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):

Fig. 2. 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. (doi: advances.sciencemag.org/content/1/8/e1500310) Illustration: Harold Greeney et al (2015)

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.

Fig. 3. Interannual comparisons of jay foraging patterns in study plots, illustrating the effect of raptor presence on the spatial distributions of foraging jays. Each point represents average height above the ground of individual jays in a single flock. Plots with active hawk nests: gray. Upper panels show pooled data from two plots that were occupied in both years, middle panels represent four plots that were not reoccupied in 2009, and bottom panels show one plot occupied for the first time in 2009. (doi: advances.sciencemag.org/content/1/8/e1500310) Illustration: Harold Greeney et al (2015)

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):

Fig. 4. Path analysis results (model fit X2 = 3.6; df = 4; P = 0.46) for the proposed causal relationships between raptor presence, jay foraging height, and hummingbird responses. “Hawk affinity” was measured as distance to the hawk nest; “hummingbird dispersion” was a measure of dispersion determined using the nearest-neighbor value, R (larger values indicate less clumping); “hummingbird nest survival” was daily survival rate (DSR). Arrows indicate positive causal relationships; the bullet-headed line indicates a negative causal relationship. (doi: advances.sciencemag.org/content/1/8/e1500310) Illustration: Harold Greeney et al (2015)

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.”