Raptor Population Index

RPI Frequently Asked Questions


1) Q. What are the criteria to be part of RPI?


A. In order to be considered for RPI, your hawk watch site must:


2) Q. How often are the trends calculated and the maps/graphs updated?


A. Our intent is to complete an analysis with trend maps and graphs every three years. Updates to conservation assessments will occur every six years for all species and for species of concern (i.e. declining) every three years.


3) Q. Can the RPI analysis account for part-time coverage (i.e., missing surveys within a year)?


A. It is best to have full coverage, however, we can still get good estimates of annual indexes and trends from incomplete data, as long as the missing data are random (not systematic) and are not too extensive. Confidence around the annual index may be affected if a larger amount of data is missing (i.e., large credibility intervals).


4) Q. If my site is missing a year of data, how does this impact the analysis?

A. Although it is better to have data for all years, we can still calculate a trend based on the years that are available. Of course, the trend estimate will be more accurate if all years are represented. One or two missing years in a long set of data will probably have little effect on the trend estimate, but the reliability of the estimate will be strengthened if more years are included and this will be reflected in tighter credibility intervals used to determine statistical significance.


5) Q. Will data gaps such as caused by COVID or other factors (between and within seasons) impact my site’s ability to contribute to RPI?


A. Data gaps, whether they represent skipped months or years, will occur in many long-term datasets. In 2020, many sites changed their raptor monitoring efforts during the Covid pandemic, whether cancelling the count season, counting limited hours or with fewer observers. Not fully knowing the extent of how Covid has and will continue to impact raptor monitoring efforts. The RPI committee will decide how to handle this before our next analysis. Although it is better to have consistent data effort, we are often still able to calculate a trend for sites with missing years or data gaps. Whatever the reason for the data gap, it is highly important that the site document the change and reason in a file within hawkcount.org. If your count effort was affected or changed in any way by the pandemic, please document these changes in your protocol document and on HawkCount.org. Please also contact HMANA’s monitoring site coordinator, Julie Brown at brown@hmana.org with any changes so we can be sure to document for future analyses.


6) Q. Why do you need 10 years of data to estimate a trend?


A.  This is based on statistical and biological experience. We know that raptor populations and migration concentrations can vary from year with prey cycles, weather influences and other factors. Consequently, it is difficult or impossible to detect a meaningful trend with short data sets. Therefore, we decided not to try to estimate trends with less than 10 years of data.


7) Q. I have 20 years of data for a single species (e.g. Broad-winged Hawk) - can these data be analyzed in RPI?


A. Yes, but we tend to give priority to sites with good data for several species. We may get around to analyzing single-species sites at some point, so please keep entering these data in HawkCount. And, these data can be useful for other conservation or research purposes.

8) Q. Do you require hourly data for RPI or can you use my daily counts as well?


A. For the first few RPI analyses, only hourly data were used, but in 2019 we were able to incorporate sites with daily data as long as they met other RPI criteria. Although we can now utilize daily, we highly recommend entering hourly data. In order to make consistent comparisons between years, we need to have counts that are standardized as much as possible. We use the hourly data to select counts from specific hours (e.g. 9 a.m. to 4 p.m.) that were covered most consistently and had the highest numbers of each species. Hourly counts since they provide more information, are more beneficial for future research by other researchers as well.


9) Q. My mid-latitude site quits counting on November 15.  Will that give adequate coverage for late season species, such as Golden Eagles, Northern Goshawk and Rough-legged Hawks?

A. A significant fraction of Golden Eagle, Bald Eagle, Red-tailed Hawk migration occurs after November 15 along with Northern Goshawk and Rough-legged Hawk. We recommend counting until November 30 at a minimum for most United States sites. South of the United States, sites may end counts prior to end of November, although Turkey Vulture flights still continue. Several mid-latitude sites continue into December to document late season eagle movements. Recent telemetry data also shows that some Golden Eagles are still migrating into late December, and there is evidence of a shift in eagle flight timing towards later dates, perhaps due to warmer conditions at high latitudes. However, December can bring snow and ice so personal safety should not be compromised for more complete data!

More than 50% of Rough-legged Hawk fall movements occur in November and December with some of the largest days  in eastern hawk-watches occurring from late October through November.  We suggest monitoring Rough-legged Hawk numbers using counts from more northern spring and fall sites where counts consistently exceed 10 or more birds a season and the count period encompasses most the migration of this northern migrant. Christmas Bird Counts also are an important tool in monitoring Rough-legged Hawk as some birds may drift south after mid-December.

10) Q. Can RPI detect whether hawks take alternate routes in different years (e.g. coastal versus mountains)?


A. Raptor migration routes tend to be fairly consistent from year to year over larger spatial scales (i.e., 100s of km), however, routes certainly vary over moderate distances (i.e.,10s of km) as a function of weather conditions. Spatio-temporal analysis of RPI data could possibly be used to detect such shifts in migration routes, but to date this has not been attempted.

Variability of annual counts can be due to shifts in migration routes between years, but also to (1) actual changes in numbers of raptors, and (2) differences in detectability of migration between years (particularly for species like broad-winged hawks that often use thermal lift to migrate at very high altitude). This inherent variability in count data is why RPI insists on a minimum of 10 years of consistently collected data before determining a trend (see question above).


11) Q. Is climate change considered in RPI’s analyses? How does it impact raptor migration and trend estimation?


A. RPI provides a statistically valid trend for migration counts from individual sites. Climate is one factor that may influence changes in numbers detected by a site but it is not considered in the analysis itself. For example, Red-tailed Hawk and Rough-legged Hawks have been documented to winter farther north in recent years compared to decades prior. Hence, some sites in the United States are observing less of these species migrating past their site. Climate changes could also affect survival or nesting success in a species and have direct impact on populations. Many researchers have documented changes in timing of migration in raptors. The impacts of climate change are complex and subject of considerable scientific attention.


12) Q. Can I use RPI indices for my own analyses, if so how do I go about obtaining those data?


A. Yes. Use of yearly indexes for other raptor related research is an RPI project goal. This form HERE gives step by step direction to access data from NatureCounts.