Age-related differences in reproductive success support the selection hypothesis in a Mute Swan population
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ORIGINAL ARTICLE
Age‑related differences in reproductive success support the selection hypothesis in a Mute Swan population Radosław Włodarczyk1 · Piotr Minias1 Received: 3 October 2019 / Revised: 30 June 2020 / Accepted: 13 July 2020 © The Author(s) 2020
Abstract In many bird species, we observe age-related differences in reproductive success, which usually increases early in life and later decreases due to senescence. At the individual level, an early-life improvement in breeding performance may be associated with experience acquired during early reproductive events (experience hypothesis). At the population level, higher average reproductive success of older age cohorts can result from a disappearance of low-quality individuals from the population (selection hypothesis). Here, we tested these hypotheses in a wild population of the Mute Swan Cygnus olor from central Poland. In 1996–2016, we collected information on breeding success of 150 marked individuals (70 females and 80 males; 590 breeding attempts). At the population level, there was an initial increase in reproductive success (1–5 years), followed by a plateau (5–8 years), and then by a decrease in older age classes. Both within- and between-individual age variation contributed to the linear increase in reproductive success at the population level, but the latter effect was much more apparent. Short-term breeders (≤ 2 breeding events) had significantly lower reproductive success than long-term breeders (≥ 3 breeding events) during their first two breeding attempts, providing support for selection hypothesis and disappearance of low-quality phenotypes. After exclusion of short-term breeders, a positive age-related increase in reproductive success lost significance, suggesting that under-specific condition (rapid population growth and strong human disturbance) experience may have a limited effect on reproductive success in long-lived species with strong pair bonds, such as the Mute Swan. Keywords Age · Mute Swan · Phenotype disappearance · Reproductive success · Selection hypothesis Zusammenfassung Altersbedingte Unterschiede im Reproduktionserfolg einer Höckerschwan-Population unterstützen die Selektionshypothese Für viele Vogelarten stellen wir altersbedingte Unterschiede im Reproduktionserfolg fest, der normalerweise in der Jugend anwächst und später im Alter wieder nachlässt. Auf individueller Ebene ist eine Verbesserung der Brutleistung in der Jugend möglicherweise mit Erfahrungen aus frühen Fortpflanzungserlebnissen verbunden (Erfahrungshypothese). Auf Populationsebene könnte ein höherer Reproduktionserfolg älterer Altersgruppen aus dem Verschwinden von Individuen minderer Qualität resultieren (Selektionshypothese). Wir untersuchten beide Hypothesen an einer wildlebenden Höckerschwan-Population (Cygnus olor) in Zentralpolen. Von 1996 bis 2016 sammelten wir Informationen zum Bruterfolg von 150 markierten Einzeltieren (70 Weibchen und 80 Männchen; 590 Brutansätze). Auf Populationsebene gab es zunächst eine Zunahme des Bruterfolgs (1–5 Jahre), gefol
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