0006-3207/97 $17.00 + .00. BOBOLINK Dolichonyx oryzivorus POPULATION DECLINE. IN AGRICULTURAL LANDSCAPES IN THE. MIDWESTERN USA.
PII:
S0006-3207(96)00066-3
Biological Conservation 80 (1997) 107 112 © 1997 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 0006-3207/97 $17.00 + .00
ELSEVIER
B O B O L I N K Dolichonyx oryzivorus P O P U L A T I O N D E C L I N E IN A G R I C U L T U R A L L A N D S C A P E S IN THE MIDWESTERN USA James R. Herkert Illinois Endangered Species Protection Board, 524 South Second Street, Springfield, Illinois 62701, USA (Received 6 June 1995; accepted 15 January 1996)
species have been in general steeper, more consistent, and more widespread than declines exhibited by other behavioral or ecological groups of bird species (Knopf, 1994). Identification of the causal factors associated with North American migrant bird declines has been problematic, however, because many species appear to be experiencing potential problems on both the wintering and breeding grounds (e.g. Robinson, 1993; Sherry & Holmes, 1993; Rappole & McDonald, 1994). Therefore, identifying the specific factors associated with migrant bird declines has been enigmatic. The bobolink Dolichonyx oryzivorus is a neotropical migrant bird that breeds in grasslands throughout the northern United States and Southern Canada and overwinters in southern South America (American Ornithologists' Union, 1983; Martin & Gavin, 1995). Historically, bobolinks bred in the extensive prairies of North America but in more recent times, a large portion of the breeding population has shifted to agriculturally associated grassland habitats, such as hay fields, pastures and small-grain fields (e.g. Bent, 1958; Graber & Graber, 1963; Sample, 1989). In recent decades, North American bobolink populations have declined significantly, a decline that has intensified in recent years in most regions (Table 1). The decline of breeding populations of bobolinks has been especially severe in highly agricultural Midwestern states, such as Illinois and Iowa (Table 1). In Illinois the bobolink appears to be the fastest declining songbird in the state (Herkert, 1993). Many hypotheses regarding the decline of bobolink populations have centered on a decrease in area of hayfield and/or the earlier and more frequent cutting of hayfields (e.g. Bent, 1958; Peterjohn, 1989; Bollinger & Gavin, 1992). Research in the midwestern and northeastern United States has recently documented serious demographic consequences of mid-season cutting of hayfields, and shown that grassland birds nesting in hay fields generally produce very few young (Frawley, 1989; Bollinger et al., 1990; Bollinger & Gavin, 1992). In this paper, I examine the dynamics of bobolink populations in an intensively farmed 37000 km 2 agricultural landscape. I present data from both a computer
Abstract
A computer model and data from the North American Breeding Bird Survey (BBS) were used to examine the effects of changes in the area and disturbance patterns in agriculturally associated grasslands on the demographics of bobolink Dolichonyx oryzivorus populations in a 37000 km 2 area in northern Illinois. Input parameters for the computer model were based on documented declines in the area of hay, pasture, and oats and on changes in the timing and frequency of hayfield cutting. Declines in the area of agriculturally associated grasslands strongly influenced the model population and resulted in an estimated overall bobolink population decline of 95% between 1952 and 1992. An increase of nearly two weeks in the median date of first cutting of alfalfa hay had a weaker impact on the model population. Field data for this area from the BBS suggest a bobolink population decline of over 90% between 1966 and 1992, a decline that was significantly correlated with documented declines in the regional area of pasture, alfalfa hay and oats. Although they cannot discount the possible effects of other factors, the model and empirical data I present demonstrate that declines in agricultural grassland habitat, and changes in the timing and frequency of hayfield cutting, have the potential to produce significant regional population declines in grassland bird species inhabiting agricultural landscapes. © 1997 Published by Elsevier Science Ltd. All rights reserved
INTRODUCTION Populations of many species of breeding birds are declining in North America, including many migratory species that breed in North America and winter in the Neotropics (e.g. Robbins et al., 1993; Peterjohn & Sauer, 1994). Although recent concern over the status of North American migratory bird species has been stimulated largely by research focused on forest bird species (e.g. Keast & Morton, 1980; Hagan & Johnston, 1992), recent analyses of North American breeding bird population trends have shown that the declines exhibited by grassland bird 107
108
J. R. Herkert
Table 1. Bobolink population trends based on data from the North American Breeding Bird Survey (BBS). BBS data include the estimated mean annual population change (trend), and the degrees of freedom (d.f.) associated with the trend estimation. Treads are estimated by the route regression method (see Geissler & Sauer (1990) for a detailed description of how BBS trends are estimated)
Strata
North America Canada New Brunswick Nova Scotia Ontario Quebec Saskatchewan United States Illinois Iowa Maine Michigan Minnesota New York New Hampshire North Dakota Ohio Pennsylvania Vermont Wisconsin
1966-1992 (%/year)
1966-1979 (%/year)
1980-1992 (%/year)
Trend
Trend
d.f.
Trend
d.f.
754 169 22 21 50 28 21 585 26 30 32 52 36 89 20 21 32 46 17 61
-5-0*** -6.1"** -8.5*** - 14.5"** -3.3*** -8.7*** - 1.0 -3.7*** - 16-6"** -9.5*** -7.1 *** -5.5*** -5-1"** -0-3 1-2 -0.7 -2.2* - 1.4 0.7 -4.1"**
881 206 19 16 52 38 28 675 29 30 43 63 41 83 19 40 34 57 19 61
- 1.3"* -0.8 -0.8 -2.2 0.4 -4.0*** 0.5 - 1-9"** -9-5*** -6-2*** 0-1 -3.1"** -0-7 0.1 2-6* -0.3 -3.1"** 1.9 - 1-8 2.7***
d.f. 1023 250 25 22 59 50 36 773 35 30 46 69 41 98 20 41 40 68 21 65
1.3* 4.4*** 7.2*** 4.1"* 4.5*** 1.7 13.7*** - 1-4 -7-7* -3-5 2.9 - 1.7 3.3 - 1.5 3.8* -4.9 -6.8*** 7.3* -4.6** -0-8
*p < 0-10, **p < 0.05, ***p < 0-01. Table 2. Bobolink density and area of habitat for northern Illinois in 1957-58
Habitat
Bobolink density/100 ha"
ha
130.9 34.6 108.7 123-5 12-0
485 600 445 100 242 900 80 900 22 000
Oats Pasture Alfalfa hay Non-alfalfa hay Ungrazed grass "Graber and Graber, 1963.
simulation model and empirical field data regarding the effects of landscape-scale land use changes on bobolink populations on their breeding grounds in the Midwestern United States.
METHODS Model development
Because breeding bobolinks are absent from southern Illinois and are significantly more abundant in northern than central parts of the state (Graber & Graber, 1963; Bohlen, 1989) my analyses focused on land use changes in the northern 25 counties of Illinois, an area of approximately 37000 km 2. Bobolink population data from 1957 to 1958 statewide field censuses of Illinois (Graber & Graber, 1963) were used to identify grassland habitats included in the analyses. Hay fields, oat fields, and pastures were the primary breeding areas for northern Illinois bobolinks in the late 1950s (Table 2), and were the three major habitats included in the
population model and analyses of field data. Data on the annual area of northern Illinois hay fields and oats were obtained from Annual Illinois Crop Reports published by the Illinois Cooperative Crop Reporting Service (Springfield, Illinois) for the period 1952-1992. Data on the area of Illinois pastures were obtained from the periodic United States Census of Agriculture conducted in 1950, 1954, 1959, 1964, 1969, 1974, 1978, 1982, 1987 and 1992 and published by the United States Department of Commerce (e.g. US Department of Commerce, 1994). Estimates of area of pasture in intervening years were obtained by interpolating between known years assuming a constant rate of change within intervals. Comparisons of Illinois hay field and oats area estimates using this averaging method, with known annual area figures for these crops, were highly correlated (r_>0.92), suggesting that these periodic data and this method provide reliable annual area estimates for agricultural habitats. Because alfalfa and non-alfalfa hayfields are cut on different timetables, area and cutting timetables for these two types of hay fields were modeled and analyzed separately. The progression of cutting for hay fields used in the simulation model was based on annual reports of the percentage of hay field area cut at 10 day intervals and published in the Annual Illinois Crop Report. Estimates for the daily progression of hay field cutting were obtained from the 10 day interval data using logistic regression (Neter et al., 1985). The time intervals between first and second cuttings of alfalfa hay fields were estimated by comparing the logistic functions from the first and second cuttings in each year, and estimating the proportion of each
Bobolink population decline in agricultural landscapes
function in which the cutting interval between corresponding cutting percentages exceeded 34 days (the estimated minimum time necessary for a bobolink whose nest was destroyed by the first cutting to raise another clutch successfully; Bollinger et al., 1990). Non-alfalfa hay fields were assumed to be cut only once during the bobolink nesting period. Based on published accounts of bobolink natural history (Bent, 1958; Erhlich et al., 1988; Bollinger et al., 1990; Martin & Gavin, 1995), birds were only allowed one successful nest attempt per year. All birds whose nests were destroyed by hay field cutting or other factors such as predation or weather ("other factors" were not differentiated in the model and were combined into a single estimated nest failure rate) were allowed a second nest attempt. Because Wootton et al. (1986) showed that the number of young fledged by bobolinks decreases as the breeding season progresses, I assumed that the average clutch size for second nest attempts was half that of first nest attempts. Clutch size for first nest attempts was set at an average of 4.5 young/nest. Based on published accounts of passerine survival rates, juveniles ( < 1 year) were assumed to survive at one-half the adult survival rate (Ricklefs, 1973). Adult survival was set near the upper limit of published accounts for adult bobolinks (60%; Martin, 1974; Wittenberger, 1978; Bollinger & Gavin, 1989). I did not have specific data on nest success rates for bobolinks in Illinois hayfields, oat fields, or pastures, so I calculated an assumed nest success rate that resulted in a stable population (sensu Henny et al., 1970) in these habitats under constant conditions (i.e. constant area and no mid-season cutting of hay fields). The nest success rate resulting from this procedure and subsequently used in the model (56.8%) was very similar to the overall nest success rate (54.5%) recently reported for bobolinks by Martin and Gavin (1995). " N a t u r a l " rates of nest failure were assumed to be constant for all habitats in the model. Although the nest success rate used in the model may not reflect actual nest success rates for Illinois bobolinks in these habitats, it does provide a good population baseline against which simulation results can be compared in the absence of actual field data. Based on patterns of bobolink densities in these habitats (Table 2), I assumed that oats, alfalfa and nonalfalfa hay fields could support comparable densities of bobolinks and that pasture could support one-half the density of oats and hay fields. The number of potential territories in each habitat was determined by dividing the total amount of habitat by the average territory size (estimated from the average densities reported in Table 2). Average territory sizes used in the model were 2 ha in hay fields and oats, and 4 ha in pasture. I assumed that the distribution of bobolinks within habitats was proportional to the annual number of potential territories in each habitat. Because bobolinks are polygynous (Bent, 1958; Wootton et al., 1986; Martin & Gavin, 1995), only females (assumed to equal one-half the total population)
109
were used in distributing birds among potential territories. Any surplus birds that were unable to find a territory, because all potential territories were occupied, remained in the population as non-breeding individuals. Based on studies of bobolinks nesting in New York hay fields (Bollinger et al., 1990), I assumed that all hay fields containing nests with eggs, and nestlings less than 13 days of age at the time of cutting, would be destroyed by hay cutting and associated factors. No adult birds were assumed to be destroyed by hay cutting. The chronology of bobolink nesting used in the model was based on the data provided by Bollinger & Gavin, 1992, but was adjusted to correspond with the known timing of bobolink nest initiation and egg laying in Illinois (Bohlen, 1989). The proportional reduction in bobolink reproduction resulting from hay cutting was determined using a modification of the function used by Bollinger et al. (1990), where the proportional reduction of bobolink nesting success as a result of first cutting of hay fields (e) was estimated as ei = Zmivi, where m i - t h e proportion of all hay fields cut on day i, and vi = the proportion of nesting bobolinks vulnerable to cutting on day i. The proportional reduction of bobolink nesting as a result of second cutting of alfalfa (r) was determined as the probability that a hay field cut on day i would not be cut on day i + 34, the estimated minimum time required between nest destruction due to hay cutting and day 13 nestlings (see Bollinger et al., 1990). In order to examine both the combined and individual influence of declines in agricultural grassland area and changes in hay field cutting timetables, I modeled the dynamics of four area/cutting scenarios. First, I modeled the bobolink population response to actual changes in the area of these habitats and actual hay field cutting timetables. Second, I used actual area figures but maintained a "1950s" hay field cutting timetable. The "1950s" hay field cutting schedule used in this simulation was determined by randomly selecting cutting schedules from actual 1952-1955 hay field cutting timetables for the full 40 year (1952-1992) simulation. Third, I maintained constant 1952 area figures and used the actual hay field cutting timetable. Fourth, I maintained a constant 1952 area and maintained a "1950s" hay field cutting schedule as described above.
Empirical data In order to assess the validity of this model, I compared changes in bobolink population numbers estimated from Illinois BBS data for the period 1968-1992 with changes in the area of these agricultural grassland habitats using product-moment correlation analysis. The BBS consists of randomly located, permanent survey routes established along secondary roads (Peterjohn & Sauer, 1993). Survey routes are 39.4 km long and consist of 50 stops at 0.8 km intervals (Peterjohn & Sauer, 1993). Each route is surveyed once annually during June and all birds seen or heard within 0.4 km of each survey point during a 3 min census are
110
J. R. Herkert 800
1-° I .
5OO•~..
-,
Ogts
g09+
Alfalfa Hay
n"
.
t.-
\
I Non-AlfalfaH. I
. - _
o
0
if'-,%.
"-.
2 200 ,
-
.........
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
,..I
::
-----'---
0.6
0: 0 •s . . . . . . . . ~ ,~- J~ / - ~ ~
-
~
~ ~ ::
:
/
-~ ~
"~
~
- ..............
I~
o0.4 -
~-
g 0.3--~
. . . . . . ~--~". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
2
IO0
a. 0.2 1950 1955 1960
0
1950 1955 1~o0 1~o5 1970 1975 1980 1~)85 1~30 1995 Year
Fig.
.
1. Area of the four agricultural grassland habitats included in the model.
recorded (Peterjohn & Sauer, 1993). In the northern Illinois study area considered here, there are 12 BBS routes. The bobolink population index was estimated using the Linear Model Method (Sauer & Geissler, 1990), with routes and years included as co-varieties. This method involves fitting a linear model to the data for the region, and then using the estimated model to calculate predicted counts for each route and year, adjusting for missing counts (see Sauer & Geissler, 1990 for a more detailed description of index estimation).
RESULTS The area of all four of these grassland habitats declined significantly between 1952 and 1992 (Fig. 1). The area of oats declined the most from 586000 ha in 1952 to 29947 ha in 1992, a reduction of 95%. Pasture showed the next greatest decline with a loss of 343000 ha ( - 7 1 % ) , followed by non-alfalfa hay which declined by 283 000 ha ( - 9 5 % ) . The area of alfalfa hay was higher in 1992 than in 1952, even though the area of alfalfa had significantly decreased in northern Illinois since reaching a peak in 1957 (Fig. 1). The median cutting date of northern Illinois alfalfa and non-alfalfa hay fields has significantly advanced over the last 40 years (F=28.8, p
