biased by ignoring tradeoffs between reproductive output and offspring fitness. Data was based on 11 years of owl surveys (1987 through 1998) including capture and banding.

Results:

We assessed reproductive status at 127 Spotted Owl territories from 1987 to 1998. We captured 143 juveniles (29.9%) that fledged as singletons, 234 (59.2%) that fledged in pairs, and 52 (10.9%) that fledged as triplets. Individual owls that fledged in pairs had higher survival in all age classes than individuals fledged as singletons or triplets. For example, juvenile survival of owls produced in pairs was 0.417 compared to 0.290 and 0.320 for singletons and triplets, respectively. Higher survival for offspring that fledged in pairs is consistent with the hypothesis that high-quality parents, or parents with access to higher-quality resources, are capable of producing many young with high survival probabilities. We suspect that the ability of parents to raise more than one offspring depends more on habitat and annual weather conditions, as well as parental quality and experience. An index of territory quality based only on reproductive output was positively correlated with indices of territory quality based on the number of recruits into the population in subsequent years (R2 = 0.23) or the lifetime net reproductive value of offspring (R2 = 0.99).

Conclusions:


  1. Our results suggested that breeding Spotted Owls incurred a cost, measured in the survivorship of their offspring, when they produced three young.

  2. However, the additional offspring compensated for this reduction in survival such that parents that fledged three young had a modest fitness advantage over parents that fledged two.

  3. Therefore, assessments of territory quality in Spotted Owls based on reproductive output provide a reasonable measure of the relative contribution of individual territories to population growth and are not seriously affected by life-history tradeoffs.

  4. Using measures of offspring fitness to assess territory quality is not possible in many parts of the species’ range because of high rates of dispersal by juveniles outside of study-area boundaries.

Full Reference:

Peery, M. Zachariah and R. J. Gutiérrez. 2013. “Life-history tradeoffs in Spotted Owls (STRIX OCCIDENTALIS): implications for assessment of territory quality” in The Auk 130(1):1−9, 2013.

The full paper is available at: http://labs.russell.wisc.edu/peery

For more information about the SNAMP project and the California spotted owl team, please visit: http://snamp.cnr.berkeley.edu/teams/owl

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