Low survival after release into the wild: assessing “the burden of captivity” on Mallard physiology and behaviour

Abstract

Captive-reared animals used in reinforcement programs are generally less likely to survive than wild conspecifics. Digestion efficiency and naive behaviour are two likely reasons for this pattern. The Mallard is a species with high adaptability to its environment and in which massive reinforcement programs are carried out. We studied physiological and behavioural factors potentially affecting body condition and survival of captive-reared Mallards after being released. Digestive system morphology and an index of body condition were compared among three groups: captive-reared birds remaining in a farm (control), captive-reared birds released into the wild as juveniles (released) and wild-born birds (wild). We also compared behaviour and diet of released vs. wild Mallards. Finally, we conducted a 1-year survival analysis of captive-reared birds after release in a hunting-free area. Gizzard weight was lower in control Mallards, but the size of other organs did not differ between controls and wild birds. The difference in gizzard weight between released and wild birds disappeared after some time in the wild. Diet analyses suggest that released Mallards show a greater preference than wild for anthropogenic food (waste grain, bait). Despite similar time-budgets, released Mallards never attained the body condition of wild birds. As a consequence, survival probability in released Mallards was low, especially when food provisioning was stopped and during harsh winter periods. We argue that the low survival of released Mallards likely has a physiological rather than a behavioural (foraging) origin. In any case, extremely few released birds live long enough to potentially enter the breeding population, even without hunting. In the context of massive releases presently carried out for hunting purposes, our study indicates a low likelihood for genetic introgression by captive-reared birds into the wild population.

Individual quality persists between years: individuals retain body condition from one winter to the next in Teal

“…Our findings lend further support to the value of the SMI as a condition index (see also Peig and Green 2010;Bók-ony et al 2012;Champagnon et al 2012), and the relationship between SMI at ringing and at recapture was tighter than when relating crude body mass measurements. Future work relating the changes in individual body condition over time to reproductive success and survival in ducks, as well as in a range of other species, is highly desirable to allow us to better understand the relationships between body condition and individual fitness.…”

Admixture between released and wild game birds: a changing genetic landscape in European mallards (Anas platyrhynchos)

“…The rate of introgression due to genetic admixture is likely low due to the poor survival of released farmed mallards (Champagnon et al 2012b) but, as suggested by this study, restocking practice involves so many birds that the Bfootprintî n the wild population may be detectable and potentially significant. With continued releases, the genetic load of farmed stocks among wild mallards will persist or grow, potentially leading to lower fitness in the latter due to introgression of genotypes that are inferior under natural selection regimes.…”

“…In addition, a change in genetic structure over time in farmed mallards is consistent with Lynch and O'Hely (2001), who argued that adaptation to captivity together with anthropogenic selection at breeding facilities will inevitably lead to a genetic alteration of the captive population. Such differences in genetics may, at least partly, explain discrepancies documented in captive ducks regarding morphology, e.g., brain volume (Guay and Iwaniuk 2008), digestive organs (Champagnon et al 2012b;Moore and Battley 2006), feeding apparatus Söderquist et al 2014), sexual behavior (Desforges and Wood-Gush 1976), mate preferences (Cheng et al 1978;Cheng et al 1979), and habituation to humans (Desforges and Wood-Gush 1975).…”

“…Released mallards have lower survival than wild-born conspecifics (Champagnon et al 2012b;Söderquist et al 2013), likely due to Bburdens of captivity^such as a less developed digestive system (Champagnon et al 2012b). Ducklings have an innate response to avoid predators (Dessborn et al 2012), a behavior that is most crucial the first weeks after hatching.…”

Diet of released captive-bred North-African houbara bustards

“…Differences in the morphology of digestive organs between captive and wild birds have often been reported as a cause and consequence of inappropriate diet leading to bird weakness or death Communicated by M. Scandura (Champagnon et al 2011;Moore and Battley 2006;Putaala and Hissa 1995). Due to the absence of parental learning, captive-bred individuals might express feeding preferences for particular food colour, shape or texture close to the food distributed in captivity (food "imprinting"), but with no or low nutritional value (Barker et al 1977).…”