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Haemig PD  (2011)  Hawks, Owls and Falcons that Protect
Nesting Birds. ECOLOGY.INFO 3

Hawks, Owls and Falcons that Protect Nesting Birds

Note: This online review is updated and revised continuously, as soon as results of new scientific research become available.  It therefore presents state-of-the-art information on the topic it covers.

In most nature books, hawks, owls and falcons are portrayed as the enemies of birds. This simplistic notion arises from the fact that many of these raptors kill and eat birds. The real world, however, is never as simple as books. Under certain circumstances, hawks, owls and falcons actually protect birds and increase their survival. This happens, for example, when certain birds build their nests close to the nests of hawks, owls or falcons, and these raptors chase away predators from their own nests, thereby incidentally chasing predators away from the other birds’ nests as well (Haemig 2001).  

In the present article, I give many examples of birds nesting with hawks, owls or falcons, and review several studies which show that these birds actually increase the survival of their own eggs and young when they nest with raptors.  In presenting this information to the reader, I group the various studies by the geographic area they were conducted in:  Japan, Europe, North America and the arctic (extreme northern parts of Europe, Asia and North America).  I was not able to include all geographic parts of the world, because detailed studies have been conducted in only a few areas.  However, I predict that this phenomena occurs worldwide, with many more species of raptors and associated birds than is presently known.

Japan

In central Honshu, Japan, Uchida (1986) found active nests of Tree Sparrows (Passer montanus) within a distance of several meters from 4 of 11 nests of the Grey-faced Buzzard-eagle (Butastur indicus),  and near 4 of 6 nests of the Honey Buzzard (Pernis apivorus).  Uchida also studied 10 nests of the Japanese Sparrowhawk (Accipiter gularis), and found active nests of the Azure-winged Magpie (Cyanopica cyana) within 10 meters of 5 of the sparrowhawk nests. However, Uchida found no songbirds nesting near or close to 19 nests of the Goshawk (Accipiter gentilis) that he visited.

In the suburbs of Tokyo, Ueta (1994ab, 1998, 1999, 2001) investigated in more detail the nesting association of Azure-winged Magpies with Japanese Sparrowhawks, and gathered convincing evidence that sparrowhawks protect magpies. In habitats where sparrowhawks lived, over 50% of magpie nests were constructed within 20 meters of a sparrowhawk nest, and over 75% were constructed within 40 meters of one. The predation rate on magpie nests increased as the distance from a sparrowhawk nest increased. For example, only 2% of magpie nests located within 20 meters of a sparrowhawk nest were lost to predators, while 50% of those located 40-60 meters, and 75% of those located 80-100 meters, were lost to predators.  If the sparrowhawks abandoned their nests, the predation rate on the magpie nests associated with them increased (Ueta 1994b).

Ueta also discovered some changes in behavior when azure-winged magpies bred in association with sparrowhawks.  Magpies that nested near sparrowhawk nests synchronized their own nesting schedule to that of the hawks, while magpies breeding away from sparrowhawk nests did not do so (Ueta 2001).  Magpies nesting without sparrowhawks usually defended their nests from predators, while those nesting with sparrowhawks usually did not, and because of this were able to feed their nestlings at a higher rate (Ueta 1999).  Finally, magpie nests built within 100 meters of a sparrowhawk nest were less concealed (had fewer leaves covering them) than magpie nests built further away.

Europe

In Spain, Blanco and Tella (1997) studied the nesting of Choughs (Pyrrhocorax pyrrhocorax) in association with Lesser Kestrels (Falco naumanni), and found that Chough nests built within kestrel colonies were over twice as successful as those built outside (most Chough nests that failed outside kestrel colonies were lost to predators).  Blanco and Tella also conducted an experiment where they placed a stuffed eagle owl (Bubo bubo) 20 meters from buildings that housed the nests of kestrels and Choughs to compare their reactions to a shared predator.  The kestrels detected the predator in all 19 trials, while the Choughs did so in only 2. Yet, even in those latter 2 trials, the Choughs took longer to detect the predator than did the kestrels.

In several European countries, the Woodpigeon (Columba palumbus) is reported to nest in association with the European Hobby (Falco subbuteo) (Collar 1978; Bijlsma 1984; Bogliani et al. 1992, 1999).  At poplar plantations in northern Italy, Bogliani et al. (1999) conducted an experiment where they set out imitation Woodpigeon nests (with quail eggs) at varying distances from hobby nests to see which would be found by predators.  After 6 days, survival of eggs in these dummy nests was checked.  During the time when the hobbies incubated their eggs, and when their chicks were less than 15 days old, the survival percentage rate of eggs in the imitation Woodpigeon nests was the same at all distances from the hobby nests.  However, after the hobby chicks were 15 days old, the intensity of nest defense by the parent hobbies increased to the point where it reduced predation on imitation Woodpigeon nests located near the hobby nests.  At that time, only 20 % of nests located less than 50 meters from hobby nests lost eggs to predators, while 70% of nests located greater than 100 meters from the hobby nests lost eggs.  Within the 50 meter zone surrounding the hobby nests, predation on the imitation Woodpigeon nests decreased the closer they were placed to the hobby nests.

Bogliani and his coworkers also examined the nesting of real Woodpigeons around hobby nests.  Up to 6 active pigeon nests were found around each active hobby nest. Pigeons nesting far from hobby nests concealed their nests in foliage, while pigeons nesting near the hobby nests did not conceal their nests.  Over two-thirds of the pigeons laid their eggs when the hobbies were incubating theirs, apparently synchronizing the nestling period of their own young with the period when the falcons most vigorously defended their nests.

Although these results show that hobbies reduced predation on Woodpigeon nests, the relationship was not always positive for the Woodpigeons.  Bogliani and his coworkers collected prey remains under the hobby nests they studied and found that Woodpigeons represented 15% of the total birds captured and consumed by the hobbies (measured by total weight).  These researchers concluded that pigeons nesting near hobbies were vulnerable to predation by the falcons, but that the benefits of enhanced nesting success outweighed the risks because other nest predators were very abundant in the area.

In Finland, the Curlew (Numenius arquata) nests in association with the Kestrel (Falco tinnunculus) (Norrdahl et al. 1995).   While the Kestrel is not large enough to prey upon adult Curlews, it does prey upon Curlew chicks (and many other small animals).   Thus, it exacts a price for its protection of the Curlews.  In a study area in the farmlands of Ostrobothnia, Finland, Norrdahl et al. (1995) found that Kestrels fed on 5.5% of the Curlew chicks hatched, while other nest predators destroyed 9% of the Curlew nests.  Thus, these researchers concluded that the Curlews were better off nesting under Kestrel protection than alone.  To further study predation on Curlew nests and protection of them by Kestrels, Norrdahl et al. (1995) constructed artificial Curlew nests (with brown hen eggs) and placed them at various intervals along 1200 meter transects originating from Kestrel nests.  They found that egg predation was higher further away from the Kestrel nests than near them.

Finish reading this review on Page 2.

The photograph at the top of the page shows a male Snowy Owl and was taken in Quebec, Canada by David Dräyer (Switzerland).  This owl protects nesting geese on the tundra from wolves and arctic foxes (see Page 2 for details).

 

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