Hummingbird eggs and babies are a favourite snack for nest-robbing jays, so what’s a mother to do to protect her family? According to a new study, it’s best to build her nest near or under a hawk nest
Tiny hummingbird eggs and babies are a favourite snack for nest-robbing jays, so what’s a mother hummingbird to do to protect her family? According to a study published recently in the journal, Science Advances, the hummingbird cleverly builds her nest near or under a hawk nest. The reason for this seemingly risky behaviour? When hawks are nesting nearby, jays forage higher above the ground to avoid being attacked from above by the hungry hawk parents. This elevation in the jays’ foraging height creates a cone-shaped jay-free safe area under the hawk nests where mother hummingbirds, their babies and nests, enjoy dramatically increased survival rates.
The enemy of my enemy is my friend
Predator-prey relationships are subtle and multi-faceted. For example, predators at one level in a food web are often are prey at the next level. The position in a food web or food chain that an organism occupies is referred to as its trophic level. Predictably, prey animals tend to avoid their predators, even when doing so restricts the prey species’ access to valuable resources, such as food, water or habitat (doi:10.1007/s10531–008–9324–5). Thus, the presence of just a few top predators has numerous effects — direct and indirect — that cascade down the trophic levels within an ecosystem. This is known as a trophic cascade.
Direct trophic effects are obvious — predators kill prey. But indirect trophic effects, where one species changes another species in some way that benefits a third species, are difficult to detect and to document. These changes include a suite of predator avoidance traits or adaptations, including behavioural, physiological, and anatomical traits that enhance fitness (doi:10.1016/S0169–5347(02)02578–8). Such indirect trophic effects are known as a “trait-mediated trophic cascade”.
A rather famous (and controversial) natural experiment was conducted when the grey wolf, Canis lupus, was restored to Yellowstone National Park in the United States in 1995 and 1996 after a 70-year absence. This restoration appeared to trigger a trait-mediated trophic cascade. According to modern-day legend, not only did the presence of these apex predators change the behaviours of the ungulates in the area, but after wolves were re-introduced, the overall variety of native plants and animals increased — and even the course of the rivers flowing through Yellowstone were affected. This particular scenario makes intuitive sense and thus, it is intellectually satisfying — until scientists looked more closely at the Yellowstone system, and realised they’d gotten it wrong.
Despite this setback, ecologists are reasonably sure that indirect trophic effects can and do alter an ecosystem, although such effects have almost never been rigorously documented.
Mexican jays are notorious robbers and destroyers of hummingbird nests
In the Chiricahua Mountains of southeastern Arizona, Mexican jays, Aphelocoma wollweberi, are midlevel predators — mesopredators — that particularly enjoy feasting on hummingbird eggs and nestlings, as you can see in this video:
The black-chinned hummingbird, Archilochus alexandri, the species whose nest was ransacked in the above video, barely weighs as much as an American penny. Thus, she can only helplessly watch when a jay, which is 40 times heavier, destroys her nest and eats her eggs or chicks. But when a pair of goshawks, Accipiter gentilis, or Cooper’s hawks, A. cooperii, moves in to the neighbourhood and builds a nest, this dire situation dramatically improves for the hummingbirds.
This is because these hawks, which are five times larger than Mexican jays, swoop down from the treetops to catch and feast upon them. But the hummingbirds are too small and too agile to make even a worthwhile snack, so the hawks ignore them. The hawks’ fondness for eating jays creates a cone-shaped “safe zone” for the hummingbirds below their nests (Figure 2):
This arrangement quickly became obvious to natural historian Harold Greeney, a postdoctoral research associate in the School of Natural Resources at the University of Arizona and founder of the Yanayacu Biological Station and Center for Creative Studies. Dr Greeney and his colleagues mapped out the spatial patterns of nest placement and documented the daily survival rates for 342 hummingbird nests in relation to the location and status (six active versus six inactive) of 12 hawk nests during three nesting seasons. The researchers also documented the three-dimensional foraging patterns of the jays in relation to the hawk nests.
Jay foraging patterns changed when hawks were present
The team found that the hawk nest, which can be 20 metres off the ground, serves as the pinnacle of a cone-shaped “jay-free space” (Figure 2) that Mexican jays rarely enter when the hawk nest is active (Figure 3). This is because the hawks sit in the treetops and launch either a horizontal or descending pursuit of their prey. Thus, to remain safe from attack, the jays must remain at least as high above the ground as the hawks.
The most successful hummingbird nests were closer to hawk nests
Of 342 hummingbird nests that Dr Greeney and his colleagues monitored, 80 percent were near hawk nests. Dr Greeney and his team documented the survival rates for eggs and fledglings and found that hummingbird nests located near inactive hawk nests lost all but 8 percent of their young, whilst those near an active hawk nest enjoyed a whopping 70 percent survival rate.
To test the nuances of this finding, the authors used a path analysis (Figure 4) to test the subtle relationships between hawk presence, jay foraging, and hummingbird nest survival. They found that the presence of nesting hawks has an indirect effect of increasing the nesting success of nearby hummingbirds because of shifts in nest-robbing behaviour of the local jays, which sought to avoid being preyed upon by the hungry hawk parents (Figure 4):
But how are the hummingbirds choosing their nest sites?
“[W]e are not sure how the hummingbirds are choosing sites, mostly because they are too small to mark them individually so we don’t know much about individual choices”, said Dr Greeney in email.
Did the hummingbirds re-use successful nest sites? Or were they purposefully seeking out active hawk nests so they could nest nearby?
“[M]y feeling is they are faithful to successful sites and this leads to a ‘buildup’ of females around hawks where more nests are successful”, explained Dr Greeney. “[I]f they are using cues, it might be the presence of other hummingbirds, or maybe the vocalizations of the hawks.”
“Although these birds likely live for a long time (15 years or more?) I think that they are too clumped for the spatial preference to be learned”, said evolutionary ornithologist Richard Prum, the William Robertson Coe Professor of Ornithology in the Department of Ecology and Evolutionary Biology at Yale University and Curator of Ornithology in the Yale Peabody Museum of Natural History, who was not part of this study.
“I would hypothesize that they have a preference to nest near hawk nests and a preference to return to successful nest sites”, said Professor Prum. “Of course, the dynamics would be aided by some social copying behavior. Young inexperienced birds would benefit from setting up near the competition if older birds know enough to set up near a hawk nest.”
Discovering natural history in our own backyards
“Great work based in awesome natural history”, said Professor Prum. “One really has to know these birds to accomplish work like this!”
“Wow — very nice”, agreed ornithologist James Van Remsen, the McIlhenny Distinguished Professor of Natural Science and Curator of Birds at the Museum of Natural Science at Louisiana State University, who was not part of the study.
“[T]hey really have worked out the system in terms of quantification of the distribution of hummer nests with respect to hawk nests, the effect of the hawks on jay movements, the risk to hummers of jay predation on their nests, and most importantly, the quantitative benefit to hummer repro[ductive] success with respect to decreased predation and increased daily survival rates of nests”, said Professor Remsen.
Professor Remsen pointed out that sharp-shinned hawks, which pose a very real threat to hummingbirds, are so similar in appearance to Cooper’s hawks, which are generally too large to prey on hummingbirds, that even experienced birders can have a difficult time separating them. Yet, the female hummingbirds easily distinguish these two species and somehow “know” which one poses no threat to their tiny families.
“How do they do that???” said Professor Remsen.
As a birder and ornithologist, I was impressed (and a wee bit amazed) by the sheer number of hummingbird nests that Dr Greeney found during the three breeding seasons when this study took place.
“Finding nests is what I do”, said Dr Greeney. He went on to explain that finding nests mostly requires patience and passion, although “practice doesn’t hurt.”
“I work mostly on neotropical birds, but I have found and described the first ever discovered nest for more than 50 species”, said Dr Greeney. Clearly, nest hunting is a passion for him.
Of course, it’s easier to gain a deeper knowledge of natural history when we actually know what we are looking for, and at.
“The value of basic natural history research, something that has been sadly devalued in the past century, is not at all diminished and there are still a myriad of wonderful stories to be told, even if from our own backyards”, said Dr Greeney.
In fact, one does not have to travel to remote field sites in exotic places to make a new discovery.
“I have been living and working in one of the most diverse spots on Earth, the eastern slope of the Andes in Ecuador, for almost 20 years”, said Dr Greeney. “And where do I find such an incredible story? In Arizona while visiting my parents.”
Further studies are being planned.
“A cool experimental test would be to follow a newly established hawk nest, or to establish new nest sites with platforms or nest boxes”, said Professor Prum.
“Yes, good followup studies would be to add and remove cues to see if you can ‘trick’ hummingbirds to nesting [near the cues]”, said Dr Greeney. “[T]hese cues could include empty nests, stuffed hawks, hawk vocalizations, or any combination of the above.”
Harold F. Greeney, M. Rocio Meneses, Chris E. Hamilton, Eli Lichter-Marck, R. William Mannan, Noel Snyder, Helen Snyder, Susan M. Wethington, and Lee A. Dyer (2015), Trait-mediated trophic cascade creates enemy-free space for nesting hummingbirds, Science Advances, published online on 4 September 2015 ahead of print | doi:advances.sciencemag.org/content/1/8/e1500310
Wolfgang Schleidt, Michael D. Shalter, and Humberto Moura-Neto (2011). The Hawk/Goose Story: The Classical Ethological Experiments of Lorenz and Tinbergen, Revisited, Journal of Comparative Psychology, 125(2):121–133 | doi:10.1037/a0022068 [£]
Som B. Ale and Christopher J. Whelan (2008). Reappraisal of the role of big, fierce predators! Biodiversity and Conservation 17:685–690 | doi:10.1007/s10531–008–9324–5 [£]
Robert E. Ricklefs and Martin Wikelski (2002). The physiology/life-history nexus, Trends in Ecology & Evolution 17(10):462–468 | doi:10.1016/S0169–5347(02)02578–8 [£]
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