Many bird species will produce 2 broods of offspring within
a season. In some species, this increases the opportunities for males to breed
and produce offspring, win for the males. However, the females don’t always
stay with the same male they produced the first brood with. For instance, if
the first brood fails to hatch at all due to neglect or simply bad luck. The
guys get blamed, and the ladies will move on to find a mate whose offspring
will actually hatch and eventually fledge.
Infanticide
is a possibility in situations like these, where if a new male takes over a
territory or wants to induce the female to brood again sooner (with him,
obviously) the males may damage the eggs beyond repair, or simply neglect the
current brood so they will fail. Some researches have, as such, become
interested in the possibility of avian stepfathers. Cases in which the males
will tolerate, and occasionally even help defend or feed, the previous male’s
brood in order to increase his own chances of siring offspring come round two
of egg laying.
In
a study done by Meek and Robert (1991) males in several pairs of Eastern
Bluebirds were removed, and in over half the cases they were replaced by other
males all of whom tolerate the previous male’s young. Several of those females
later mated again with the “stepfather”, which worked our well for the males.
Work done by Pinkowski (1977) may help explain the males tolerance to
pre-existing broods due in that there is a high level of divorce within this species
after early-season breed failures, meaning that tolerance may produce more
offspring for the male than infanticide would.
_food_for_a_10_days_old+nestling..jpg) |
| Male Eastern Bluebird feeding chicks |
A
study that was supposed to examine single mother chick rearing in Black-billed
magpies ended up observing full parental care by replacement (or stepfather)
males when they could not be caught and removed (Dunn and Hannon 1989). Four of
the eight stepfather replaced the previous male when the female was no longer
fertile, so there wasn’t even a chance that the stepfather may have had partial
paternity for the brood. This phenomenon of stepfathers in black-billed
magpie’s has also been observed several other times outside of studies.
 |
| Black-billed Magpie |
Clearly
not all males are very tolerant of another male’s brood monopolizing a female
they could mate with. Tolerance to the offspring isn’t quite the same as an
actual father’s love either. Or caring. Close enough in the animal kingdom.
However, as the magpie shows humans may not be totally isolated in caring for
another guys kids, though goodness knows there is a LOT of research needed to
be done to totally understand the motivations of the feathered stepfathers for
not just kicking those eggs out.
References
Dunn, P. O. & Hannon, S. J., 1989. Evidence for obligate
male parental care in black-billed magpies. Auk
106: 635-644
Meek, S. B. & Robertson, R. J., 1991. ‘Adoption of young
by replacement male birds: an experimental study of eastern bluebirds and a
review’. Animal Behaviour 42:
813-820.
Pinkowski, B. C. 1977, ‘Breeding adaptations in the eastern
bluebird.’ Condor 79:289-302.
Pictures
Bluebird: http://www.carolinabirdclub.org/gallery/Duyck/eabl.html
Magpie: http://www.fws.gov/northdakotafieldoffice/bhotline/nd_birding_hotline_rep_jan08.htm
Really interesting blog you got! The magpies are just incredible birds, they are so smart, and as far as I know, they are monogamous, they usually stick to each other year after year, at least where I come from. Do you think this might matter in parenting something that is not their own? I mean, if a female find a new male while she still have young, and he helps her and show her he is off good material, then she might choose him as the new partner, and then he can have young with her for many years... So what I'm asking; is there a chance that monogamous birds are more likely to broad someone else's young if they know they might be the new partner, compared to birds that change partner often?
ReplyDeleteThat is a VERY interesting question! It seems that not a lot of testing has been done on stepfathers in birds directly testing it, a lot of data is sort of anecdotal, but it would be a great topic to directly compare between monogamous and polygamous species. In one review paper of these studies I read it seemed that the researchers believed that the option for divorce in the polygamous species is what was driving the tolerance for the males, as with some species an early brood failure will result in the female ditching that male. But monogamous birds could actually go all-out and care for the young as a sign of a good mate as you said. Clearly someone needs to test this!
DeleteFascinating post this week! Do stepfathers provision their step kids at a lower rate than biological fathers? Is it likely that there is a higher incidence of stepfathers tolerating another male’s offspring because the sex ratio in this population of Eastern bluebirds is male-biased? Did the researchers provide an indication as to why magpie males will care for another male’s young, even when they are unlikely to sire a brood with that female? Great!
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