Hi Peter,
Back in my university days, I supported a PhD student from University of
Washington who was examining this very topic in house wrens - the so-called
Tropical and Northern subspecies of House Wren. I spent many glorious days
in the Monteverde Rainforest sticking my hand in nest boxes, counting eggs
and monitoring fledgling success. Botflies, scorpions and snakes loved to
live in the nest boxes - but I'll leave that story for another time.
Essentially, the theory at that time which we set out to prove (or disprove)
was that the tropical subspecies, due to intense predation pressure, was
better off putting energy into laying fewer eggs per clutch but more
clutches in a longer breeding 'season'. As you refer to, predators such as
arboreal snakes or coatimundi are generally observed to take the whole
clutch when they raid a nest, so through simple probability of getting hit
by a predator you are better off producing multiple smaller clutches. In
contrast, in temperate zones, with a shorter breeding 'season' and lower
predation pressure, it is more efficient as a wren to lay more eggs in a
single clutch or perhaps two clutches. Although, I don't think raccoons
would agree with this theory, it did bear true at the time of our study and
I'm sure there are several scientific papers which can be searched for to
support this notion.
Sorry, just had time to give a brief account. Hopefully this helps.
Certainly brought back fond memories of birding the cloud forests of Costa
Rica - thanks !!!
Cheers,
P.
On Fri, Dec 12, 2008 at 8:37 AM, Peter Shute <> wrote:
> The article says:
> "Increased predation pressure experienced by open-nesting birds also causes
> them to lay smaller clutches than cavity-nesting birds, literally having
> fewer eggs in one basket to spread the risk."
>
> I don't understand that. If the young are more likely to be taken,
> wouldn't it be better to have more of them?
>
> Or are they saying that it's better for such birds to have fewer young
> because if the nest is discovered then they all get eaten, not matter how
> many there are?
>
> Peter Shute
>
> wrote on Friday, 12 December 2008 10:12
> AM:
>
> > The following item may be of interest to ornithologists ...
> >
> > Regards, Laurie
> >
> > http://www.sciencedaily.com/releases/2008/12/081208203911.htm
> >
> > Why Some Bird Species Lay Only One Egg
> > ScienceDaily (Dec. 9, 2008) - Why do some species of birds
> > lay only one egg in their nest, while others lay 10 or more?
> >
> > A global study of the wide variation among birds in this
> > trait, known as the "clutch size," now provides biologists
> > with some answers. The study, published in the current issue
> > of the journal PLoS Biology, combined data on the clutch
> > sizes of 5,290 species of birds with information on the
> > biology and environment of each of these species.
> >
> > <snip>==========www.birding-aus.org
> > birding-aus.blogspot.com
> >
> > To unsubscribe from this mailing list,
> > send the message:
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> > ==========www.birding-aus.org
> birding-aus.blogspot.com
>
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--
Mr. Paul Rose
Head of Department - SS Science
Environmental Co-ordinator
Prem Center for International Education
PO Box 1 Mae Rim
Chiang Mai 50180
Thailand
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