On the Advantage of Being Different: Nest Predation and the Coexistence of Bird Species Thomas E. Martin PNAS 1988;85;2196-2199 doi:1.173/pnas.85.7.2196 This information is current as of February 27. E-mail Alerts Rights & Permissions Reprints This article has been cited by other articles:.pnas.org#otherarticles Receive free email alerts hen ne articles cite this article - sign up in the box at the top right corner of the article or click here. To reproduce this article in part (figures, tables) or in entirety, see:.pnas.org/misc/rightperm.shtml To order reprints, see:.pnas.org/misc/reprints.shtml Notes:
Proc. Nati. Acad. Sci. USA Vol. 85, pp. 21%-2199, April 1988 Ecology On the advantage of being different: Nest predation and the coexistence of bird species (community structure/density-dependent predation/resource partitioning/search images) THOMAS E. MARTIN Department of Zoology, Ariona State University, Tempe, AZ 85287-151 Communicated by Jared M. Diamond, December 23, 1987 ABSTRACT A long-standing debate in ecology centers on identifying the processes that determine hich species coexist in a local community. Partitioning of resources, here species differ in resource use, is often thought to reflect the primary role of competition in determining coexistence of species. Hoever, in theory predation can favor similar patterns. This theory premises that predators increase their search intensity ith increasing density of prey. One set of experiments reported here supports this premise based on predators that search for bird nests. A second set of experiments documents that preda. tion rates are loer hen nest sites are partitioned among different sites than hen the same number of nests are placed in similar sites. Moreover, predation rates on experimental nests are more similar to rates on real nests hen experimental nests are partitioned among different sites. These results provide support for a hypothesis that nest predation is a process that can favor coexistence of bird species that partition resources, here nest sites are the resources. The publication costs of this article ere defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance ith 18 U.S.C. 1734 solely to indicate this fact. 2196 Local processes determine the number and types of species that coexist in a local'community, ithin regional and historical constraints on availability of species (1). Competition is an example of a local process. In fact, resource partitioningi.e., differences among coexisting species in their use of resources-is commonly observed and often thought to reflect the primary role of competition in determining coexistence of species (2-5). Hoever, the importance of competition has been hotly debated, creating a need to examine alternative processes (6-9). Predation is an alternative process that can affect coexistence of species by mediating competitive interactions or by eliminating species (1-14). More importantly, theory suggests that predation can also favor resource partitioning similar to competition (15-17). Thus, this theory provides an alternative explanation for a common pattern (i.e., resource partitioning). Here, I test hether nest predators exhibit the behaviors that can favor resource partitioning among coexisting bird species. Nest predation is probably an important agent of natural selection for birds because such predation commonly is a primary limit on reproductive succes's (18). Hoever, the effect of predators depends on their searching behavior (17). Predators, in searching for bird nests, must examine some potential nest sites that are not occupied by eggs (no reard). Increased reard frequency should reinforce search behavior. Consequently, both search intensity and the proportion of nests lost to predators should increase ith the frequency or density of occupied nest sites (reard rate). Moreover, predators can enhance their search efficiency by specialiing on prey types and learning search images (19-22). If predators do not discriminate among species ith similar nest sites, then the proportion of nests lost to predators ill increase for individual species ith the cumulative density of all similar species (ref. 17; also see refs. 23 and 24). Thus, such behavioral responses by the predators, if they exist, can favor coexistence of species that use different nesting sites (partitioning of resources, here nest sites are the resource) to minimie cumulative density effects (17). Here, I report experiments that support the premise that predator search intensity increases ith the frequency of occupied nest sites (reard rate). I then sho that, for a given density of nests, nest predation is reduced hen those nests are placed in sites that differ (partitioned nesting space) than hen placed in similar sites (unpartitioned space). STUDY AREA AND METHODS Study Area. Experiments ere conducted in four mixedconifer drainages in central Ariona at -23-m elevation. These drainages ere of the same vegetation type and general location as other sites on hich I have been studying nest predation (25, 26). Nest predators ere identified using cameras outfitted ith infrared light beams that trigger the camera hen the beam is broken. Ten cameras ere set up at artificial nests that ere in the same drainages as experimental nests but hich ere not included in the results presented here. Preliminary results based on 11 pictures indicated that the nest predators ere red squirrels (Tamiasciurus hudsonicus) and gray-neck chipmunks (Tamias cinereicollis). These results coincide ith my impressions on these sites from observations of birds chasing these to predators. Hoever, other predators are also on the sites [e.g., long-tailed easels (Mustelafrenata) and Steller's jays (Cyanocitta stelleri)] and undoubtedly account for some nest losses. Experiments used artificial icker nests baited ith quail (Coturnix coturnix) eggs. Artificial nests have been successfully used in other studies to examine predation rates (27, 28). Moreover, I previously tested potential biases associated ith using artificial nests in the habitats studied here and found that nests that simulate the appearance and position of real nests can elicit predation responses that reflect real trends (29). Experiment 1. This experiment tested the response of predators to reard frequency by modifying the frequency of egg-occupied nests in three treatments. In all treatments a constant number of nests (seven) as placed in small hite firs (Abies concolor) in 1-m-diameter circles (referred to as clumps). Ten clumps ere used in each treatment and placed about 25 m apart. Treatments ere separated by 1 m. One, three, and all seven nests in each clump contained eggs in treatments one, to, and three, respectively. Thus, this design used 7 nests per treatment ith 1, 3, and 7 nests containing eggs in treatments one, to, and three, respectively. Experiment 2. This experiment tested the effect of nest space partitioning on predation rates. Four species ere simulated. One nest type simulated an orange-croned arbler (Vermivora celata) and as placed in a small de-
Ecology: Martin pression in the ground under the stem of a deciduous shrub (usually big-toothed maple-acer grandidentatum) (termed ground species). The second nest type simulated a hermit thrush (Catharus guttatus); it as covered ith moss and placed about 1 m above ground in a small hite fir (termed fir species). The third type as unmodified and placed 1 m above ground in maple [termed maple (1m) species], simulating a MacGillivray's arbler (Oporornis tolmiei). The fourth type as placed 3 m above ground in maple [termed maple (3m) species] simulating a black-headed grosbeak (Pheucticus melanocephalus). Nests ere placed at =1-m intervals along to parallel transects (long continuous strips of sample area) that ere a minimum of 25 m apart; exact distance beteen transects and among nests on a transect varied depending on the availability of a suitable nest site. The same spacing as used in both of to treatments (to be described) to maintain constant density beteen treatments. Both treatments ere included in each of three spatial replicates. Treatments ere separated by 1 m in each replicate, and spatial replicates ere separated by 3-5 km. Experiments in all three spatial replicates ere initiated at the end of May hen egg-laying activity peaks. The experiments ere then repeated in late June hen many birds ere renesting. This design used 8 nests per spatial replicate (4/treatment) or 24 nests per temporal replicate, ith 48 nests used overall. Treatment one simulated a four-species assemblage (partitioned nesting sites). The four nest types ere alternated sequentially along the to transects in each replicate, using 4 nests per replicate. Treatment to simulated a singlespecies assemblage (unpartitioned sites); 2 nests of one species ere placed along the transects folloed by a 1-m buffer and then a second set of 2 nests of a different species for a total of 4 nests in this treatment. This design, hich only tested to single species per temporal replicate, as used to maintain constant sample sie (4 nests) beteen treatments and because of time and space constraints on putting out additional nests. The ground species as tested in both temporal replicates to test for any temporal biases. The other single species tested in the first and second temporal replicates ere the fir and maple (1m) species, respectively. Thus, three of the four species ere tested as single species. Statistical Tests. Predation rates on artificial nests ere measured by examining loss of eggs from nests every 3 days during 15 days of exposure to predators. Differences among treatments ere examined by comparing the percentage of nests remaining ithout having lost any eggs to predators. These data ere arcsine-transformed and analyed by one-, to-, or three-ay, repeated measures, analysis of variance (ANOVA). One-ay analyses ere used for the first experiment. To-ay analyses ere used in the second experiment for those comparisons that did not include temporal replications. Otherise, three-ay analyses ere used. To- and three-ay interactions ith treatment effects ere not significant (P >.1 in all cases). Predation rates on real nests of the four species being simulated ere measured by examining loss of eggs every 3-4 days and using the Mayfield method, hich measures the proportion of nests that are lost to predators per day (3, 31). Differences in these daily mortality rates beteen artificial and real nests ere tested ith the test (31). RESULTS Experiment 1. Predation rates increased (F - 1.7, P <.1) ith increased reard frequency hen based on the percentage of clumps remaining ithout any nests losing eggs to predators (Fig. la). Given that the number of nests at risk increased across treatments, then such results may Proc. Natl. Acad. Sci. USA 85 (1988) 2197 -J 6 9 12 DAYS EXPOSED FIG. 1. Predation rate as a function of the ratio of egg-occupied nests to total nests in a clump. The three treatments include one, three, and seven nests containing eggs in each clump. (a) Percentage of clumps remaining ithout any nests in the clump losing eggs to predators. (b) Percentage of intact nests remaining (no egg loss to predators). arguably reflect an increasing probability of random encounters of nests across treatments. Hoever, the risk to individual nests is shon by the percentage of intact nests (no egg loss) that ere remaining at each nest check; the results sho that even individual nests had a greater rate of predation (F = 6.8, P <.2) ith increases in reard frequency (Fig. lb). Experiment 2. Predation rates ere greater (F = 515.89, P <.3) in the single-species treatment across all spatial and temporal replicates (Fig. 2). The higher predation rates could ~ m Lii LA- O ~ -O Multiple species 1 *@-4 Single species ( io 1 W L1Z'~O_ L.,4-- - 4!O 2. 15 Temporal Replicate 2 3 h 6 9 12 15 3 6 9 12 1 FIG. 2. Percentage of nests that lose no eggs to predators. Each column is a temporal replicate, and each ro is a spatial replicate. The multiple species treatment represents the combined predation rate for four species; the single-species treatment combines to sets of single species in each temporal replicate.
2198 Ecology: Martin Proc. Natl. Acad. Sci. USA 85 (1988)!R A I I- 3: l:: I- C) LL EL a Ground Species 1 t --O Multiple species 81 K 61 \ 1 2 O+-~. 4 2 ol ol \X---o --. =- * t oi '1~ 3 6 9 12 15 3 6 9 12 15 b 1op- Fir Species Multiple species Maple (1 m) Species Temporal Replicate 2 -@ Single species -- --.~ *l- 1-.~ 3 6 9 12 15 3 6 9 12 15 FIG. 3. Percentage of nests that lose no eggs to predators. Each column is a temporal replicate, and each ro is a spatial replicate. (a) Data in each treatment are based only on the nests that simulated the ground species in all spatial and temporal replicates. (b) Data from both treatments are based only on the nests that simulated the fir species for the first temporal replicate and the maple (im) species for the second temporal replicate. have occurred because the species used in this treatment have containing eggs. These results complement other data shoing that predation rates on real hermit thrush nests are loer generally higher predation rates than those in the multiplespecies sample; this hypothesis is unlikely because three of hen greater numbers of hite firs (unoccupied potential the four species ere tested in the single-species treatment. nest sites) surround the nests (26). Thus, data from both real Hoever, comparison of predation rates for the individual and artificial nests support the premise that predators modify species used in both treatments allos direct examination of their search behavior in response to the frequency of occupied nests that they encounter. this potential bias. Such comparisons sho that predation rates are unequivocally greater in the single-species treatment The second set of experiments documents that predators (Fig. 3); predation rates ere higher (F = 3.6, P <.4) for can exhibit the behaviors necessary to favor partitioning of the ground-nesting species in both temporal replicates (Fig. 3a) and for the fir species (F = 52.43, P <.2) and nesting space; predation rates ere reduced hen nests maple ere (im) species (F = 18.83, P <.5) in temporal replicates 1 partitioned among different sites. These results do not seem and 2, respectively (Fig. 3b). Moreover, predation rates in the to be an abnormal response of predators to artificially multiple-species treatment ere much more similar to predation rates on real nests than in the single-species treatment species treatment ere similar to predation rates on real high nest densities given that predation rates in the multiple- (Table 1). Predation rates can differ among years (unpublished data), but such fluctuations should not influence the species treatment as compared ith single-species treatment nests (Table 1). The loer predation rates in the multiple- general pattern documented here because the results are may arise because the four nest types ere so different that based on controlled treatments. different predator species specialied on each nest type; these predators may then have simply responded to the different DISCUSSION densities of individual nest types in the to treatments. The first experiment documents that predators can increase Photographs of predators at the to nest types (fir and their searching intensity ith increasing frequency of nests ground) monitored ith cameras suggest this to be an unlikely Table 1. Mean ± variance in predation rates, measured as daily mortality rates, for the artificial nests in the multiple- and single-species treatments and for real nests of the four species simulated in the treatments Daily mortality rate, nests lost to predators per da-y Multiple-species Single-species treatment Real nests treatment Ground.75 ±.1 *.27 ±.2 (17)t **.21 ±.4 Maple (im).5 ±.1.47 ±.2 (22) **.168 ±.4 Fir.68 ±.1.81 ±.5 (19) *.154 ±.4 Maple (3m).35 ±.1.12 ±.1 (8) Predation rates on real nests are based only on nests monitored during the same season as experiments (1987) and only during the egg stage. *, P <.2 and **, P <.1 in comparisons beteen adjacent numbers. tsample sie in parentheses, hich equals the number of nests observed.
Ecology: Martin explanation; both red squirrels and chipmunks ere detected taking eggs at both nest types (unpublished data). In addition, observations suggest that these same predators also take eggs from the other to nest types (personal observation). The loer predation rate in the multiple-species treatment may instead occur because multiple prey types inhibit development of search images and thereby reduce foraging efficiency (17, 21, 22). Separation of these to possibilites ill require more photographic data and additional experiments. Nonetheless, these experimental results sho that there can indeed be an advantage to being different; nest predation is significantly reduced hen nest sites differ. The to major predator species, or their sibling species, are idespread throughout forested North America (32), providing a basis for expecting the behavior documented here to be idespread. Moreover, because nest predation is commonly the primary source of nesting failure in avian systems (18), the predation behavior documented here may represent a process that commonly favors coexistence of species that partition nesting sites. Some evidence does indeed indicate that partitioning of nesting space may be common (17). Such patterns are supported by analogous arguments and evidence that predation may favor coexistence of species that differ in appearances or escape behaviors (23, 24). Other processes clearly may be acting concurrently ith predation, and the relative importance of these processes ill vary over time and among habitats and areas. Hoever, the results documented here suggest that predation may provide an alternative explanation for some patterns of resource partitioning. Moreover, previous ork on predation has focused on the loss of independent juveniles and/or adults, even though reproductive success is an important component of fitness. This study emphasies that predation on eggs and dependent young can constitute another important level of selection. These points are underscored by demonstrating them on birds because predation effects have been ignored in birds more than any other taxonomic group (11). Indeed, studies of birds provided much of the original impetus for the long-held vie that resource partitioning induced by competition as a primary and idespread cause of species coexistence. Finally, some data sho that birds are relatively invariant in their nesting heights among geographic regions (17). This evidence indicates that nest site differences among coexisting species do not necessarily reflect local coevolution. Instead, the differences may reflect selection for coexistence of species ith nest sites that Proc. Natl. Acad. Sci. USA 85 (1988) 2199 already differ due to differences in their individual evolutionary histories (17). Such effects emphasie the importance of considering historical and regional processes in the structuring of communities (1, 17). I thank J. Connell, J. Diamond, D. Levey, M. Douglas, T. A. Marko, M. A. Neton, R. E. Ricklefs, J. J. Roper, T. W. Schoener, and D. Simberloff for helpful comments on earlier drafts. K. Donohue, M. Godin, T. McCarthey, and J. Soliday provided able assistance in putting out and monitoring the artificial nests. This ork as supported by Whitehall Foundation, Inc., and National Science Foundation (BSR-8614598). 1. Ricklefs, R. E. (1987) Science 235, 167-171. 2. Schoener, T. (1974) Science 185, 27-39. 3. Diamond, J. (1978) Am. Sci. 66, 322-331. 4. Schoener, T. (1983) Am. Nat. 122, 24-285. 5. Martin, T. (1986) Curr. Ornithol. 4, 181-21. 6. Wiens, J. (1977) Am. Sci. 65, 59-597. 7. Connell, J. (1983) Am. Nat. 122, 661-696. 8. Simberloff, D. (1983) Am. Nat. 122, 626-635. 9. Strong, D. R., Jr., Simberloff, D., Abele, L. G. & Thistle, A. B., eds. 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(1975) Evolution 29, 313-324. 25. Martin, T. (1988) Ecology 68, 74-84. 26. Martin, T. & Roper, J. (1988) Condor 9, 51-57. 27. Wilcove, D. S. (1985) Ecology 66, 1211-1214. 28. Loiselle, B. A. & Hoppes, W. G. (1983) Condor 85, 93-95. 29. Martin, T. (1987) Condor 89, 925-928. 3. Mayfield, H. (1975) Wilson Bull. 87, 456-466. 31. Hensler, G. L. & Nichols, J. D. (1981) Wilson Bull. 93, 42-53. 32. Hall, E. R. (1981) The Mammals of North America (Wiley, Ne York).