October 2, 2018, BioMed
Central
Blue tit eggs that were laid in
urban parkland were 5% larger than eggs laid in a nearby forest, which could be
due to differences in the amount of calcium available to birds in urban and
forest environments, a study published in Frontiers in Zoology has
found.
Researchers at University of Lodz
investigated the difference in blue tit egg size and how many eggs were
laid per clutch in
an urban parkland and a forest environment ten
kilometres apart. They found that in the urban environment, tits laid bigger
but fewer eggs, while tits in the forest laid smaller and rounder, but more
eggs.
Professor Jerzy Banbura,
corresponding author of the study, said: "There is a certain difference in
egg volume that you would expect from a trade-off with clutch size, which means
there are either bigger eggs but fewer eggs in a clutch, or a greater number of
smaller eggs, depending on the resources available to the birds. However, what
is surprising about our findings is that the difference in egg volumes between
the urban and forest sites went beyond the difference we expected from that
trade off."
The researchers also found that
eggs laid in the forest were more spherical than eggs laid in the urban
environment. Just like egg and clutch size, this may be due to limited amounts
of calcium; more spherical eggs have a stronger structure than less round eggs,
but need a smaller amount of calcium.
Professor Banbura, said:
"Poor availability of calcium in the forest environment we studied- for
example due to smaller numbers of shelled snails—may have created a pressure of
natural selection towards producing smaller eggs, more spherical eggs, so that
the limited calcium was used more economically. The larger eggs and smaller
clutch sizes in urban areas may reflect the greater availability of calcium in
this environment."
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