LI E. RAFLY OF THE UN IVER5ITY or ILLl NOIS cop, 2 './.lUHAL HISTORY SURVEY STATE OF ILLINOIS Willi.im G. Stratfon, Governor IS^k V DEPARTMENT OF REGISTRATION AND EDUCATION Vera M. Binks, Director yr' ■^ f y I.,' \ f- \L\ \V ^ k j: >► «;' HAWKS AND OWLS: Population Trends From Illinois Christmas Counts Richard R. Graber Jack S. Golden A ." 'Vvl ^r;.-> •Ar/'^' • > •V • ■■ /L'3 V <<-U a. • f ^v- ■• /.,", ■ ■y:- m- '. r^Jrs. V'y n**^^ ,^'' If'^ ::^:>-»i>:'' » I •irf'i n /- Biological Notes No. 41 Printed by Authority of the State of Illinois March, I960 7>' NATURAL HISTORY SURVEY DIVISION Harlow B. Mills, Chief Urbana, Illinois Fig. 1.— Saw-whet owl. Smallest owl (7-9 inches) in eastern North America. Because it seeks dense cover in daMime, the saw-whet owl was reported relatively infrequently even in northern Illinois, where the population is probably highest in the state. HAWKS AND OWLS: Population Trends From Illinois Christmas Counts RICHARD R. GRABER AND JACK S. GOLDEN In 1900 the Audubon Societies, parent organizations of the National Audubon Society, inaugurated the Christ- mas bird census. This census was introduced and briefly outlined in the societies' official organ, Bird-Lore mag- azine (Chapman 1900:192), and two or possibly three aims were suggested. The first stated purpose was that the Christmas census would serve as a harmless "hunt" or entertainment for the participants. The second, in the words of the article, was that it would "also con- stitute, in a measure, a census of Christmas bird-life. A third purpose was implied, that is, conservation of wildlife. The Christmas census or count that has been repeat- ed annually for more than half a century has constituted a numerical record of winter bird life, but opinions of its value as a census vary greatly. Stewart (1954) re- viewed some of these opinions and suggested methods for eliminating several major variables in the Christmas census. Ilickey (1955) urged that the Christmas census be considered a sport and implied that it should be largely disregarded as a scientific tool. Though major faunal changes are a part of our writ- ten history, there is no record of winter bird life in the United States comparable to that provided by the Christ- mas census. Fille/. ^ g ." ^30 Q. /■< fl» > 2 - ~ TTl : 1 o t ■ z ?? mr 1 LjJ 3 1 1 M O m^y- fT ^ LlJ rrr SSL, 1 ■:-:■:: ■:■: b^ kU .1. ' f-TH r-r-f1 LJ- llllll^^ 1 I] p ^ m ~ E CE ^ i] vTT ^ lilies i IILL ] m *"■ im>i Lllll^:;:! ,.,:- ^^. ''<'. '^.> '°o^. ^o^ "^e. '\ 'W ^, 'o, •e. % e^ ^^. '^ "^er ^^/ '^ ^, > ^. 'e, Fig. 5. — Frequency of occurrence of the relatively common species of hawks reported on Christmas counts in northern, central, and southern Illinois, 1903-1955. „ 70 r I- 60 3 o o ^ 50 o 40 tr uj 30 a. o 20 UJ LjJ fmr-n imv-i m NORTH - 301 COUNTS m CENTRAL- 123 COUNTS E] SOUTH - 103 COUNTS —f^ J!k 13 E3 r/^ hr '^. 13 m TM '^. '^. ^K ^^r Fig. 6. — Frequency of occurrence of the relatively common species of owls reported on Christmas counts in northern, sntral, and southern Illinois, 1903-1955. ables and still have a relatively complete and continu- ous record; however, most of the variables were present tliroughout the census period considered here and were probably not operating in any one direction to produce undue bias. The Habitat Variable. — From the earliest censuses, observers recorded a variety of data on the census localities, such as hour of starting and returning, char- acter of the weather, direction and force of the wind, and temperature. Not until the early 1940's did census takers begin providing information on the kinds of habi- tat covered. This deficiency in basic data would be a serious deterrent to an evaluation of the trends in rap- tor populations were it not for the fact that central and northern Illinois have relatively uniform terrain. Culti- vated land constitutes over 90 per cent of the habitat in these regions, and most of the censuses were taken in counties with less than 9 per cent of the land in woods (King 8. Winters 1952:22). In the consideration of numerical changes in raptor populations, emphasis is placed on data from the re- gions of the state which have been under heavy culti- vation throughout the period 1903-1955. In northern and central Illinois, the observer was never far from culti- vated farm land, and most of the birds of prey reported on counts were probably seen in or from this general kind of habitat. In southern Illinois, the part of the state with the most land in forest, birds reported were seen in a great- er diversity of habitat than in northern and central Table 1. — Number of raptors per census-party-hour recorrleii and southern Illinois, 1946-1955. Illinois. Woodland species of raptors would be expected to appear more frequently in southern Illinois than in the other zones. The Locality Variable. —Census reports showed that raptors were not uniformly distributed throughout Illi- nois, table 1. Census localities in southern Illinois averaged higher numbers of raptors than those in the central and northern zones. For the period 1919-1955, larger numbers of raptors (an average of 2.51 birds per party-hour) were reported for Grafton than for anv other Illinois locality. The large number at Grafton was brought about in part by winter concentrations of bald eagles along the Mississippi and Illinois rivers in recentyears. Similarly, Moline, situated on the Mississippi River in the north, has had large numbers of eagles and of other raptors in recent years. Through the years a given locality, in relation to other localities, has tended to show a consistent rank with regard to the density of its raptor populations. In central Illinois, Urbana regularly has reported higher raptor numbers than Springfield. Rockford consistently has reported the highest raptor numbers in northern Illinois. Variation in reported numbers from different localities in a relatively small geographic area is shown in the records of Chicago, River Forest, south- eastern Cook County, and Joliet, table 1. Though certain localities, such as Springfield and Urbana in central Illinois, have had fairly Ion<» records of Christmas counts, no single locality has provided a complete record of counts from 1900 to 1955. In analyz- ing the general trends of hawk and owl populations, it on Christmas counts at different localities in northern, central. Number o i Raptors Recorded Per Census-Party-Hour Year Northern Cen tral Southern Moline Rockford Chicago River Forest South- eastern Cook Co. Joliet 'r^?f Urbana Bird Haven Grafton Murphys- boro 1946 0.75 1.25 0.14 0.40 0.69 1.14 0.33 1947 0.50 2.00 0.14 0.32 0.50 1.75 1918 2.39 0.30 0.10 0.13 1.26 0.24 0.88 2.00 1.95 1949 4.38 0.36 0.12 0.30 0.47 0.37 1.22 1.53 0.58 1.00 1950 1.62 0.16 0.12 0.47 0.50 1.00 0.83 3.35 0.83 1951 2.13 0.49 0.36 0.22 0.33 0.10 0.27 1.25 2.94 2.41 1.29 1952 0.50 1.20 0.44 0.11 0.18 0.44 5.69 2.47 3.27 1.40 1953 1.66 0.22 0.10 0.17 0.52 0.83 2.04 3.41 0.66 1954 1.00 O.IR 0.25 0.14 0.37 0..35 0.66 2.38 2.73 0.86 1955 .... 2.04 0.37 0.35 0.83 0.65 1.01 2.92 2.00 0.80 2.50- 2.00 NORTHERN ILLINOIS — -<>-— BIRDS PER OBSERVER-HOUR O BIRDS PER PARTY- HOUR O BIRDS PER MILE a tr. m LU CD Z I.50 1.00- '^r 1 I I I n / o I I II II 1 1 I II I I I I 1 1 II I I I I I I 1 1 I II 1 1 I I II I >5 '^d /« ^5 /«< /s. Fig. 7. — Average numbers of raptors reported on Christmas counts in northern Illinois, 1905-1955, expressed as birds per observer-hour, birds per party-hour, and birds per mile. Knowing that the southern region of the state is more heavily forested than the central and northern, we would expect higher frequencies there for such wood- land birds as the red-shouldered hawk and the barred owl and lower frequencies for such prairie-loving species as the short-eared owl and the rough-legged hawk. These relative frequencies appeared as expected, figs. 5 and 6. In winter certain raptors tended to concentrate at latitudes farther south than in summer, regardless of habitat, as though responsive to the temperature dine. The Cooper's hawk, marsh hawk, and sparrow hawk oc- curred with increasing frequency from nbrth to south. Conversely, the snowy owl and the saw-whet owl oc- curred with decreasing frequency from north to south. Frequencies reported for the sparrow hawk and some other species tended to increase in later years, but these apparent increases do not necessarily indicate real increases in raptor populations. The increases re- ported in later years probably resulted largely from was necessary to use data from a number of localities in each section of the state. Analysis of Frequency of Occurrence Probably the simplest quantitative treatment of Christ- mas census data is the analysis of frequency of occur- rence of each species, the percentage of the total num- ber of counts in wiiich the species was reported. The frequencies of occurrence for the species of raptors recorded in the period 1903-19.5.5 in southern Illinois were based on 10.1 counts and in 1905-1955 in central and northern Illinois on 123 and.301 counts respectively, figs. 5 and 6. If the Christmas census has any value in making determinations of relative abundance of species of rap- tors from early to later years, it should become apparent from an analysisof frequency of occurrence and density, the latter determined by number of birds per observer- hour, birds per party-hour, and birds per mile. 4.00- 3.50 3.00- 2.50- 2.00- a cr UJ ffi 3 1.50- 1.00- .50- ■BIRDS PER g BIRDS O- BIRDS CENTRAL ILLINOIS OBSERVER -HOUR PER PARTY-HOUR PER MILE '9. 's. '/o "/^ '9, ^9 ^S I I I I ^9. ^9. Fig. 8. — Average numbers of raptors reported on Christmas observer-hour, birds per party-hour, and birds per mile. three changes in the Christmas census methods: (1) attempts by observers to "better the records of pre- vious years; (2) increases in the number of observers; (3) wider coverage of the census locality areas through the use of automobiles. CHANGES IN RAPTOR POPULATIONS Although there is considerable fluctuation from year to year, a general decline in winter populations of rap- tors is apparent from early to later years. Peaks repre- senting years of high population are lower in later years. There are considerable gaps in the record for southern Illinois, particularly in the early years, so that no def- inite trend showed there, but the population trend of rap- tors in northern and central Illinois shows the same gen- eral decline when expressed by three different units of measure— birds per observer-hour, birds per party-hour, and birds per party-mile traveled, figs. 7 and 8. counts in central Illinois, 1905-1955, expressed as birds per Stewart (1954:187) stated that, as a unit of measure for the Christmas census data, number of birds found per mile of travel was more meaningful than number found per hour of observation. As mileage was not given on all of the early counts this record is incomplete. Eifrig (1939:5) and Nice (1945:9-10) both pointed out a decline in hawk populations in Illinois. If there has been a decline in hawks, has it been general for all raptors and all species? If all raptor populations have declined, what has caused the decrease? To answer the first question, we must look at the data on those individual species for which the record is most nearly complete. For the sparrow hawk, frequency and density have been highest in southern Illinois, figs. 10 and 11. .Al- though the record was incomplete for early years, the highest density peaks came in those years, with the peak in the late 1920's lower, and peaks in the 1940's and 1950's lower still. No definite trend is evident for Fig. 9. —Golden eagle. Comparable in size to the bald eagle. The golden eagle was reported only twice on Christmas counts, both times in southern Illinois. 10 100 CO 90 I- z =) 80 o o LlJ O q: LlI >- o 70 h 60 50 40 30 uj 20 o S 10 u. W^ CENTRAL ILLINOIS ^ SOUTHERN ILLINOIS ^^ X ■■m kV> 1 i^ x7 -m L:a I ^9, % '% '^/5 /Sp /So <9, •^^O "^5 "^O /S. /s % /S. Fig. 10. — Frequency of occurrence of the sparrow hawk in central and southern Illinois calculated for 5- year periods be- inning in 1905 and ending in 1954. The year indicates the beginning of the 5-year period. CENTRAL ILLINOIS SOUTHERN ILLINOIS Fig. 11. — Average numbers of sparrow hawks reported per observer-hour in central and southern Illinois, 1903-1955. 11 Fig. 12. — Bald eagle (immature). About 3 feet in length when mature. In Illinois, the bald eagle was found principally along the major rivers, especially along the Mississippi River. 12 100 - 30 o z UJ 20 Z) o UJ 10 (C CENTRAL ILLINOIS ttu^^^^^UMil %s '% '^''^ '% '% '% "% '% '^^5 '^^O Fig. 13. — Frequency of occurrence of the red-tailed hawk in central Illinois calculated for 5-year periods beginning in 1905 and ending in 1954. The year indicates the beginning of the 5-year period. .10- .00- .90-:, 45 40- ^30- 03 u. O a: 20- UJ CD 2 Z 10- 0- CENTRAL ILLINOIS I I I I I I I I I I I I I I I I t I I I I I I I I I I I I I I I I I I I I I I I I I I I I I ^9r. %^ ^"9/45- ^^P^ ^^^^ ^^=t ^S* ^9^ ^Sc ^^'i ^9if ^O^ 'O 'S <0 - o z UJ o UJ 100 90 80 70 60 50 40 30 20h 10 ED CENTRAL ILLINOIS ^ SOUTHERN ILLINOIS 1 Ba. '^o^ '% '^/5 '% '^es '% '^ '^5 /5. ^9. ^5 /«.. Fig. 16.- ning in 1905 ■Frequency of occurrence of the marsh hawk in central and southern Illinois calculated for 5-year periods begin- and ending in 1954. The year indicates the beginning of the 5-year period. .35 .30-1 .1 5-1 o CD 10- tr UJ CO ,15- o-[ i CENTRAL ILLINOIS SOUTHERN ILLINOIS I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I '^-- '^/n '% '^en '^3. '^3r I I I I I I I I I I I I I I I I i^~rm~]" Os '^/o "'s -eo '^ o z o UJ 50 40 - 30 - 20 10 CENTRAL ILLINOIS SOUTHERN ILLINOIS X '^^o '^/5 '^eo '^'?5 '^ > <9> '9^ <9-, ''S* Fig. 19. — Frequency of occurrence of the red-shouldered hawk in central and southern Illinois calculated for 5-year periods beginning in 1905 and ending in 1954. The year indicates the beginning of the 5-year period. .25 .20- w 15 o tr. m S loH Ul m 5 .05- o-[ ixist^ CENTRAL ILLINOIS SOUTHERN ILLINOIS I M I I I I I I I I I I I I I I I I I I I I I I I I I I I I I II I II I II I I I II I I I I I |l I I I I '9. '/5 '% '^^d /« '% Fig. 20. — Average numbers of red-shouldered hawks reported per observer-hour in central and southern Illinois, 1903-1955. 17 Fig. 21. — Barred owl. Approaches the size of the great horned owl. The barred owl was the most frequently reported owl on Christmas counts in Illinois and, like the great horned owl, was seen with greatest frequency in southern Illinois. 18 If) \- z 3 O o z UJ o 100 90 80 70 60 50 S 40 Q. > 30 h o LlI o UJ (T Ul 20 - 10 - CENTRAL ILLINOIS ^9r '% '^/5 '9^ '^es '^^ /9 % /»<,. ^9 % '9.. Fig. 22. — Frequency of occurrence of the rough-legged hawk in central Illinois calculated for 5-year periods beginning in 1905 and ending in 1954. The year indicates the beginning of the 5-year period. 1.00- .90- .80- .50; .40- c .30 m ii. o a: .20 Ul OD 3 Z .10- 0- CENTRAL ILLINOIS I I I I I I r ^9 I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I [ Os "^S '^s '9>^ '9, /«< Fig. 23. — Average numbers of rough-legged hawks reported per observer-hour in central Illinois, 1908-1955. 19 Fig. 24. — Long-eareH owl. Smaller (13-16 inches) than the great horned owl. The long-eared owl was reported with greatest frequency in northern Illinois. Long-eared owls favor evergreen plantations for winter roosting sites; several owls may roost close together. 20 central Illinois, where the winter sparrow hawk popu- lation has always been relatively low. The record for the red-tailed hawk is fairly uniform except for a high density peak in central Illinois in the early 1930's, figs. 13 and 14. Again the pattern of peaks is higher in early years than in later years. The record for the marsh hawk shows no consistent declining trend in central and southern Illinois, figs. 16 and 17. However, for later years there is a sugges- tion of decline in central Illinois and of increase in the southern region. Like that for the marsh hawk, the census record for the red-shouldered hawk shows no consistent declining trend in the state, figs. 19 and 20. In the record for central Illinois, high density peaks occurred in early years (before 1930); in southern Illinois, red-shouldered hawks have increased in recent years. The rough-legged hawk was seen in central Illinois with increasing frequency from 1900 to about 1920, after which the frequency tended to fall until about 1945- 1950, fig. 22. The density of rough-legged hawk popu- lations tended to follow the pattern of frequency; the population has decreased decidedly from 1900 to recent years, fig. 23. Both early and late in the period, numbers of rough- legged hawks fell to zero in some years; it is the peaks that best show the decline. A 3- to 5-yeeir cycle has been suggested by Wallace (1955:218) for both old and new world rough-legged hawks. The relatively complete record for central Illinois shows a fairly consistent 4- year cycle. In the 1930's rough-legged hawk numbers fell so low that annual fluctuations did not show. This level continued until the mid-1940 s, when the r\,umbers increased somewhat. Though the record for the barred owl is very incom- plete and has the largest gaps in the early years, the population trend shows the familiar pattern, the popu- lation being higher in early years and lower in later years. Records for other species of owls are even less complete, and their population trends were not evaluated. Although winter populations of raptors as a group have generally decreased, the decrease is not reflected in each species of raptor. The decrease in hawk populations has been attrib- uted by Nice (1945:10) to both shooting of hawks and destruction of habitat. The effect of habitat destruction is difficult to evaluate because there are not sufficient statistical data on the extent of habitat change in Illi- nois. Also, the total effect of a change in habitat on anyone species of raptor is virtually unknown. A given change maybe detrimental to one species and beneficial to another. Before we can evaluate the effect of habi- tat change, we need more precise information both on the ecology of all species of raptors and on the kind and amount of habitat change. The effect of shooting would be most apparent among the less wary species and those species that are most frequently encountered. The rough-legged hawk, a rela- tively lethargic species, has suffered the most severe decrease in numbers of any of the species here con- sidered. The red-tailed hawk and the sparrow hawk also have declined in numbers, and they are the species which have been reported with greatest frequency. Returns from raptors banded in other states help to point out man's role in the population decline. Cooke (1941:151) said, "It is probably a safe estimate that nine-tenths of the recoveries Qjanded birds of preyj represent birds killed." Lincoln (1936) presented evi- dence that 75 per cent of the band recoveries for eastern species of raptors represented birds killed by man. Even for a species as rigidly protected as the bald eagle the proportion of banded birds killed by man is high— 60 per cent as figured from data published by Broley (1947:7). These figures are difficult to evaluate. They are biased on one hand because the most likely source of band returns are birds taken by shooting or trapping and on the other because many bands probably go unreported. Since 1900 the laws concerning protection of hawks and owls in Illinois have changed greatly. Until 1919 there was no explicit protection for any species of hawk. In 1919 most raptors gained legal protection, except the accipiters, duck hawk, pigeon hawk, and great horn- ed owl. These were left unprotected until 1959, when the Illinois legislature passed a law protecting all spe- cies of hawks and owls. A few other states (Connecti- cut, Indiana, and Michigan, for example) give complete protection to hawks and owls, but most states continue to permit the killing of certain species of birds of prey- usually the accipiters and the great horned owl. Such limited protective laws for raptors are only partially effective; they ultimately depend upon the shooter's ability to identify the different species of raptors, and field identification of birds in this group is difficult for the untrained observer. Christmas census data from Illinois show that ob- servers encountered the protected species seven times as often as the species that were unprotected; conse- quently, it is the former group that is likely to suffer at the hands of the shooter as long as any species goes unprotected. During the period 1919-1959, when Illinois had protective regulations covering only some species of hawks and owls, the protected species declined in numbers in spite of legal protection. 21 SPECIES RECORDED ON CHRISTMAS COUNTS Although the winter records for many of the species of raptors are too incomplete to show definite popula- tion trends, data for these species are of interest to students of Illinois birds and are summarized in the following annotated list. In general, the annotations are very brief, but a few species, such as the bald eagle and the broad-winged hawk, are discussed in some detail. The commonest Illinois raptors, which have been discussed in the previous section, are in- cluded with summary statements. No attempt was made to evaluate the authenticity of the records; they are reported here as they appeared in the published counts. Turkey vulture, Cathartes aura. Recor