Using Artificial Nests to Study Nest Predation in BIRDS

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Using Artificial Nests To Study Nest Predation IN BIRDS

JAMES R. BELTHOFF

est predationoccurs when either eggs or Why Study Predation on nestlings within nests of birds are destroyedor con- sumed by predators.Because of the loss of their off- Artificial Bird Nests? spring, such behaviorreduces the reproductivesuc- cess of the parent birds, and their lifetime fitness Studies with artificialbird nests are useful for declines. Untold numbers of bird nests are depre- illustratingbasic ecological principles,such as how dated each year, and ecologists and wildlife man- predation occurs differently in different habitats agersare interestedin the factorsthat affectthe like- (Rudnicky& Hunter,1993; Leimgruber,McShea & lihood of nest predationor success. Rappole, 1994), effects of land use changes such as those resulting from habitat fragmentation(Bayne & Hobson, 1997) on predation,and the effects of distance from edge and edge type on predation(i.e., James R. Belthoffis Professorof Biology at Boise State the so-called"edge effects," where predationincreas- University, Boise, Idaho 83725; e-mail: ibeltho@boise es near ecotones [Paton, 1994]). The coevolution state.edu. between predatorsand their prey also is of interest

NESTPREDATION INBIRDS 105 to biologists and can be examined with artificial nests. This coevolution sets up an "arms race" between the predatorsand their prey, IN as naturalselection increases the abilitywith which predators detect and captureprey, but selection also improves the prey's capacity to avoid detection (Krebs & Davies, 1993). Because many organ- isms prey on bird nests, including reptiles (e.g., snakes), mammals (ranging in size from small mice to largercarnivores), and many Downloaded from http://online.ucpress.edu/abt/article-pdf/67/2/105/52223/4451794.pdf by guest on 26 September 2021 types of birds, there is strong selection on nesting birds to place nests in areas that will elude detection (such as areas with greater cover, in cavities in trees), to distrib- ute nests in spaced rather than clumped distributions, or to cryptically color their eggs. Each of these factors can be examined in studies using artificialbird nests (see Major& Kendal, 1996). In this article,I describe Figure1. simple and effective field Artificialnest containing three eggs made of clay designed to resemblethat of a Brewer'sSparrow exercises that demonstrate duringa study of nest predation. factors affecting predation on bird nests. With instruc- tor guidance, students in stores or their wholesale suppliers, such as high school biology or college-levelbiology, ecology, Howell's Home and Floral (www.howellson- animal behavior,wildlife managementor ornithology line.com; Portland,OR) and resemble those of laboratorycourses can collaborateto design field exper- many small passerinebirds (Figure 1). iments related to nest depredation.Hypotheses about * nest location (ground, shrub, tree branch, cavity) or Plasticinemodeling clay for molding eggs (Van amount of cover,egg number or color, habitat type, or Aken International, Rancho Cucamonga, CA; naturalvs. disturbed environmentscan be formulated multiple colors are available from local craft and examined. I have used these exercises in introduc- stores) in -0.45 kg (1 lb) packages. For small tory animalbehavior and conservationlaboratories but, passerineeggs (e.g., 1-2 cm in length), each pack- with little or no modification,they are suitable for an age makes approximately200-400 eggs. arrayof other courses within the biologicalsciences. * 26-gaugewire for securingnests to substrate * 50 m or 100 m measuring tapes for laying out Materials transectsor measuringheights or other aspects of artificialnests The following materials will be needed for this exercise: * Small plastic vials for transporting depredated eggs to laboratory for follow-up analyses of * Artificialbird nests to resemble those of avian beak and tooth impressions left by predators species in your area. Nests 7-10 cm (3-4 in) in (optional) diameter are commonly available from craft

106 THEAMERICAN BIOLOGY TEACHER, VOLUME 67,NO. 2, FEBRUARY2005 * Dissecting (IOX) microscope for examining desired size. Generally,it is best to secure the artificial tooth and beak markson clay eggs (optional) nests to the particularsubstrate (shrub, tree branch, ground) so wind or the predators themselves do not * Pin flags or plastic flaggingtape to markthe gen- dislodge nests. We use 26-gauge which takes eral location of artificialnests deployed (option- wire, only a moment to weave through the nest material and al). In general, these markersshould be placed around the branchof a shrub or tree. some distancefrom the nests themselvesto avoid cuing predatorsto the nests. * Vinyl gloves for making and deploying nests to Some Factors & Questions of eliminatehuman scent contamination Interest

Experimental Design Effects of Different Colored Eggs Egg Construction Do white eggs experience more predation than eggs of differentcolors, including colors that are Eggs of appropriatesize and shape aremolded from more cryptic? the modeling clay by hand (Figure 1). Vinyl gloves are Downloaded from http://online.ucpress.edu/abt/article-pdf/67/2/105/52223/4451794.pdf by guest on 26 September 2021 used to help prevent human scent from being trans- Effects of Egg Number ferredto eggs, as human scent could alter rates of predation. A guide, such as Baicich& Harrison(1997), is Do nests with only one or a few eggs attractfewer helpful for providingdescriptions of eggs for common predatorsand end up successfulmore than nests species in your area. with more eggs?

Nest Deployment Effects of Nest Height Artificialnests can be deployed along linear tran- Do nests closer to or on the ground experience sects, such as every 10 m along a 100 m transect,or they greaterpredation than those higher in the shrub can be distributed within replicated study plots of or tree canopy?

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NESTPREDATION INBIRDS 107 The color of clay available for making eggs

What To Let Them Determine (With Your ~ ~ Supervision) so~~~4, -~ What are the hypotheses (null and alternate) being examined? Birdbeak marks How many nests to deploy Mammalteeth marks and in what configuration

How many eggs to place in Downloaded from http://online.ucpress.edu/abt/article-pdf/67/2/105/52223/4451794.pdf by guest on 26 September 2021 Figure2. each nest Twoclay eggs that have been disturbed by predators.The egg onthe lefthas teeth marks char- acteristicof a mammaliannest predator, while the egg onthe righthas impressions left by the The criteriaused to deter- beakof a bird.Both types of marksare characteristic of nests that students would categorize as mine whether a nest is suc- depredated. cessful or failed General Effects of Nest Distribution Table1. Procedure Resultsof artificialnest study to test the null hypothesis that Do nests in Our laboratory periods nestsuccess and predation are independent ofnest position clumped distribu- are three hours in duration, tions experience (onthe ground or in the canopy of a sagebrushshrub).Values and this exercise generally greater predation representthe number ofnests in each category. can be completed in two peri- than those that are ods. During the first period, randomlyspaced? NEST FATE students discuss and formu- late specific hypotheses to be Effects of Edge -_ -_m examined and then develop Ground 15 28 the experimentaldesign to be Do nests closer to used, which takes 30-40 min- Shrub 8 36 the edge of a habitat utes. We then visit the field patch experience site, which in our case is greater nredation- about 20 minutes from cam- perhapsbecause of edge effects? pus, to deploy nests. The travel time is also used for modeling the clay eggs, although I usually solicit a few Effects of Predator Type student volunteers to arriveat class approximatelyan Arebirds, mammals,or other groups more likely hour early to help me get this started. Students work to prey on ground nests or nests within other in groups of three to deploy the nests: One student treatmentgroups? This can be assessed through records the data (nest number and location, nest follow-upexamination of marksin clay eggs from height, color or number of eggs, and other factors spe- depredatednests. cific to the hypothesis being examined), another flags the nest locations so that nests can be relocated, and the third secures the nests and places the eggs. Spring What To Tell the Students is the best time for this exercise, as birds are building and tending nests, and presumably their predators How many nests are availablefor their group are accustomed to finding nests at this time, but we The basic approachto deploying nests, such as have conducted the exercise in the fall with success as how to place the eggs within them and how to well. After one week, we inspect the nests for signs secure nests to substrate of predation.

108 THEAMERICAN BIOLOGY TEACHER, VOLUME 67, NO.2, FEBRUARY2005 Collection of Data Students relocateand retrieveeach of the nests and score them as either successful or failed.Although students may discuss and establishvarious criteria, we con- Meet... sider intactnests, where all eggs arepresent and show no signs of disturbanceby potential predators,to be successful. Failednests are those in which eggs exhibit teeth or beak marks or in which eggs have been eitherpartial- ly or completely removed. Tooth and beak marks are sometimes helpful for determiningthe type of predator that visited the nest (Figure 2). We use small, plastic vials to store markedeggs for examinationunder a dis- s~~~' secting microscope in the laboratory,which sometimes .~~~ helps furtherdecipher predatortype. Some researchers

deploy automated cameras at nests to more accurately Downloaded from http://online.ucpress.edu/abt/article-pdf/67/2/105/52223/4451794.pdf by guest on 26 September 2021 determine the identity of predators,but that is likely beyond the scope of a laboratoryexercise for introducto- Jan Leach, Ph.D. ry classes. * Professor,Colorado State University * Molecular biologist who studies Results plant-microbe interactions *Fellow of The American Nest Position Phytopathological Society *Institute Adjunct Scientist, We recentlyobtained the followingresults concern- International Rice Research Institute ing depredationof artificialnests in relationto nest posi- *Grateful student of elementary teacher tion in a sagebrush(Artemisia tridentata) steppe habitat Frieda Mason who encouraged her in which artificialnests were placed on the ground (i.e., interest in science at the base of a shrub or near other suitablecover) or in the canopy of the shrub (at approximatelythe height of each student'sthigh). Nests were 7.6 cm in diameterand Science Teachers Make contained 3 eggs modeled from light green clay (Figure A Difference. 1) to resemblethose of Brewer'sSparrows (Spizella brew- help eri),which is a common sagebrushnesting species in our Enrich your biology classes and area.Students deployed 87 nests along 100 m transects inspire the next generation of plant at 10 m intervals,of which 64 (73.6%) were unsuccess- health scientists. Everything you need, ful because of predation,and 23 (26.4%) werejudged as from lesson plans to laboratory experiments, successful (Table 1). Evidenceof bird (beak marks) and is available free of charge at our website. small mammal (teeth and claw marks) predation was detected from impressions in eggs similar to those in Visit www.apsnet.org/education. Figure2.

Data Analysis We used Chi-Squarefor 2 x 2 contingencytable to ask whether fate (successful or failed) is independent of the particulartreatment factor. More advanced classes may use logistic regression,generalized linear models, or APS" Healthy HealthyWorld Cox Regression (survival analysis) analysis to examine Plants.* nest fate in relationto multiple factorsof interestor any potentialinteractions among them. For the data in Table 1, there was no differencein predationbetween shrub nests and ground nests (contingencyanalysis: X2 = 3.12, p = 0.077).

NESTPREDATION INBIRDS 109 Clumped vs. Spaced Distribution actual eggs increase the realism of the experiment, recent research indicates that quail eggs may underesti- A second study examined the proportion of nests mate predation by small mammals such as deer mice depredated when clumped vs. scattered within 100 m x (Peromyscus maniculatus), because the shells of quail 100 m plots within sagebrush steppe habitat. Clumped eggs are larger and thicker than eggs of many passerine nests were placed within 5 m of a neighbor whereas scat- birds (see Roper, 1992; Bayne & Hobson, 1999). tered nests were greater than 20 m from the nearest neigh- Nonetheless, quail eggs offer a viable alternative for the bor. Each plot (n = 16) contained 10 ground nests. We cal- experiments described, and they can be obtained from culated the percentage of nests in each plot that were a variety of poultry farms (e.g., http://www.north depredated, and then compared average rate for plots in west-gamebirds.com/eggs.htm). Alternatively, brown both treatments using a Wilcoxon ranked sums test (Zar, chicken eggs can be used to resemble larger gallina- 1999). In this case, the null hypothesis of no difference in ceous birds that nest on the ground, such as grouse or proportion of nests depredated was rejected, as the mean pheasants. proportion of failed nests was greater for clumped nests (0.82 ? 0.16 [SD]) than in nests spaced further apart (0.51 ? 0.08 SD; Wilcoxon test, z = 3.20, P = 0.001). Acknowledgments Downloaded from http://online.ucpress.edu/abt/article-pdf/67/2/105/52223/4451794.pdf by guest on 26 September 2021 I thank Chris Canon, Catherine Rideout, and Rex Discussion of Experimental Sallabanks for assistance in obtaining materials for these studies and Ian Robertson for suggesting this arti- Results & Educational Value cle. Two anonymous reviewers also provided comments Results of artificial nest experiments often show that that improved the manuscript. predation rate varies as a function of nest placement, habitat type, land use, and other factors. Such results act References as a springboard for discussion of how birds may better conceal their nests, how the search paths of predators Baicich, PJ. & Harrison, CJ.O. (1997). A Guide to Nests, Eggs, may differentially encounter clumped vs. scattered nests, and Nestlings of North American Birds, 2nd Edition. New or how egg color might increase or decrease the visibili- York:Academic Press. ty of nests to We also discuss student predators. opin- Bayne, E.M. & Hobson, K.A. (1997). Comparing the effects of ions about how realistically the artificial nests estimate landscape fragmentation by forestry and agriculture on actual predation rates. These discussions often revolve predation of artificial nests. Conservation Biology, 11, around the fact that actual nests would have live, adult 1418-1429 birds to (1) defend the nests against predators which Bayne, E.M. & Hobson, K.A. (1999). Do clay eggs attract pred- may reduce predation, or (2) cue predators to the where- ators to artificial nests? Journal of Field Ornithology, 70, abouts of nests, which may simultaneously increase 1-7. rates. We also discuss how well the students "hid" the nests (i.e., how much cover there was) and if they believe Krebs,J.R. & Davies, N.B. 1993. An Introductionto Behavioural this was a factor in their chances of being depredated. Ecology,3rd Edition. Oxford, United Kingdom: Blackwell Scientific. The exercises are suitable for all levels of classes Leimgruber, P., McShea, WJ. & Rappole, J.H. (1994). because offer the to make them as com- they flexibility Predation on artificial nests in large forest blocks. Journal plex as necessary. Beginning classes in biology, such as of WildlifeManagement, 58, 254-260. in high schools, can examine one factor's effect on predation rate, such as nest position or nest distribution, as Major, R.E. & Kendal, C.E. (1996). The contribution of artifi- illustrated above. More advanced classes, such as upper cial nest experiments to understanding avian reproduc- tive success: a review of methods and conclusions. Ibis, level college biology classes, can discuss multiple 138, 298-307. hypotheses, issues related to experimental design, and more complex data analyses. In either case, my experi- Paton, P.W.C.(1994). The effect of edge on avian nest success: ence has been that students are excited to construct the how strong is the evidence? Conservation Biology, 8, eggs from clay, deploy the nests, and eagerly examine 17-26. their fate upon return trips to the study area. Roper, Jj. (1992). Nest predation experiments with quail eggs: too much to swallow? Oikos, 65, 528-530. Additional Follow-up Exercises & Rudnicky, T.C. & Hunter, M.L. (1993). Avian nest predation in clearcuts, forests, and edges in a forest-dominated land- Questions scape. Journal of Wildlife Management,57, 358-364. Students can discuss the pros and cons of using Zar, J.H. (1999). Biostatistical Analysis, 4th Edition. Saddle quail eggs rather than clay eggs. For example, while River, NewJersey: Prentice Hall.

110 THEAMERICAN BIOLOGY TEACHER, VOLUME 67, NO.2,FEBRUARY 2005