Background
Distinguishing between Western Wood-Pewee
(Contopus sordidulus) and Eastern Wood-Pewee (Contopus virens)
typically requires hearing the birds sing, or a careful series of measurements
in the hand.
On 14
September 1999, RRBO banded an adult female pewee which had an appearance
and measurements highly suggestive of Western Wood-Pewee. An important
measurement was not obtained, and other measurements, while closer to Western
Wood-Pewee (WEWP) than Eastern Wood-Pewee (WEWP), were not completely conclusive.
One characteristic that triggered the questioning
of this bird’s identity was the extent of dark coloration on the underside
of the bill. In adult pewees, this extent rarely reaches beyond 3.5 mm
in EAWP, and is nearly always more extensive than 3.5 mm in WEWP. In young
birds, there is more variation and some EAWP have more extensive dark coloration.
The 1999 pewee had a virtually completely dark under-bill.
On 30 August 2005, RRBO banded another pewee
that had a dark underbill. As shown in the photo below, the conservative
extent of the dark color was 7.6 mm. However, skull ossification
indicated the bird was a hatching-year. This meant that the dark underside
of the bill was not as suggestive as it would be on an adult, and any plumage
characteristics would not be helpful. Still, it prompted further investigation.
Methods
Measurements taken on the 2005 pewee
The two species differ in tail length and
wing proportions. There are four measurements needed to help distinguish
between EAWP and WEWP. They are:
Wing Chord
Tail Length
Tail Clear
Wing Tip
All measurements were taken multiple times
to the nearest half-millimeter with standard wing rules and a dial caliper.
Wing Chord and Tail Length are straightforward, standard measurements.
The Tail Clear and Wing Tip measurements require an unreasonably steady
hand. Read how I made measurements.
Calculations
Two numbers are derived from these measurements
are calculated. They are the ratio TC/TL and the difference between the
tail clear and wing tip measurement (TC-WT). I derived the mean values
for actual measurements taken, and calculated an error
array to account for possible inaccurate measurements.
Results: Major
For pewees that are unsexed, the following
combination of measurements identified 100% of study skins correctly (n=130)
based on a key developed by John Hubbard:
TC/TL together with
either TC-WT and/or Tail Clear
TC/TL
UM-D bird = Mean value is 47.2% for both
sets of measurements (range 47.2 to 48.8 for actual measurements, 43.2
to 51.2 in error array)
EAWP should be >52.8%
WEWP should be <48.4%
Conclusion: WEWP. Mean value, 5 out of 6 values
for actual measurements, and 44 of 66 in the error array are within the
WEWP range. None of the values from actual measurements or the error array
fit EAWP.
TC-WT
UM-D bird = Mean value is 8.3 for actual
measurements (range 4 to 11) and 7.0 for error array (range -0.5 to 14.5)
EAWP should be >11.5
WEWP should be < 6.0
Conclusion: Inconclusive, suggestive of WEWP.
Mean value of actual measurements closer to WEWP, and 5 of the 6 values
fall between the two species. Mean value from error array also closer to
WEWP, with 26 of the 66 values within WEWP, 10 within EAWP, and the rest
in between. However, this result is half of an and/or statement. See next
item.
TC
UM-D bird = mean value 29.5 for actual measurements
(range 29 to 30.5) and for error array (range 27 to 32)
EAWP should be >34.0
WEWP should be <30.0
Conclusion: WEWP. Both mean values are within
WEWP. 5 of 6 actual values are within WEWP. 44 of 66 values in the error
array are within WEWP. None of the actual values or those in the error
array are within EAWP.
The bottom line: Two out of three required
major characteristics, which have been used to identify 100% of pewee study
skins, conclude this bird was a WEWP.
Results: Minor
Four things are considered minor characteristics
in aiding with the identity of pewees.
Bill color
As mentioned in Background, the extent of
the dark coloration on the underside of the bill tends to be <3.5 mm
in EAWP. In WEWP, it tends to be >3.5 mm. It was 7.6 mm in the UM-D bird.
This characteristic is not as helpful in hatching-year
birds such as the UM-D bird. But bills this dark in EAWP are still rare.
You can read some results of an examination of bill color
in study skins.
Wing Tip
A Wing Tip measurement of >28.5 mm would
indicate WEWP, while <18.5 mm would indicate EAWP. The mean value of
actual measurements of the UM-D bird was 21.2, with a range of 19.5 to
25.5. The mean value of the error array was 22.5, with a range of 17.5
to 27.5. These values are closer to EAWP.
Wing Chord
WEWP have longer wings than EAWP, but it
varies by individual and sex. Wing Chord of the UM-D bird was 81.5 mm.
If it were >88 mm, it would indicate WEWP, if it were <76 mm it would
indicate EAWP. But because of the variation and the unknown gender of the
UM-D bird, this character is not very helpful.
Therefore, bill color and tail length lean
towards WEWP, wing tip towards EAWP, and wing chord is unhelpful, especially
not knowing the sex of the bird.
Summary
Measurements and bill color indicate that
the bird banded at RRBO on 30 August 2005 was a Western Wood-Pewee. (Although
evidence was quite compelling for the bird banded in 1999, without comparable
measurements we can only focus on the 2005 bird.)
Several scenarios are possible. First, these
measurements can accurately identify study skins, but are not as good with
live birds. Second, these measurements did not use a large enough sample
size and may not even truly identify study skins. Third, these measurements
are correct and this bird was a WEWP.
Perhaps WEWP are more common in the east than
reported, since they are not frequently banded by most banding stations,
and it is unlikely most banders take the time to go through all these measurements.
Therefore, some WEWPs may go undetected.
