The American Kestrel story

Drew WeberAnalysis, Conservation Issues, distribution, General News and Info, Migration, Ranges and Distributions, Science6 Comments

American Kestrel - male (Photo by Alex Lamoreaux)

American Kestrels have experienced some pretty severe declines in parts of their range. This was most recently brought to my attention while reading Clay Sutton’s article ‘Killy Killy No More’ in the November 2011 edition of Birding. This article was perhaps inspired by the fact that American Kestrel was the 2011 ABA Bird of the Year. More locally, there was a recent discussion in the PA Birders Facebook group on the declines that the American Kestrel is experiencing in our region. I find this an interesting topic and decided to look at some of the different data that was available.

There are multiple monitoring schemes across Pennsylvania that allow us to track different trends throughout the year, hopefully providing a more complete story on the plight of the American Kestrel in our state.

Spring migration

Spring hawk watches offer a good place to start looking at trends of American Kestrels. The number of kestrels counted migrating past hawk watches in the spring can give an idea of the number of kestrels wintering further south and breeding to the north. You might also imagine that you can get a sense of winter survival from spring counts. Since many of the kestrels migrating past may not be breeding in Pennsylvania, the numbers may not actually be indicative of a trend in our state. I looked at the Raptor Population Index, which you can also find on Hawkcount‘s page for individual hawk watches. There are few spring hawk watches that have been active for a long period in PA, so I also show the trend for Montclair Hawk Watch in New Jersey where I worked for a spring. Data from both hawk watches show a significant linear decline over the years that data was collected.

Breeding season

Data from both of Pennsylvania’s Breeding Bird Atlas’s show a decline in breeding activity. The first atlas in the 1980’s had observations in 2939 blocks while the most recent atlas completed just last decade fell short with just 2570 blocks recording observations. While this is at a rough scale, only showing whether there is breeding activity in a block, it does appear to show a contraction of the range in the southeast and a filling in of the range in central Pennsylvania. However, it doesn’t give us an idea of the number of breeding kestrels (if there are multiple kestrels breeding in a block). The 2nd PA Breeding Bird Atlas should be coming out sometime in late 2012 which will provide additional analyses of breeding trends.

Breeding Bird Survey routes also help to show what is going on with kestrels during the breeding season. Across the entire state, there appeared to be a slight downward trend but the trend was not significant. The BBS site also allows you to see the trends for different Bird Conservation Regions. I looked at 3 different regions that encompass parts of Pennsylvania– Appalachian Mountains, Piedmont and the New England/mid-atlantic Coast. Below are the graphs for these three regions with the 1966-2009 trend estimates and the credible interval for the estimate. One thing that quickly becomes apparent is that as you move from west to east (or less developed to more developed), the trend goes from non-significant (overlapping zero) to significant declines.

I see this as an indication that spreading development and urbanization may be a strong factor behind the declines we are seeing.

Fall migration

Fall raptor migration is a big deal in Pennsylvania, with many hawk watches putting full-time effort into tracking the southbound movement of birds. Looking at results from three sites show mixed trends, with only Hawk Mountain showing a significant annual decline. Waggoner’s Gap actually appears to be increasing slightly but the trend is not significant. Similar to spring counts, fall counts can be thought of as an indicator of how birds to the north of us are faring, however the fall data is indicative of what happened during the breeding season. If different watches are showing different trends, maybe the watches are sampling different parts of the northern population.

Winter

The Christmas Bird Count is perhaps the best indicator of how American Kestrels are faring in the winter. The graph below shows the kestrels per party hour since 1980. There is a definite decline noticeable on the charts. In talking to birders across the state, it seems that these declines are primarily in southeastern PA where some of the counts now struggle to get a single kestrel for the entire count! Other counts in central PA seem to be more stable. Number per party hour can be a difficult metric to understand over time, as changes such as increased participation may cause a false downward trend in the data for a conspicuous species such as American Kestrels. What I mean by this is that,

PA Christmas Bird Count trend for American Kestrel

Overall, it seems that American Kestrels are declining in the more urbanized southeastern part of the state. This could be a direct result of increase in development, or it could be some other cause such as increase in Cooper’s Hawks that may predate on kestrels. Other mentioned causes I have seen include pesticides and increasingly intensive agricultural practices.

I am be interested in hearing your theories for the kestrels decline, so please leave a comment.

Sources

  1. Hawk Mountain species account: American Kestrel
  2. National Audubon Society (2010). The Christmas Bird Count Historical Results [Online]. Available http://www.christmasbirdcount.org 2.20.2012
  3. PA Breeding Bird Atlas data
  • Phil Brown

    Does mercury pollution come into play with the American Kestrel as it has with Rusty Blackbirds? I’ve seen several recent articles speaking to the high levels of mercury in birds in general from eating bugs that have elevated levels of mercury and the effects this has on bird behavior in general and their inability to raise young. Kestrels with their diet of bugs, small mammals and birds would seem to fit perfectly into this type of problem. Have their been any studies done with Kestrels & mercury?

    • I am not sure, Phil. I think waterways are an important pathway for mercury to find its way into animals which is why Rusty Blackbirds are suffering since they are associated with swampy areas.

  • Josh Schultz

    I remember a while ago that I looked at some data provided by Hawk Mountain which showed the affect of DDT on Raptor populations.  While DDT clearly had a deleterious affect on the large raptors, Eagles, Buteos, etc. American Kestrels seamed to benefit from the introduction of DDT.  I have no good hypothesis on why this could be other than competition with larger raptors, which doesn’t seem probable to me, but I see this decline in the American Kestrel, as other birds recover from DDT’s affect as further conformation of the counter-intuitive link between DDT and American Kestrel populations.  Obviously this is all speculation and the correlation could be nothing but a coincidence, but I still find it rather striking.  

    On a happier note, I see Kestrels all over the place when I drive around my area of Lancaster county, so perhaps this is simply a result of habitat loss, as more farmland is turned into apartment buildings and neighborhoods.  Whatever the reason I hope this cool little raptor will stick around for a long time.

    • I have seen some interactions between kestrels and Cooper’s Hawks. These guys tend to do well as suburbanization occurs which may be a bad thing for kestrels

  • Judy S.

    In western Wisconsin, just based on casual observations of kestrel nest box occupation, it would seem that kestrels have better breeding success in large areas that are, for one reason or another, not intensely agricultural.  Competition from starlings for kestrel nest boxes seems higher in farmed areas, whereas starlings seem less prevalent in large, unfarmed regions.  This runs counter to arguments from some years ago that encouraging kestrel populations would facilitate their predation on starlings and thus help control starling populations.  We don’t see that happening here.  Starlings often kick the kestrels out of the nest boxes.  Data on this are sparse, though. 

    • I would think a starling would be pretty big for a kestrel to take down…