Theoretically ‘trait-mediated trophic cascades’ occur when the behaviour of top predators affects the behaviour of smaller predators, and therefore the species upon which these predators feed. This is essentially what is seen in ‘Tom & Jerry’ cartoons, with the Mouse, Jerry, able to escape the Cat, Tom, by hiding in the kennel of the Dog, Butch. However such behaviour is notoriously hard to detect in wild populations, not least because Human intervention has removed top predators from many environments. One group that theoretically ought to be particularly prone to such effects are Birds, whose nesting success is strongly reduced by the presence of predators, and who are therefore extremely sensitive to such factors.
In a paper published in the journal Science Advances on 4 September 2015, a group of scientists led by Harold Greeney of the Yanayacu Biological Station & Center for Creative Studies, the Department of Biology at the University of Nevada, Reno and the Department of Natural Resources, University of Arizona, discuss the results of a study into the behaviour of Black-chinned Hummingbirds, Archilochus alexandri, Mexican Jays, Amphelocoma wollweberi, Northern Goshawks, Accipiter gentilis and Cooper’s Hawks, Accipiter cooperii, around the Southwest Research Station in the Chiricahua Mountains in Cochise County in southeastern Arizona.
The study area comprised open woodland, with stands of tall trees, predominantly Juniper, Juniperus deppeana, Oak, Quercus spp., and Sycamore, Platanus wrightii, reaching between five and thirty meters in height, surrounded by open grassland. This grassland is maintained by grazing, but is dotted with saplings of the larger trees as well as other types of shrubs and bushes.
Mexican Jays are a major predator of Hummingbird nests, but are themselves prey to both forms of Hawk. The Hawks preferred method of attack is to wait on a high perch for prey then fall onto it from above in a rapidly descending pursuit. Attacks in which the Hawks chased the Jays from the same or lower heights were far less successful. Neither Hawk species showed any interest in the much smaller Hummingbirds.
A Mexican Jay removing the eggs from the nest (middle-lower right) of a Black-chinned Hummingbird. Greeney et al. (2015).
All tall trees are theoretically hazardous to the Jays, but trees with a Hawk’s nest (both species prefer to nest high in tall trees) presented an obvious source of danger. Greeney et al. found that the Jays avoided a conical zone beneath such nests, presumably to avoid predation. Hummingbird nests within these conical zones were therefore far less likely to be predated, and the Hummingbirds appeared to nest in clusters beneath the Hawk nests (80% of Hummingbird nests were found within the conical zones), though whether they responded to the presence of the Hawks or simply returned each year to sights they had found to be safe (Hawk nests are re-used for many years) was unclear.
Stylized graphical model of cone-shaped space surrounding active Hawk nests, within which Hummingbird nests had significantly higher survivorship. Data on the locations of Jays in relation to each plot’s Hawk nest were pooled across plots and were used to generate the shape of the cone, using the lowest individual jays detected during the study and superimposed on a fictional landscape representative of the study area. Yellow, Hawk nest; green, successful Hummingbird nest; red, depredated Hummingbird nest. Greeney et al. (2015).
Hummingbird nests within the cones were far more successful than nests outside the zones, fledging a significantly higher number of chicks. Four Hawk nests were abandoned during the study, and the success rate of the Hummingbird nests within the zones beneath these abandoned nests fell to almost zero, suggesting that the improved fledging rate was indeed due to the presence of the Hawks.
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