Pair-living

Canarian Egyptian vultures, like all other vulture species, form lifelong pair-bonds. Although they do not display strict, year-round, pair-living habits, pair bonds are maintained throughout the year, most commonly through reciprocal allopreening. Especially outside the breeding season, these allopreening sessions can be remarkable elaborate, sometimes taking the form of mutual feather maintenance involving preening of tail and wing feather (in addition to neck and head feathers), usually while the receiving partner is sitting.

The tight pair bonds of Canarian Egyptian Vulture are further illustrated by joint activities such as day-time resting (i.e., so-called contact-sits, see photos above) and regular records of pairs mud bathing together (photos below, left panel).

A unique feature of Egyptian vultures are the performance of joint flights, whereby both members fly in close distance of each other, usually at less than a few meter distance. This strong synchronization of activities may in part be driven by males trying to secure paternity, however, it likely also fulfills an important role in pair-bond maintenance. For example, based on movement data from a pair with both members carrying GPS-trackers, it appears that these joint flights are made on a daily basis from the start of the breeding season until egg-laying (i.e., from early December to late February), but also during the chick-rearing phase, although at a much lower frequency, and occasionally also outside the breeding season. The tight partnerships of the species are remarkable and suggest strong cooperative relationships during reproduction. In this line, male and female Egyptian Vultures share most of their reproductive task. An interesting form of cooperation among pair members was witnessed on August 2016. During observations on social interactions at the central feeding station, we noticed an adult female walking around restlessly with a piece of meat in her beak for more than ten minutes. After she found her partner, she handed over the food (see video still above), after which the male flew away immediately, probably to feed their two fully grown nestlings (i.e., 90-100 days old). This behaviour was noticed twice that day, and raises intriguing questions about the cooperative capacities of Canarian Egyptian Vultures

Reproductive prospecting

Prospecting movements performed by unpaired birds during the breeding season are among the many fascinating habits of Canarian Egyptian Vultures. Prospecting forms a key component of the strong seasonal socio-spatial dynamics characterizing this population. During the non-breeding season, individuals tend to aggregate; many activities are then confined to the centre of Fuerteventura (Figures below panels b). However, during the breeding season, breeders move to their territories (left figure below, panel a), and unpaired birds engage extensively in prospecting (right figure below, panel a). Their movements often cover the entire island of Fuerteventura, and even regularly include flights to the nearby island of Lanzarote.

Since the spatial extent of these prospecting activities typically peaks around egg-laying (see figures below), we believe that these flights partly serve to keep track of potential territory vacancies. For example, as shown below in panel b, a female recruits into a vacant territory in the middle of the breeding season (April/May). We observed similar phenomena after poisoning events, when sudden vacancies may be filled up within a day after the territory owners died; these anecdotal observations suggest that existing territories are precious and preferred resources. Explorative movements often also continue after egg-laying, possibly because this allows vultures to collect further information about the quality of territories (and their owners). Importantly, vultures start explorative activities at the age of 3 (panel a below), which is well before they acquire sexual maturity (4-5 years). Explorative flights thus also allow young birds to gain knowledge about reproductive environments.

Reproductive Trios

The Canarian Egyptian Study population is unique given the high incidence of polyandrous (one female and two males) and polygynous trios (one male and two females). Same-nest polygamy is rare in birds, but seems relatively common in vultures, especially in Bearded Vulture, but possibly also in Cinereous Vulture and California Condor.

The causes and consequences of these alternative reproductive strategies are, however, poorly understood. In Canarian Egyptian Vulture, there appears to be no direct link between the formation of trios and overall population breeding density indicating that ‘outsiders’ joining an existing pair are not necessarily constrained from independent breeding. However, trios are typical formed in older, productive territories, located in the center of Fuerteventura (13 out of 15 trios; 87%), suggesting an important underlying role of territory and/or individual quality in trio formation.

Preliminary analyses revealed that polyandrous and polygynous trios last on average two years. Intriguingly, however, in some cases polygenous trios turn into long-term ‘coalitions’ (ranging between 5-9 years). These trios have a higher annual reproductive success compared to the population mean (0.9 vs 0.4 fledgling per year), which may be partly due to additional females contributing to parental tasks (incubation, nest attendance, food provisioning). Interestingly, primary females are the genetic mother in most cases (70% out 15 young raised by four polygynous trios).

In the left panel below, all members of a long-term polygenous trio are feeding together at a carcass at the central feeding station (male marked as 1TJ (right), alpha female marked as 1H4 (left), beta female marked as 22J (middle). This trio is together for 10 years (2011-2020).  Although the beta female is usually lower ranked than the alpha female, in this specific case, all members are among the the top-ranked birds of the population. The male is an exceptional case because it has a social position above many females.

Polyandry is characterized by high levels of sexual conflicts among males. Intriguingly, primary males seem unable to get rid of the new male intruding their territory. These conflicts are likely responsible for the low reproductive output with only two cases where fledglings were produced out of 11 breeding attempts. In the right panel above, the primary male (middle bird, marked as 0U5) is mate-guarding the female (left, marked as 0RU)). During the fertile period (end of February/start of March) this male was standing next to her most of of the time, in this case while she is feeding. In this specific trio, the beta male invented various way to impress the female. In one occasion, he offered her a piece of meat and jumped on top of her to copulate (but was rejected). In another occasion, this trio arrived together at the feeding station, and shortly after their arrival, the male started to collect nesting-material and followed the female with a beak full of wool for more than 20 minutes. These males are in conflict now for five years. This illustrates their willing to invest in territory acquisition (and delay reproduction) when expected fitness returns are high (i.e., a high quality territory and/or partner). Although beta males and females sometimes become new owners of these territories, this is not always the case. Overall, the precise behavioral mechanism underlying these peculiar breeding strategies are little understood. All trios have their own story and the way social relationships develop seem to a large extent depend individual-specific characteristics.