Breeding Data
How many eggs does a bird lay?
A clutch of eggs is the total number of eggs laid in one nesting
attempt. Clutch sizes vary within and among species. For example, an
Ash-throated Flycatcher may lay between 4 or 5 eggs in an average
clutch, whereas a Wood Duck may lay 7 to 14 eggs per clutch. Virtually
all songbirds lay one egg per day, usually in the early morning, until
the clutch is complete. In some larger species there are two- to
three-day intervals between the laying of successive eggs, but still no
more than one egg is laid in a day.
How do we study clutch size?
Collecting the data needed to study clutch size involves monitoring
nests on a regular basis, counting the number of eggs, and recording as
much information about the nest as possible.
How many eggs do Western Bluebirds lay (what is their average clutch
size)? This table shows the range of average clutch sizes reported by
participants of The Birdhouse Network for Western Bluebirds across
their breeding range for the years 1997-2003.
How does that number vary across regions? Looking at 2003 data from the
table, we find that birds in many of the regions show similar clutch
sizes for that year (Colorado, Idaho and Oregon: an average of 5.3 eggs
per nest; California 4.9 eggs per nest). Birds in some regions,
however, laid many fewer eggs that year (Washington: 2.5 eggs per
clutch; New Mexico: 3.0).
How does the average number of eggs per clutch vary from year to year?
Looking at the previous year (2002) we see the opposite trend, with
birds in Washington laying the most eggs on average. What might
contribute to such variation?