Hawkwatch Data for 2006
October 21, 2006: (Note: the final audited data is below).
Once again, I would like to thank Hornsby Bend for running the hawkwatch this year and the Travis Audubon Society for funding it for my second year. The help from visitors scanning clouds for the tiniest of specs made the slowest of days tolerable and the busiest days exciting, while keeping stress levels to a minimum. The most valuable thing I have learned coming out of this years watch is how little we really know about the migration of the Swainson's hawk and how important long-term data collection is for determining trends in central Texas raptor migration. Communication this year between birders from the areas surrounding Austin and educating them on what days would probably provide large kettles and streams also helped in creating a better picture of what was happening. Upon reviewing the 2006 Hornsby Bend hawkwatch data and discussing the results with the other hawkwatches, I will offer some possible explanations for our results.
Beginning with total numbers and species numbers, we ended 2006 with a total of 8,398 total migrating raptors. Although the numbers recorded for the Hornsby Bend Hawkwatch are often between 10,000 and 17,000, Swainson's hawks have mainly comprised the greatest numbers, where once again this year was dominated by Turkey vultures. While this years results showed surprisingly similar comparisons to last years in some aspects, it shared other similarities with the previous years. For total species we had 16 different raptor species recorded this year, which is consistent in that every year has always been between 16 and 18 total. Only the single Prairie falcon, Ferruginous or Zone-tailed hawk usually alters this number from year to year.
For the second year in a row, I now wanted to determine the differences in numbers compared over the past years. The greatest factor in decreased numbers was again the missing Swainson's hawk migration over Hornsby Bend. Yearly past averages were 8,000-11,000 and for the second year in a row we had less than 1500. 2001 was the only other notable deviation with only 2104. The other main difference was the record high number of Turkey vultures (5201) and Black vultures (818) this year without increased numbers of most other species coming through. Also notable species, which were up in numbers compared to years past, are Osprey (38) when ten seems to be about the average. Broad-winged hawks (436) were at an all-time high compared to the average of 100-200. I find this number to be quite misleading due to the fact that 330 of these came over in one kettle. The increase of Bald Eagles (5) was very interesting when formerly getting only one during a hawkwatch was an exciting event. The other enigma that seems to ask many questions is the decrease in Mississippi kites. The first two years of the hawkwatch averaged over 800 kites a season. The next two counts were very close together with about 340 birds and my two years have been very close together at about 78.
After looking at the Veracruz River of Raptors, Mexico website, I was able to notice that they had record low counts of Swainson's and Broad-winged hawks, with numbers coming in about 400,000 shy for each species. They also had a record low for Turkey vultures with about 700,000 unaccounted from their yearly average. After conferring with the Corpus Christi and Smith Point hawkwatches, I have found their results varied also. Some species being up by 200% and others down by 50%. In another statistic, Broad-winged hawks, which make up over 80% of Corpus Christi's total numbers, seem to be right on average for the year.
Finally, I would like to conclude by completely throwing out my hypothesis from last years study that the hurricanes alone may have decreased species numbers. From the discussions among other hawkwatchers, average surrounding weather conditions can greatly affect where the birds decide to come over. Sightings of large numbers of Swainson's hawks in all directions encircling Hornsby help to show how much more comprehensive coverage is needed to accurately predict a species abundance. Once again we may not be able to correctly determine if trends are being formed, but the data collected this year will hopefully provide answers in the future. Hopefully any lessons learned will not be in vain.
Until next year,