Decomposing fecundity and evaluating demographic influence of multiple broods in a migratory bird

Relevance of breeding season fecundity as a driver of population dynamics has been highlighted by many studies. Despite that, knowledge about how brood type specific (i.e. first, second or replacement) fecundity affects demography of multiple-brooded species is limited. In fact, estimation of brood type specific fecundity is often challenging due to imperfect detection of nesting attempts.

In a new paper published on the Journal of Animal Ecology, Simone Tenan, research scientist at the Cnr Institute of Marine Sciences (Cnr-Ismar), in collaboration with the Swiss Ornithological Institute and the Museum Heineanum (Germany), examined the demographic contribution and the feedback on population density of different components of fecundity, along with other vital rates, in a facultative multiple-brooded migratory bird.

Researchers used a novel formulation of a fecundity model that allows incorporating reproductive data for which information on the type of brood was unknown in some cases, and embedded it into an integrated population model to obtain consensual estimates of all demographic rates, including brood type specific fecundities, reproductive success probabilities, and proportion of breeding pairs that performed a second or replacement brood. Then, they conducted transient life table response experiments on IPM estimates to account for non-stationary environments. The model was applied to two twenty-year data sets collected in a Swiss and a German local population of wrynecks (Jynx torquilla).

Brood type specific fecundities and temporal patterns of brood type specific probabilities of success, number of successful and unsuccessful first broods, probability of starting a second or a replacement brood, and proportion of pairs that performed a second or a replacement brood differed between the two populations. However, changes in immigration rate and apparent survival were the dominant contributors to temporal variation and large sequential changes in realized population growth rates in both populations. In the Swiss population, a second brood fecundity declined when population size increased.

The study provides insight into the reproductive processes that affect population dynamics and mediate density-dependent fecundity in a migratory bird. In addition, the analytical approach proposed can be used in other studies of multiple-brooded species to maximize the use of available fecundity data through the estimation of unknown brood types, thus favouring a better understanding of the demographic contribution of brood type specific fecundity.

Per informazioni:
Simone Tenan
CNR – Institute of Marine Sciences
[email protected]

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