For the past 21 days (Dec 26, 2002 - Jan. 15, 2003), whenever possible we have attempted to catch and band American kestrels (Falco sparverius) - the only New World kestrel. All of the data collected on each individual bird is entered into our field notebook as the birds are captured and banded. In the evenings Jill enters the data into an Excel spreadsheet for safe keeping in the event of the loss of our field notebook. This evening we were able to start playing with some of the data from the Excel file and the trends that appeared we found to be very interesting. From our number crunching, theories were born, only to be crushed a half hour later. We are far away from our kestrel literature, maybe some people could jump in and help us out with some of our findings (which may just be common knowledge).

Distribution of gender in wintering kestrels and population segregation:

To date, we have captured and banded 90 kestrels from Lamar Colorado down to south Texas. Overall, 60% were male. We were able to divide our kestrel trapping areas into 4 distinct zones, each about 250 miles south and 80 miles east from the previous zone. In the northern three zones (around Lamar, Colorado; around Turkey, Texas and a transistion zone further south but not including south Texas) kestrels were pretty much divided up equally - with males representing 46-50 % of the kestrels captured. Then once we started sampling kestrels in what we call the south Texas zone (San Antonio south to the lower Rio Grande Valley), male kestrels suddenly started to represent 85% of all kestrels captured and banded.

Our first theory to spring from the Excel files was the obvious; that somewhere further north from where we'd started trapping was an area where a trapper would capture 85% female wintering kestrels and then a transistional area where the ratio would be evenly distributed males to females and then further south (San Antonio, TX and southerly) male kestrels would be the predominant gender of wintering kestrels.

Then, we started looking at wing chords and the theory we'd hatched fell apart. We soon realized that the further south, the smaller the wing chords became for both male and female kestels. In the Lamar, CO area the mean female wing was 200 mm (n=8), males were 193 mm (n=6). In the Turkey TX area the mean female wing was 202 mm (n=14), males were 190 mm (n=11). Going another 250 miles south and 80 miles east into Texas, the mean female wing was 200 mm (n=6), males were 192 mm (n=6). However, in south Texas we found the mean female wing to be 191 mm (n=5) while males were 187 mm (n=26). This left us with the conclusion that since wing chords of both sexes were decreasing with the latitude, we were getting into different populations of wintering kestrels. Our first theory would only have worked out if we could locate a population of smaller kestrels either further south or west where 85% of the trapped kestrels were females. At this point, we are clueless as to where the females of these smaller kestrels are wintering, much less where this population breeds.

Variation of male kestrel tail coloration and patterns:

After handling 54 male kestrels in rapid sucession, we were surprised at the variability in their tail feather coloration and patterns. After handling these small falcons for more than 48 years combined experience (between the two of us), we would like to describe the typical male kestrel tail as one that the tail feather has a white terminal band of approximately 8 mm adjacent to a black subterminal band about 22.5 mm wide. The remainder of the tail feather is a deep rufous color, except for the outer tail feathers which have a black and white the outer edge with the inner edge being the same deep rufous color as the rest of the tail feathers. See photo, Typical male kestrel tail.

TYPICAL MALE KESTREL

TYPICAL MALE KESTREL TAIL

Now for some of the variations we have noticed.

Terminal bands range from the typical white to gray to light rufous. Subterminal bands we have found to be fairly uniform in all variations. The remainder of the tail feather, which we called solid rufous above for the typical male kestrel, has wild swings in coloration and patterns. One pattern that we have documented in 2 of the individuals is a black dotted bar approximately 18mm above the black subterminal band. One of these birds started yet another row of black dots another 15 mm higher up on the tail.
See photo ATYPICAL male kestrel tail # 1

ATYPICAL Male Kestrel Tail # 1

Another variation that we have documented in several individuals is repeats of terminal and subterminal bands up higher on the tail, giving the spread tail the appearance of being a black and white barred tail with the exception of the 2 central tail feathers which remain rufous above the subterminal band. See photo Atypical male kestrel tail # 2.

ATYPICAL Male Kestrel Tail # 2

A rare variation is where the tail feathers are not symetrical in coloration or patterns. The feathers are of different lengths and the subterminal band was incomplete on a central tail feather. See atypical male kestrel tail # 3.

ATYPICAL Male Kestrel Tail # 3

Lance & Jill Morrow
Falcon State Park
on the Texas/Mexico border

 

 

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