Action: Use appropriate populations to source released populations
- A replicated study from Sweden and a small study from France found that birds sourced from populations distant from where they were released were less successful than birds from the area.
- In Sweden, released white storks Ciconia ciconia from North Africa produced fewer than half the chicks as those that naturally re-colonised, whilst both studies found that storks and little bustards Tetrax tetrax were less likely to migrate than birds originating in the release area.
Some bird species follow instinctive migration patterns, which vary between populations (Berthold et al. 1984), and populations are likely to be adapted for local environmental conditions. If individuals from one population are released into another area, this could result in maladaptive behaviours, with birds migrating in the wrong direction, or breeding at the wrong time. Sourcing individuals for release from the area that they will be released into may, therefore, improve success rates.
Berthold, P. (1984) The endogenous control of bird migration: a survey of experimental evidence. Bird Study, 31, 19–27.
Supporting evidence from individual studies
A replicated study in southern Sweden in 1989-2005 (Olsson 2007) found white storks Ciconia ciconia that naturally re-colonised the region in 1989 from the nearest remaining population (in northeast Europe) and their direct descendants fledged over twice as many chicks as birds descended from a reintroduced population which originated in north Africa (average of 1.9 fledglings/pair for birds descended from wild birds vs. 0.9 fledglings/pair for birds descended just from reintroduced birds). In addition, birds with wild ancestry were significantly more likely to migrate than birds only descended from captive individuals (11 of 18 storks confirmed as migrating had some wild ancestry, as did eight of ten storks that probably migrated. A total of 101 storks in the population had some wild ancestry, compared to 189 descended solely from captive storks). The original reintroduction was of 15 birds from a breeding centre in Switzerland, of which eight bred, leading to 470 descendants between 1980 and 2005. Approximately 82% of the current Swedish population is descended from four captive birds. A total of 12 native birds re-colonised, with 14% of the total population being descended from four of these.
A small study in southern France and Spain in 1997-2007 (Villers et al. 2010) found that six little bustards Tetrax tetrax originating in Spain but hand-reared and released in France did not migrate to Spain (with the possible exception of one bird that could not be tracked). By contrast, 13 out of 21 wild adults from France (62%) and six of eight hand-reared French chicks (75%) migrated. The authors conclude that hand-rearing does not affect migration probability, but that genetic origin appears to.
- Olsson O. (2007) Genetic origin and success of reintroduced white storks. Conservation Biology, 21, 1196-1206
- Villers A., Millon A., Jiguet F., Lett J.-M., Attie C., Morales M.B. & Bretagnolle V. (2010) Migration of wild and captive-bred little bustards Tetrax tetrax: releasing birds from Spain threatens attempts to conserve declining French populations. Ibis, 152, 254-261