Discussion
Our results compare some major fitness components
between ‘‘pure’’ and hybrid partridges (A. rufa 9 chukar)
in a native wild population in Spain. Given that
hybrid females had the same probability of laying a
clutch than ‘‘pure’’ ones, had a similar hatching
success, but laid larger clutch sizes, hybrid females
might spread their genotypes more efficiently than
‘‘pure’’ ones in wild populations. Thus, genetic introgression
may be extending through red-legged partridges
range by wild breeding, not only by releases of
hybrid birds (Blanco-Aguiar et al. 2008). In fact,
although the number or partridges known to be
released in our study area is relatively low (about
2000 birds/year), and restricted to two game estates of
relatively small size and high hunting pressure (see
details in Casas and Vin˜uela 2010), we found a
relatively high percentage of hybrids in the whole
population (29% of breeding birds). Hence, some of the
hybrid birds we have monitored might have originated
from backcrosses in the wild, although initial introgression
must have been generated by human releases
of farm-bred birds, most likely via releases in two of
the game estates in the study area, because chukar and red-legged partridges ranges do not overlap (McCarthy
2006). Our results contrast with those reported by Potts
(1989) for British populations, who did not found any
remarkable difference in clutch size, fertility or chick
survival between chukar, hybrid or red-legged partridges,
but, for reasons unknown, chukar and hybrid
partridges had lower productivity than red-legged
partridges. However, in that study there was not genetic
identification of birds, so information could be incomplete
or skewed. Alternatively, the effects of releases
and hybridization might differ depending on prevailing
environmental conditions experienced by wild populations,
but additional research comparing UK and Spain
would be necessary to test this possibility.
Larger clutches of hybrid females might be a
consequence of artificial selection in farms, or strictly
due to greater genetic laying ability of hybrids (Potts
1989; Nadal 1992). Thus, larger clutch size of hybrid
birds could simply reflect a farmed origin, independently
of hybridization. Larger clutches could be
associated with increased predation risk in birds (Skutch
1982), but we have not found this to be the case in our
study population (Casas et al. 2009; Casas and Vin˜uela
2010). In this study, we did not find significant
interannual variations in clutch size in nests incubated
by females. The significant year-to-year variation in
clutch size reported in this species (Casas et al. 2009)
may be mainly explained by clutch size variation in
nests incubated by males, and in the proportion of
females that lay in two nests (one incubated by the male,
and the other one by herself; Green 1984; Casas et al.
2009). We found that hybrid males tended to incubate
more often than ‘‘pure’’ ones, although the difference
was not significant. The fact that hybrids may breed as
well as, or even better than ‘‘pure’’ ones, in wild
populations have important implications regarding the
persistence and possible spread of hybrids in nature. It
stresses out the widespread concern about the genetic
integrity of red-legged partridge populations (Potts
1989; Aebischer and Potts 1994).
We found that hybrid partridges survived less well
than ‘‘pure’’ ones in wild populations. Hybrid birds
suffered higher mortality that was attributable mainly
to predation, in particular by carnivores. ‘‘Pure’’ birds
were predated mainly by carnivores too, but also by
raptors. These results should be interpreted cautiously,
given that foxes often scavenge on dead
birds, so we could underestimate mortality due to
other causes. Predation is typically the main cause of
mortality in farm-reared released gamebirds and
partridges (Leif 1994; Gorta´zar et al. 2000; Putaala
and Hissa 2003), most likely because of inappropriate
antipredator behaviour in captive-bred birds (McPhee
2003). Partridges are particularly vulnerable to predators
during the first weeks after release (Gorta´zar
et al. 2000; Pe´rez et al. 2004), and our results suggest
that this may be the case in the longer-term too.
Although survival rate was lower in hybrid birds,
enough hybrids seem to survive, recruit and breed to
be maintained in natural populations at a relatively
high rate. This might be because despite surviving
less well, those hybrids that recruit might be more
productive than ‘‘pure’’ birds (greater laying capacity).
An alternative explanation is that there is a
continuous influx of hybrid birds into wild populations
because of repeated (annual) and massive (3–4
millions/year in Spain) releases of captive-bred birds,
and because of a high prevalence of hybrids in farms,
with poor or no genetic control (hybrids have been
detected in c. 63% of farms used for releasing
partridges; Blanco-Aguiar et al. 2008). Thus, even
although few released birds would survive in the
wild, massive releases conducted over many years in
the same area would allow hybrid partridges to be
maintained in wild breeding populations (Barbanera
et al. 2009b). Summarizing, the lower survival rate of
hybrids could limit their spread into wild populations
in the long term, but our data do not allow a complete
demographic evaluation, because this lower adult
survival could be compensated by a higher productivity.
Data about chick survival of hybrid versus
‘‘pure’’ partridges are urgently needed to develop a
complete demographic model, allowing a full evaluation
of hybrid performance in the wild.
Hybridization is very important for the speciation
and evolution of animals, but may also be an undesired
consequence of human perturbations, and thus a
general ‘‘hybrid policy’’ that applies to all situations
and to different species would be very difficult
(Allendorf et al. 2001). Providing appropriate and
helpful recommendations is probably better on a caseby-case
basis. Gamebird restockings are spreading
introgression into wild populations (Puigcerver et al.
2007; Blanco-Aguiar et al. 2008; Barbanera et al.
2010). According the classification of Allendorf
et al. (2001), our case of hybridization would be of
‘‘type 5’’, i.e. widespread anthropogenic introgression,
with the associated recommendations of maintainin and expanding the remaining ‘‘pure’’ populations
(Allendorf et al. 2001). We cannot rule out completely
that fitness (breeding and survival) differences
between hybrid and ‘‘pure’’ partridges might be due,
in part, to domestication effects rather than effects
associated with the introgression of chukar alleles. We
have found that even in hunting estates were restocking
with farm-reared partridges are performed (which
populations could contain both ‘‘pure’’ and hybrid
farm-bred partridges), the survival was lower in
hybrids than in ‘‘pure’’ ones. Nevertheless, further
investigation is needed (1) to disentangle the relative
effects of domestication or hybridization on fitness, for
which purpose additional studies in captivity or semicaptivity
should be useful to study the fitness differences
between ‘‘pure’’ and hybrids farm-bred birds,
and (2) to study the long term hybridization effects.
Nonetheless, according to Spanish law (law
42/2007, Natural Heritage and biodiversity), the
Spanish public administration ensure that releases
of species for hunting purposes does not involve a
threat to conservation of these or other species, in
genetic or demographic terms, so management strategies
should be implemented to control genetic
introgression and to help recovering the original
genetic structure of populations by (1) stopping
releases of hybrid birds; (2) implementing an effective
inspection protocol of partridge farms and game
estates where partridge releases are performed, and
(3) promoting management plans to avoid releases in
those areas where restocking programs have not yet
been performed, and where there is reliable evidence
that populations consist of non-hybridized individuals.
Gamebirds other than red-legged partridges can
hybridize in the wild with introduced, non native
birds that originates from hunting restocking programs
(e.g. common quail Coturnix coturnix hibridizing
with non native japanese quail Coturnix
japonica or their hybrids; Puigcerver et al. 2007).
Our work should stimulate more research not only on
the occurrence of hybrids in the natural populations
of other species, but also where possible on the fitness
of hybrids in order to better understand the risks to
native populations.