Table Of ContentWilson Bull., 115(3), 2003, pp. 270-276
ARE RESULTS OE ARTIFICIAL NEST EXPERIMENTS A VALID
INDICATOR OF SUCCESS OF NATURAL NESTS?
EDUARDO T MEZQUIDA* ^^ AND LUIS MARONE^^
—
ABSTRACT. Studies using artificial nests to assess rates orpatterns ofnest predation have increasedduring
recent decades. However, the critical assumption that temporal or spatial patterns ofrelative predation rates of
artificial nests parallel those of natural nests has begun to be questioned. We compared the daily survival rate
for 365 artificial nests with that estimated for 295 open nests ofpasserines, and analyzed the trends in survival
rates for both nest types among three breeding seasons and three species of nest plants in the central Monte
Desert, Argentina. Daily survival rate for artificial nests (0.737) was significantly lower than that for natural
nests (0.925). Trends in survival ratesofartificial nestsoveryears andamong speciesofnestplantsonlypartially
reflected those of natural nests. Our results support the well established finding that artificial nests may not
accurately estimate actual rates of nest predation. Artificial nests may provide an additional source of data to
natural nests when testing ecological hypotheses, but artificial nest experiments should be carefully designed
(i.e., realistic) and should attempt to identify predators ofartificial and natural nests to validate theexperimental
results. Received 24 October 2002, accepted 10 March 2003.
The main cause of nest mortality in most 1998, Sloan et al. 1998, Wilson et al. 1998,
bird species is predation (Ricklefs 1969, Ro- Zanette 2002) or more intense predation on
tenberry and Wiens 1989, Martin 1993, Mez- natural nests (Chamberlain et al. 1995, Davi-
quida and Marone 2001). However, it often is son and Bollinger 2000). However, this is not
difficult to obtain the appropriate number of necessarily a problem for comparative studies
natural nests to assess different factors influ- on artificial nests. Despite the fact that abso-
encing rates of nest predation (Reitsma et al. lute predation rates may differbetween natural
1990). For this reason, the use of artificial and artificial nests, inferences derived from ar-
nests to estimate nest predation rate has in- tificial nests can be valid if relative predation
creased during the last few decades (reviewed rates follow a similar and consistent trend in
in Major and Kendal 1996, Sieving and Will- both types of nests. This assertion, commonly
son 1998, Schmidt and Whelan 1999). Major used to justify the artificial approach, usually
and Kendal (1996) summarized the diverse, has not been tested and the results at hand are
and sometimes nonrigorous, held techniques conflicting. In some studies, rates ofpredation
used in these types of experiments that at for both nest types did not follow the same
times make analysis of pattern consistency pattern (Willebrand and Marcstrom 1988,
difficult. Macivor et al. 1990, Buler and Hamilton
Absolute rates of predation on artificial 2000), while trends were similar for artificial
nests commonly have been found to differ and natural nests in other studies (George
from those of natural nests. Some authors 1987, Ammon and Stacey 1997, Wilson et al.
have found similar predation rates in artificial 1998). Moreover, when a series of treatments
and natural nests (see references in Major and was considered in the study (e.g., types of ar-
Kendal 1996), but most studies have shown tificial nests, spatial and temporal variations),
either lower predation pressure for natural the relationship between the success of artifi-
compared to artificial nests (Bechet et al. cial and real nests usually differed among sub-
sets ofdata (Butler and Rotella 1998, Davison
' Grupo de Investigacion en Ecologia de Comuni- and Bollinger 2000, Weidinger 2001a, Zanette
dades de Desierto (Ecodes), UF&EV, lADIZA, CC 2002).
507, 5500 Mendoza, Argentina. In this study, we examined whether preda-
-Center for Advanced Studies in Ecology and Bio- tion on artificial nests reflects that of natural
diversity, Santiago, Chile. nests of passerines in an area of southern
Sta^tCCeouUrrnrrieevns.tp,oanLddadisrneCgsrsua:cuetDhseo,prt;N,eM-omfa8i8Bl0i:0o3l-o8g(y)(,)lN,eUwSAM.exico pSroeudtahtiAomneroifcaa.rtiWfieciaclomapnadrenaatbusroallutneesrtast.esWoef
[email protected] also compare trends in predation rates for ar-
270
8
Mezc/uicia and Marone • ARTIFICIAL NEST EXPERIMENTS 271
TABLE 1. Nests of 15 species of open-nesting passerines from Nacunan made up the sample of natural
nests used for survival rate calculations.
Numberof
Species nests
Ringed Warbling-Finch, Poospiza torquata 95
Gray-crowned Tyrannulet, Serpophaga griseiceps 74
Cinnamon Warbling-Finch, Poospiza ornata 53
Greater Wagtail-Tyrant, Stigmatura budytoides
1
Many-colored Chaco-Finch, Saltatricula multicolor 16
Crowned Slaty-Flycatcher, Empidonomus aurantioatrocristatus 12
White-banded Mockingbird, Mimus triurus
1 1
Black-crowned Monjita, Xolmis coronata 4
Scrub Flycatcher, Suhlegatus modestus 4
Golden-billed Saltator, Saltatoraurantiirostris 2
Yellow-billed Tit-Tyrant, Anairetesflavirostris 2
Fork-tailed Flycatcher, Tyrannus savana 1
Rufous-collared Sparrow, Zonotrichia capensis
1
Vermilion Flycatcher, Pyrocephalus ruhinus 1
White-tipped Plantcutter, Phytotoma rutila 1
Total 295
tificial and natural nests both temporally opossum (Thylamys pusilla, Didelphidae) and
(across three breeding seasons) and spatially the little grison (Galictis cuja, Mustelidae)
(across three species of nest plants), and dis- also have been confirmed as nest predators
cuss the validity of using trends observed in (Mezquida and Marone 2002). Other potential
artificial nests to infer trends in natural nests. predators include the white-eared opossum
(Didelphis albiventris, Didelphidae), Molina’s
METHODS
hog-nosed skunk {Conepatus chinga, Mustel-
We conducted this study in the Biosphere idae), South American grey fox {Dusicyon
Reserve of Nacunan (34° 03' S, 67° 54' W), griseus, Canidae)—, and small snakes.
Mendoza province, Argentina. The reserve is Natural nests. During spring and summer
located in an alluvial plain ofthe central Mon- (October to March) from 1996-1997 to 1998-
te Desert and comprises an open algarrobo 1999 (i.e., three breeding seasons), we located
(Prosopis flexuosa) woodland with scattered open nests ofpasserines by systematic search-
chanar trees (Geoffroea decorticans), abun- es in the vegetation and by following adult
dant tall shrubs >1 m tall (Larrea divaricata, birds (Martin and Geupel 1993). Most nests
Capparis atamisquea, Atriplex lampa), low were placed in chanar and zampa {Atriplex
shrubs <1 m tall (mainly Lyciurn spp.), and lampa), two plant species widely used for
perennial grasses (Pappophorum spp., Trich- nesting in this area (Mezquida and Marone
loris crinita, Setaria leucopila, Digitaria cal- 2001). All nests included in our sample ofnat-
ifornica, Aristida mendocina). Annual forb ural nests (see Table 1) were found in the
cover is highly variable from year to year and shrub-midstory (<1.5 m) or canopy-subcano-
usually lower than grass cover. The climate of py (>1.5 m) strata. Nests placed in these two
Nacunan is dry and temperate, with cold win- strata showed similar survival rates (Mezqui-
ters. Most precipitation (78% of annual rain- da 2000). Each nest was visited every 1—3
fall) occurs during the spring and summer days until it either failed or fledged young.
months (October to March; Marone et al. Nests that fledged at least one young were
2000 considered successful. A nest was considered
).
Birds such as the Chimango Caracara {Md- depredated when all the eggs or young nest-
vago chimango) and Brown Cacholote {Pseu- lings disappeared (see Mezquida and Marone
doseisura lophotes) appear to be the most im- 2001 for further details).
—
portant nest predators in the reserve, although Artificial nests. Concurrent with our stud-
some mammals such as the small fat-tailed ies of natural nests, we conducted artificial
272 THE WILSON BULLETIN • Vol. 115, No. 3, September 2003
nest experiments. In the open woodland hab- by predation and using this, we calculated dai-
itat, we established two or three divergent ly predation rate (m^). Daily survival rate to
transects (>200 m apart at origin) along predation (Sp) was equal to 1 — nip. We esti-
scarcely used tracks inside the reserve. On mated total exposure days as the interval be-
each transect, we tied small pieces of colored tween the day when the nest was found active
tape to vegetation, alternating on the right and (i.e., with eggs or nestlings) and the fledging
left side of the track, at 40-50 m intervals. ofthe young or halfway between the two sub-
One nest was placed more than 20 m from sequent visits between which a nest failed. We
each mark in a perpendicular direction to the counted total exposure days for artificial nests
transect. To aid in locating nests again, we tied in a similar way. Nest mortality (mainly due
a new mark to the vegetation, in most (>80%) to predation) for natural nests was very high
cases >10 m from the nest, following sug- during our study (usually <80%; Mezquida
gestions by Major and Kendal (1996). Artih- and Marone 2001). Therefore, to increase
cial nests were open cup natural nests col- sample size and the confidence of survival
lected during the previous breeding season, rate estimates, we calculated the daily survival
mainly Ringed Warbling-Finch (Poospiza tor- rate of natural nests during the egg and nest-
quata) and Cinnamon Warbling-Finch {P. or- ling period as a whole, because daily survival
natd) nests. All of the nests we used in the rates did not differ significantly between the
experiments were small cups made of grasses two stages (Mezquida and Marone 2001). We
with an external diameter of 6-7 cm and an used the Hensler and Nichols (1981) test to
external height of5-6 cm. We placed the nests test for differences in daily survival rate be-
in three plant species, chahar, jarilla (Larrea tween artificial and natural nests. We com-
spp.), and zampa, trying to imitate natural po- pared daily survival rates for artificial and nat-
sitions. Range of nest heights in each plant ural nests among years and nest plants using
were 1-2 m (chafiar), 0.5-1.0 m (jarilla), and methods described by Aebischer (1999). We
0.5-1.0 m (zampa). Each nest was baited with tested the goodness-of-fit of different models
two fresh quail {Coturnix sp.) eggs. Quail for each two-way comparison (i.e., nest type
eggs are bigger than those of the local pas- X year and nest type X nest plant) and used
serines. However, the use of these eggs in our likelihood-ratio tests for comparisons between
experiments does not appear to have biased models.
our results (see Discussion). Total exposure
RESULTS
period was 12 days for each experiment, sim-
ilar to the mean incubation period of the pas- We distributed 365 artificial nests in the
serine assembly breeding in the area (Mez- open woodland, 353 of which were depredat-
quida and Marone 2001). We checked the ed during 1,344 exposure days. The sample of
nests every two days taking care to approach natural nests included 295 nests of 15 bird
the nests from different directions to avoid species, although most (95%) nests were of
creating obvious paths for potential predators. the seven most numerous species (with more
When we detected the presence of a potential than 10 nests each; Table 1). During a total of
predator, we did not approach the nest until 2,628.5 exposure days, 197 natural nests
the predator moved away. Artihcial nests with failed due to predation. The resulting daily
both eggs missing or damaged were classihed survival rates were significantly lower for ar-
as depredated. Therefore, nests with one or tificial nests (daily survival probability ± as-
both eggs undamaged at the end of the ex- sociated variance = 0.737 ± 1.4 X lO”'^) than
periment were considered successful. Artifi- for natural nests (0.925 ± 2.6 X 10“^; Hensler
cial nests and eggs were removed when the and Nichols test, Z = 14.4, P < 0.0001).
nest was considered depredated or after the The full model that included the main ef-
exposure int—erval if survival occurred. fects ofnest type, year, and the interaction be-
Analyses. We calculated the daily survival tween them (deviance = 567.5, df = 654, P
rate of natural and artihcial nests using the = 0.99) fitted well. The comparison ofthe full
Mayheld method (Mayheld 1975), and its as- model with a model with main effects only
sociated variance (Johnson 1979). For natural (deviance = 1378.3, df = 656) showed that
nests, we included only nest mortality caused there was a significant interaction between
Mezqiiida and Marone • ARTIFICIAL NEST EXPERIMENTS 273
Artificial Natural Artificial Natural
1996 1997 1998 Chanar Jarilla Zampa
FIG. 1. Daily survival rates ofartificial nests were FIG. 2. Daily survival rates ofartificial nests were
always lower than those of natural nests during the consistently lowerthanthoseofnatural nestsinallnest
three years studied,butwasespecially lowforartificial plants, but the relative rate forartificial nests placed in
nests during 1998. Daily survival rates, 95% confi- zampawashigherthan thatofnatural nests.Thus,both
dence ranges (Hensler and Nichols 1981) and number nests types showed nonparallel trends. Bars represent
ofnests are shown for artificial and natural nests dur- the daily survival rate and the 95% confidence range
ingthreebreeding seasons (austral springandsummer) of each nest type in three plant species at Nacunan,
in the Reserve ofNacunan, Argentina. Argentina. Sample sizes are indicated for each group.
vival rates ofnatural nests in chanar and zam-
nest type and year (D = 810.8, df = 2, P < pa were lower than that in jarilla (Fig. 2).
0.0001). A model that included the main ef-
fect ofnest type (deviance = 610.0, df = 658, DISCUSSION
P = 0.91) showed a good fit, whereas a model Our results, in agreement with other find-
with just temporal effects was rejected (devi- ings, showed that artificial nests sufferedhigh-
ance = 768.2, df = 657, P = 0.002). Thus, er predation pressure than natural nests (Sloan
the daily survival rate of artificial nests was et al. 1998, Wilson et al. 1998, Zanette and
consistently lower compared to natural nests, Jenkins 2000). However, this is not surprising
but was particularly low during 1998, causing because most studies comparing absolute rates
an interaction between nest type and year of predation between artificial and natural
(Fig. 1). nests have found differences. Many factors
Similarly, a full two-way model with inter- might explain the discrepancy in the proba-
actions to examine the effect of nest type and bility of every nest type being predated (re-
species of nest plant on daily survival rates viewed in Major and Kendal 1996). Although
showed a satisfactory fit (deviance = 535.6, studies designed for one or a small number of
df = 624, P = 1.00). The likelihood-ratio test bird species could better simulate natural con-
that compared the full model with a two-way ditions, our artificial nests were reasonably re-
model without interactions (deviance = alistic. We did not use artificial structures
1306.9, df = 626) was significant (D = 771.4, (e.g., wire baskets, tennis balls) to simulate
df - 2, P < 0.0001), indicating a significant nests, but natural nests in order to avoid arti-
nest type X nest plant interaction. A model ficial biases (Martin 1987). Further, we placed
=
specifying just nest type effects (deviance the nests in plants and positions based on pre-
569.5, df = 628, P = 0.95) showed a good vious knowledge ofthe nesting ecology oflo-
fit, whereas a model with just nest plant ef- cal passerines in the study area.
fects (737.4, df - 627, P = 0.002) fitted poor- In contrast, the use of quail eggs in the ar-
ly. The daily survival rates were lower for ar- tificial nests mightjustify some criticism sug-
tificial nests in all nest plants, but increased gesting that our predation rate estimates are
from chanar to zampa, whereas the daily sur- biased. The size of quail eggs (mean length:
274 THE WILSON BULLETIN • Vol. 115, No. 3, September 2003
32.7 mm, n = 60) was larger than the mean we used the same general area to place arti-
egg size for bird species included in the sam- ficial nests in successive years, probably fa-
ple ofnatural nests (19.9 mm, n = 15 species; cilitating predators to learn to detect our prof-
Mezquida 2000), which could have increased fered nests.
the detectability of eggs in artificial nests. Another comparison showed that the sur-
However, some small-mouthed nest predators vival rate of artificial and natural nests dif-
are unable to break quail eggs (Roper 1992, fered depending upon the species ofplant that
Haskell 1995), and others do not appear to supported the nest, but patterns were not fully
consume this type of egg (Marini and Melo similar for both nest types. The survival rate
1998). Hence, the use of commercially avail- of artificial nests was lowest in chanar and
able eggs might under- as well as overestimate highest in zampa. Chanares are small trees
predation rate on artificial nests. The results with small and dispersed leaves, while zampas
of specific experiments conducted at the Re- are medium-sized and compact shrubs with
serve ofNacunan suggested that this potential abundant evergreen leaves. Thus, the pattern
bias was a minor one; the predation rate for of predation on artificial nests suggests that
artificial nests baited with eggs that more ac- the degree of nest concealment might have
curately resembled eggs ofthe local passerine played a role. Moreover, birds appear to be
species was similar to the predation rate for major nest predators at our study area (Mez-
nests baited with quail eggs (Quse 2001). The quida and Marone 2002), and these visual
pattern of egg coloration also differed be- predators probably were very efficient at de-
tween quail and natural eggs, although previ- tecting our artificial nests, mainly those placed
ous studies indicate that egg color does not in chanar that were higher and less concealed.
seem to affect predation rate (Jobin and Pie- Other studies also have suggested that the de-
man 1997, Weidinger 2001b). gree ofconcealment is an important factorthat
Even though results of experiments using affects success of nests unattended by parents
artificial nests do not generally offer realistic (Gotmark et al. 1995, Major and Kendal 1996,
indicators of actual nest predation, this might Cresswell 1997). Natural nests also suffered
not invalidate their use if patterns or trends high predation pressure, but rates for nests
(i.e., relative rates of predation) among treat- built in chanar and zampa were similar. In
ments are similar in both nature and experi- contrast to artificial nests, the relative inci-
ments. However, studies comparing the suc- dence of terrestrial predators (i.e., mammals
cess of artificial and natural nests in different and reptiles) on natural nests in zampa pre-
treatments (e.g., habitats, patches, years) have sumably would be higher, due to the presence
obtained conflicting results, as in the present of parents and the activity at the nests (Dav-
study. Our results show that patterns of pre- ison and Bollinger 2000).
dation of artificial nests across years and spe- In conclusion, our results for a location of
cies of nest plants did not closely parallel central-western Argentina add new evidence
those found for natural nests. In our first com- to the well established pattern that artificial
parison, the survival rate of artificial nests nests are not always a trustworthy indicator of
during the first two breeding seasons followed predation rates on natural nests. Whether ab-
the same trend as in natural nests, but the pat- solute rates of predation on artificial nests are
tern differed during the next breeding season. higher or lower than those on natural nests
The decreased survival rate of artificial nests probably depends upon characteristics of the
overtime might be a consequence ofpredators artificial setup (e.g., nest site and position,
learning to find artificial eggs (Major and nest type) and the kind of predators involved.
Kendal 1996, Vander Haegen and DeGraaf Our study also showed that temporal and spa-
1996). For example, Buler and Hamilton tial trends in daily survival rates of artificial
(2()()0) also observed an increase in predation nests did not closely correspond with those of
of artificial nests between two consecutive natural nests. Differences suggest behavioral
years, contrary to predation of natural nests, responses by predators and that, at least in
and suggested the potential for learning by some situations, both types of nests may not
predators (mainly crows). In Nacunan, we did attract the same predators. We recommend
not use the same transects for later trials, but that future studies using artificial nests should
Mezciliida and Marone • ARTIFICIAL NEwST EXPERIMENTS 275
be realistic (i.e., based on the nesting ecology land vs. mainland: relation to nest predation level.
of the target species) and concurrent with a Ecology 68:1393-1400.
study on natural nests, and should attempt to Gotmark, E, D. Blomqvist, O. C. Johans.son, and J.
Bergkvkst. 1995. Nest site selection: a trade-off
identify predators of both nest types to vali-
between concealment and view of the surround-
date the experimental results. In addition to ings? J. Avian Biol. 26:305-312.
natural nest experiments, carefully designed Haskell, D. G. 1995. Forest fragmentation and nest
artificial nest experiments may provide an in- predation: are experiments with Japanese Quail
dependent source of data (i.e., a measure of eggs misleading? Auk 1 12:767-770.
potential predation pressure) that help explain Hensler, G. L. andj. D. Nichols. 1981.The Mayfield
how avian species may be adjusting to differ- method of estimating nesting success: a model,
estimatorsand simulation results. Wilson Bull. 93:
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the field work. Some students aided in the collection MacIvor, L. H., S. M. Melvin, and C. R. Griffin.
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