Transcript Predation of artificial nests placed in native vs. invasive shrubs

Predation of artificial nests placed in invasive vs. native shrubs and trees
Todd Tracy, Jesse Nieuwenhuis, Jackie Knibbe
Northwestern College, Orange City, Iowa
ABSTRACT
This study is part of a larger project exploring the effects of an invasive introduced
species, European buckthorn (Rhamnus cathartica), on forest communities in
northwestern Iowa. In July 2007, we placed artificial nests, each containing a Coturnix
quail egg and a clay egg, in European buckthorn, invasive bush honeysuckle (Lonicera),
and native shrubs and saplings at Oak Grove County Park near Hawarden, Iowa. Nests
were subsequently checked for removal of the quail egg or damage to either egg by
predators. No quail eggs were broken or removed by predators during the study, but all
but one clay egg and some quail eggs showed scratches and peck marks by day 19.
Using egg tampering as a measure of nest survival, we found no difference in nest
survival among the three treatment groups, suggesting that buckthorn and honeysuckle do
not negatively impact late-season nesting success of forest birds in this area.
RESULTS
We found no significant differences in nest survival among the three groups of
shrubs (Fig. 3). A higher proportion of nests survived through day 7 in the
honeysuckle treatment (4/5) than in either the buckthorn treatment (2/7) or native
shrub treatment (5/10; Table 1), but our small sample sizes preclude performing a
Chi-square goodness of fit test on the data. In contrast to our 2006 early-season
study, in which 21 of 22 nests were depredated within 24 hours of the onset of the
experiment, we found in our current study that all nests survived the first day, and
only 4 of 22 nests were depredated by day 2 of the experiment (Table 1). Also
unlike our 2006 study, in which all depredated nests had their quail eggs removed
by predators, no quail eggs were removed nor even breached by predators in this
experiment. Nevertheless, quail eggs did exhibit scratches as evidence of
tampering (Fig. 2), and associated with such scratches were scratches, gouges,
and peck-marks on the accompanying clay eggs (Figs. 2 & 4). Calculated daily
nest mortality rates (DMR) ranged from 0.136 to 0.206 (Table 1).
Figure 1. Experimental nest in a honeysuckle shrub.
METHODS
This study was performed from July 17 through August 6, 2007, in a 75-acre
upland forest at Oak Grove County Park near Hawarden, Iowa. Twenty-two
woven craft nests were wired to branches in buckthorn (n=7), bush
honeysuckle (n=5; Fig. 1), and native shrubs and trees (American plum,
hackberry, Juneberry, chokecherry, white ash, and apple; n=10). Nests were
positioned from 1 to 2 meters above the ground and were placed a minimum
of 20 meters apart within the forest. During initial placement and subsequent
nest checks, we avoided leaving dead-end scent trails (which can lead
ground predators straight to nests) by walking away from the nest in a
different direction from which we approached. A Coturnix quail egg and a
wired-down clay egg were placed in each nest, and nests were checked
using a hand-held mirror (Fig. 2) on days 1, 2, 3, 7, and 19 for the removal or
damage of either egg. Because of inclement weather and the fact that the
study site was so far from Orange City, we were unable to check nests on a
daily basis after day 3 and therefore use conservative estimates of nest
survival by treating the nests as being depredated on the day following the
last day eggs were known to be unmolested. Daily nest mortality rates were
calculated by dividing the number of nest failures by the total number of days
the nests survived. An ANOVA was performed using Microsoft Excel
comparing nest survival duration in buckthorn, honeysuckle, and native
trees/shrubs.
Figure 2. A hand-held mirror was used to check the status of eggs in higher nests.
The date of earliest evidence of egg tampering was recorded for each nest, and all
but one nest showed tampering by day 19.
Egg "survival" in nests placed in invasive and
native shrubs/trees
14
Number of days until egg
tampering
INTRODUCTION
An “invasive species” is a rapidly spreading species introduced, either
directly or indirectly, intentionally or unintentionally, into a new geographic
area by humans, with negative ecological and/or economic
consequences. Because the new environment lacks the interspecific
interactions (i.e., competition and predation) that kept their populations
controlled in their native areas, invasive species tend to ‘take over’ their new
area, supplanting native species and, in many cases, causing the extirpation
or extinction of these natives. Despite the fact that invasive species are one
of the largest and most easily preventable causes of the loss of biodiversity
on our planet, most Americans have never heard of them and are completely
unaware of their existence, except perhaps for the dandelions in their lawn.
Unfortunately, as we in our ignorance aid the spread of invasives, their
expansion across the globe accelerates, wreaking ecological havoc
everywhere from the alpine tundra of Colorado to the icy terrain of Antarctica.
The European or common buckthorn (Rhamnus cathartica) and glossy
buckthorn (Rhamnus frangula) are shrubs/trees initially introduced to the
U.S. from Europe in the 1800’s for ornamental purposes. Both species are
considered to be invasive and have been shown to alter soil properties
(Heneghan et al. 2002, Heneghan et al. 2006), outcompete native saplings
for light and other resources (Fagan & Peart 2004), and possibly even poison
other plant species via purported allelopathic capabilities (Vincent 2006). In
their Illinois study comparing songbird nest predation in European buckthorn,
amur (bush) honeysuckle (Lonicera mackii), and native shrubs, Schmidt and
Whelan (1999) concluded that robins nesting in both buckthorn and
honeysuckle “experienced higher nest predation than nests built in
comparable native shrubs,” although their data appear to support their
conclusion only for honeysuckle.
In this study, we used artificial nests, each containing a Coturnix quail
egg and a wired-down clay egg, to test our hypothesis that late-season nests
built in buckthorn and honeysuckle shrubs would experience increased
predation compared to nests built in native shrubs and trees. We had
performed a similar early-season study in Northwestern College’s 5-acre
forest near Alton, Iowa, in mid-April 2006 and found that the quail eggs in all
but one of 22 artificial nests were pilfered by predators within the first day of
the study. Since other studies (e.g., Maier & DeGraaf 2000) have shown that
predators such as chickadees and mice that are too small to breach quail
eggs will depredate small passerine eggs, we used clay eggs to detect
predation attempts that might otherwise have gone undetected if we had
used only quail eggs. We also used the wired-down clay eggs in an attempt
to identify predators that either breached or completely removed the quail
eggs.
Figure 4. Examples of damage to eggs observed in our study. Scratches on quail egg suggest
a depredation attempt by a small mammal, while the small peck marks on the clay egg (see
arrow) suggest a depredation attempt by a small bird such as a black-capped chickadee or
house wren. The deep gouge and hole in the clay egg suggest a depredation attempt by a
larger bird. No quail eggs were breached or removed during the study.
12
12
10
8
7
5
F = 0.46
p = 0.64
6
4
2
0
Buckthorn
Honeysuckle
Native species
Nest placement
Figure 3. Egg “survival” time (+/- 1 SD) in nests placed in European buckthorn, bush honeysuckle, and native
trees and shrubs. Sample sizes are indicated above bars. No differences were found among the three
treatments (results of ANOVA are shown).
Buckthorn
Honeysuckle
Native shrub (Species)
3 days
3
4
4
4
8
8
2
8
8
8
8
DMR = 0.206
DMR = 0.147
2 (Wild plum)
2 (Apple sp.)
2 (Wild plum)
4 (Wild plum)
4 (Chokecherry)
8 (Juneberry)
8 (White ash)
8 (Hackberry)
8 (Chokecherry)
20 (Apple sp.)
DMR = 0.136
Table 1. Number of days of egg “survival” for each nest. Values greater than 3 days were conservatively
estimated based on status of the eggs on days 3, 7, and 19. DMR = daily nest mortality rate = (# of failed
nests)/(total # of days all nests survived).
DISCUSSION
Schmidt and Whelan (1999) conclude from their study of the nesting success of
robins and wood thrushes that both avian species “experienced higher nest predation
[in European buckthorn and Amur honeysuckle] than nests built in comparable native
shrubs. However, they also state in their results that “nests built in Rhamnus did not
have higher DMRs than natives for either species [of bird].” Given the lack of clarity
in their argument and the tenuousness of their conclusion about buckthorn’s effect on
native bird species, we decided to investigate the effect of buckthorn and
honeysuckle by using artificial nests containing quail and clay eggs. While an earlier
attempt to perform a similar study resulted in all but one nest being depredated within
24 hours, all nests in our current study survived until at least the second day of the
study, and one nest survived the full 19 days of the experiment. We found no
significant differences in nest survival among the three treatment shrub types.
DMRs in the Schmidt & Whelan study ranged from ~0.015 for native shrubs to
~0.06 for robins nesting in bush honeysuckle. Our DMR values were all considerably
higher than those of Schmidt and Whelan, suggesting that our nests may have been
more easily found by predators than nests built by birds themselves. This is certainly
likely, as surely a bird would be better able to find a secure place for a nest than
would a human, and indeed, the nests themselves would be less conspicuous than
our craft nests. Furthermore, a real nest would be defended by the parents and
would be camouflaged by the cryptic plumage of the parent incubating the eggs.
Unlike our 2006 study, all of our nests survived until at least day 2 of the study, and
the majority survived at least a week, so we do not believe that scent trails or other
inadvertent cues were used by predators, or at least not to the same extent as in our
previous study. The difference between the findings of our two experiments may
simply relate to the fact that most trees and shrubs have not leafed out by mid-April,
and thus eggs in that study were in plain sight of avian predators, while in our current
study, many of our nests would not have been directly visible from above. Another
explanation for the difference in nest survival between our study and that of Schmidt
and Whelan is that our nests were considered to have failed as soon as either egg
showed evidence of tampering (because a small predator might have been able to
breach a smaller egg), while Schmidt and Whelan considered a nest to have survived
if a single fledgling was produced, a scenario that would not preclude earlier attempts
by predators to breach and/or steal eggs from the nest.
Schmidt and Whelan conclude in their study that robins are selectively using
honeysuckle shrubs for nests despite the greater likelihood of nest failure in
honeysuckle. In our current study we found no evidence of any birds nesting in any
of the shrubs used, and indeed, we do not know whether birds would even choose to
nest in buckthorn and honeysuckle in our forest. While it would be inadvisable to
generalize from our present findings, it would be safe to conclude from our data that
we found no evidence to support our hypothesis that late-season nests built in
buckthorn and honeysuckle shrubs would experience increased predation compared
to nests built in native shrubs and trees. Furthermore, despite Schmidt and Whelan’s
conclusion that birds nesting in buckthorn experience increased nest predation, we
would argue that such a phenomenon has never been experimentally demonstrated.
REFERENCES
Fagan, M. and Peart, D. 2004. Impact of the invasive shrub glossy buckthorn (Rhamnus frangula
L.) on juvenile recruitment by canopy trees. Forest ecology and management 194: 95-107.
Heneghan, L., Clay, C., and Brundage, C. 2002. Rapid decomposition of buckthorn litter may
change soil nutrient levels. Ecological Restoration 20(2):108-111.
Heneghan, L., Fatemi, F., Umek, L., Grady, K., Fagen, K., and Workman, W. 2006. The invasive
shrub European buckthorn (Rhamnus cathartica, L.) alters soil properties in Midwestern U.S.
woodlands. Applied Soil Ecology 32: 142-148.
Maier, T., and R. DeGraaf. 2000. Rhodamine-injected eggs to photographically identify small
nest-predators. J. of Filed Ornith. 7:694-701.
Schmidt, K., and Whelan, C. 1999 Effects of exotic Lonicera and Rhamnus on songbird nest
predation. Conservation Biology 13: 1502-1506.
Vincent, M. 2006. Allelopathic effects of the fruit of European buckthorn, Rhamnus cathartica.
Honours thesis. Department of Biology, University of Winnipeg.
This research was funded by a Northwestern College scholarship grant and by a grant from the Iowa Science Foundation, administered by the Iowa Academy of Science.