Predation of artificial nests placed in invasive vs. native shrubs and treesTodd Tracy, Jesse Nieuwenhuis, Jackie Knibbe

Northwestern College, Orange City, IowaABSTRACT

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.

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).

Egg "survival" in nests placed in invasive and native shrubs/trees

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Buckthorn Honeysuckle Native species

Nest placement

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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.

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.

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.

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).

Buckthorn Honeysuckle Native shrub (Species)3 days 2 2 (Wild plum)

3 8 2 (Apple sp.)

4 8 2 (Wild plum)

4 8 4 (Wild plum)

4 8 4 (Chokecherry)

8 8 (Juneberry)

8 8 (White ash)

8 (Hackberry)

8 (Chokecherry)

20 (Apple sp.)

DMR = 0.206 DMR = 0.147 DMR = 0.136

Figure 1. Experimental nest in a honeysuckle shrub.

INTRODUCTIONAn “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.

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.

RESULTSWe 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).

DISCUSSIONSchmidt 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.

REFERENCESFagan, 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.