Would you see the third eagle? A note on alternative family structures
"Cooperative or ‘communal’ breeding occurs when more than two birds of the same species provide care in rearing the young from one nest.”
According to current knowledge, only a few percent of bird species are cooperative breeders. But the number of undetected cases might be high.
Screenshot and Photomontage: maertha/Latvian WtE cam http://dabasdati.lv/lv/kameras2015
Would we notice a third parent bird in or around a White-tailed Eagle nest where we usually expect to see a pair? Experiments show that it´s even possible to overlook a gorilla
... “Cooperative groups may be overlooked in raptors (...) for several reasons:
(1) there are few raptor studies involving banded birds;
(2) many species have large territories, making it difﬁcult for observers to see all individuals residing in the territory;
(3) many raptors are sensitive to disturbance and will avoid their nest or behave abnormally when a human observer is present; and
(4) distance between territories may make it difﬁcult to monitor a large number of nests in enough detail necessary to detect cooperative behavior. In addition, because most raptors are thought to be monogamous, additional birds may be viewed by casual observers as transients and thus not recorded
.” (Kimball et al 2003)
The most creative raptor concerning family structures seems to be the Madagascar fish eagle: “35 percent of the known breeding population exhibits cooperative breeding strategies”.
(For more information see e.g.: Population status of the Madagascar Fish Eagle Haliaeetus vociferoides in 2005–2006, G. Razaﬁmanjato et al. or The Eagle Watchers/Madagascar Fish Eagle, Ruth E. Tingay.) There are also some reports of Bald Eagle (Garcelon et al. 1995) and other eagle breeding trios. Gunnar Bergo, for example, describes such a group of Golden Eagles in Norway: “All three birds in the 'trio' took part in all breeding activities, but only two were sexed with certainty.”
Occasionally the White-tailed Eagle cooperates in a similar way. As yet I found these sources:
- Scotland: “The establishment of the reintroduced population was slow at first. No eggs were produced until 1983 when one of two breeding attempts involved a trio (a pair with a second female); both clutches were damaged as a consequence. Such trios have since been observed in the Scottish population several times, and may persist for several years.” White-tailed Eagle. John A. Love in: The Birds of Scotland, 2007
Unspecified: “Some eagles breed in polyandrous trios (one female with two males); this occasional behavior has been reported for some white-tailed sea eagles and Spanish imperial eagles”. The Eagle Watchers: Observing and Conserving Raptors around the World edited by Ruth E. Tingay and Todd E. Katzner, 2010
Ireland: “White-tailed Eagles have successfully hatched chicks across four counties in Ireland. Eight pairs of White-tailed Eagles have nested and laid eggs with five nests successfully hatching chicks in counties Clare, Cork, Galway and Kerry. In the last few weeks’ chicks hatched in nests on Lough Derg at Mountshannon, Co. Clare, at Glengarriff in West Cork, and in Killarney National Park, Co. Kerry. Pairs also successfully hatched chicks at a nest in Co. Galway for the first time and at another site in Kerry. Three other pairs nesting in Kerry failed to hatch successfully. Interestingly two of these ‘pairs’ were made up of trios: two males and a single female at one site and two females and a male at another, both in Kerry!” Press release. Golden Eagle Trust, 2 June 2015. Update on the Kerry breeding trio, 20 September 2016 https://www.facebook.com/permalink.php? ... 4808950774
Update, 16 August 2017. UK/Scotland. Also interesting: “A male sea eagle has successfully raised chicks from two different nests in East Scotland for the first time ever in Scotland. The eight year old male sea eagle, known as Turquoise Z, has been travelling between Angus and Fife visiting two nests, more than 28 miles apart, and raising chicks with two different females.” Read more at: http://www.fifetoday.co.uk/news/environ ... -1-4532872
- Bergo, G. Territorial behaviour of Golden Eagles in Western Norway. Brit birds 80: 361-376, August 1987
- Drew, T., Vo, M. L., & Wolfe, J. M. (2013). The invisible gorilla strikes again: Sustained inattentional blindness in expert observers. Psychological Science, 24(9), 1848–53
- Garcelon, D. K., G. L. Slater, C. D. Danilson, and R. C. Helm. 1995. Cooperative nesting by a trio of Bald Eagles. Raptor Research 29:210-213
- Kimball R. T., Parker P. G., Bednarz J. C. 2003. Occurrence and evolution of cooperative breeding among the diurnal raptors (Accipitridae and Falconidae). Auk 120, 712-729
Edit: The third eagle in the Latvian nest in the picture above was added by me, but theoretically it can happen ... Stay vigilant and question everything ;-).