**4. Discussion**

Before commenting 2062+ owl calls recorded in Malawi and The Gambia below is short summary for each species on what the handbooks [7, 13–17] and papers [18–23] say about the calling and main breeding times of these species:

*African Barred Owlet*: Late afternoon one can hear a repetitive, mournful "*krroo, krrooo, krrooo*" call rather like that of a Ring-necked Turtle Dove *Streptopelia capicola* (Previous Cape Turtle Dove). It lacks the vivacity of the Pearl-spotted Owl's crescendo whistle [13]. Another book says: "Its call is a series of 6–10 low, whistled notes with a half-second interval between notes: '*hue-hue-hue'*; usually 2–6 second pauses between series. Series may rise and fall in volume and sometimes one series follows immediately by another. Calls mainly at night but sometimes in daylight" [7]. A third book describes the voice: "Repeated fairly high-pitched series of notes '*purr purr piu piu piu piu'*, rising slightly in volume; also 2-syllabled slightly trilled '*prr-purr, prrpurr'*, second note slightly higher than first." [14]. In Malawi, the breeding season is October [7] and Southern Africa from September to October [14].

*African Scops Owl*: This owl calls regularly in the evening at dusk, the female and male answering each other with an insect-like ventriloquial "*prrrup*" at approximately five-second intervals. It is a call that carries over a considerable distance and, if one is sitting by a campfire, it immediately enhances the whole bushveld atmosphere. Often, however, one may not recognize it as an owl at all [13]. The call resembles insect voices so much that most likely it was missed often on noisy African nights, especially in Malawi. In The Gambia, breeding in September [7] and Southern Africa from September to November, but also in June in Zimbabwe [14].

*African Wood Owl*: Songs almost every night and loud calls are audible over at least 500 m and show sufficient individual variation for an observer to discriminate between some individuals [19]. The male song is typically described as a rhythmic "chuckle" sequence of clear hoots, "*hoo-hoo,hu,hu, hu,hu -hu,"* the last five syllables delivered unevenly with a syncopated rhythm. Female has a higher-pitched "*eeyow"* to which the male answers by a low gruff "*hoo* or *woo"* depending on the listener's

*The Vocal Activity of Twelve African Owl Species DOI: http://dx.doi.org/10.5772/intechopen.103887*

interpretation [20]. The breeding season in Sierra Leone is February [7] and in Southern Africa August to November but in Zimbabwe also one April record [14].

*Barn Owl*: In South Africa, the calling in the garden intensified during February and the beginning of March. The call most often heard is a drawn-out tremulous screech—*schrreeee*—an eerie sound which serves a variety of functions: for territorial advertisement, courtship, and a contact call [13]. The male is said to screech more often at beginning of the breeding season when courtship chases are common [7]. In The Gambia breeding September–April [7] and Southern Africa from February to May, but breeding is possible in all months [14].

*Eurasian Scops Owl*: Palearctic migrants wintering in Africa call very little during their stay making them difficult to study [15]. However, a soft frog-like croaking "*drrrr…drr..*" was heard in The Gambia and first labeled as owl x. Other owl x voice was a Little Owl *Athene noctua* like "*kiev, kiev, kiev*" notes. Only in Hungary we realized that the Gambian owl x was *O. scops* [21]. During the winter survey in Italy, it was found that the few Eurasian Scops Owls wintering there responded more strongly to playback of the Little Owl than they did to Eurasian Scops [22].

*Giant or Milky Eagle Owl*: The normal call is a very deep double hoot "*oop-poop*," almost as deep as the call of the Ground Hornbill *Bucorvus leadbeateri*. These two species could be confused, as both may roost and call from the riverine Acacia Forest at dawn and dusk. Luckily, the owl has a variety of other calls. The presumed male emits a series of short, grunting hoots "*uh.uhu-uh-uh"* and is answered by a deeper "*uhuh"* from his mate. This duet once continued for 15 minutes [23]. Distinctive gruff hooting call is described also as "*hook-hook"*; uttered with inflated throat at variable rate and volume but in series of 1–5 hoots with an interval between series [7]. The call is said to carry for up to far as 5 km, but this is hard to believe [16]. In The Gambia breeding November–February [7] and Southern Africa from March to September, mainly June–August [14].

*Greyish Eagle Owl*: The song of male Greyish Eagle Owl recorded in Mali and Ivory Coast consists of two clear syllables and has been rendered as *"kuo-wooh"* [16] or *"koowhoo"* [17]. The first syllable is rather explosive, and the second syllable is somewhat downward inflected, lower-pitched, and extended. This call is uttered in intervals of several seconds [13], and it is not like that of the Spotted Eagle Owl [17]. Breeding mostly from November to April almost throughout the species range [17], but in The Gambia, two well-studied nests had eggs in March [11].

*Northern White-faced Owl*: A disyllabic call is mellow fluting "*po-proo"* at 4–8 s intervals [7]. The first note is a very short, longer second note following 0.6 s [16]. In other words that voice has been described livelier as "*cuk-coooo*": a brief note, followed less than a second by another note, somewhat louder, elongated and descending. This motif sounds rather pigeon-like and is repeated at intervals of 5–12 (average 6.49) s [17]. The main call is quite different from the Southern White-faced Owl. In The Gambia, breeding takes place in February–April, but there are records also from the October–December period [7].

*Pearl-spotted Owl*: The sheer volume of its whistling call "*tiu, tiu, tiu, tiu"* is amazing for such a small bird. Notes began softly but increased gradually in intensity before achieving a penetrating crescendo. Sometimes a second owl would join in antiphonally. In addition, *"too-woop* and *tee-weep"* calls, the latter higher-pitched call that of the female; these soft calls are used by the pair to maintain contact [13]. In The Gambia breeding February–April and Malawi August–September [7].

*Pel's Fishing Owl*: Its main call is a deep sonorous hooting preceded or followed by low grunt "*hooommmmm-hut*"; repeated horn-like "*hoom-hoom"*; resonance from inflated air sacks. Sometimes male and female call in duet, male starting with grunting "*uh-uh-uhu*" building up to high "*hoommm"*; female answering by deeper hoot [17]. The hoot carries over a great distance, up to three kilometers on a calm night. Unlike many other owls, they do not become vocal at dusk and call mainly from midnight to dawn and the hoot is used throughout the year as a contact call [24]. In Nigeria, a young in a nest in February [7], in Zimbabwe breeding April, May, and October and in Botswana mainly February–April [14].

*Southern White-faced Owl*: A nuptial display consists of the bubbling hoot, and the male may approach the female along a branch while bobbing his head up and down and hooting [13]. A pair defends territory by calling; male calls regularly at dusk and dawn but also the night; female may join in. The bubbling polysyllabic "*popopopopopeeu"* has a very fast stutter at first and the second part is more mellow, fluting, and rising in pitch. In other words, the same voice has been described as a rapid series of 5–11 (average 9) hoots, the last one being somewhat higher and accented, repeated at intervals of 7–15 (average 9) s. This song may be written "*w-h-h-h-h-h-h-h-oo*," pronounced as rapidly as a man may do [18]. It is quite different from the Northern White-faced Owl [7]. In Southern Africa breeding May to November, in Malawi peak is in August [14].

*Spotted Eagle Owl*: Increased hooting during the courtship period, the male's "*hoo-hoo"* being answered by the female's triple hoot "*hoo-hoohoo*," the middle "*hoo"* higher, so that the call has a pleasing cadence. Usually, the pair would duet, the female answering her mate immediately so that it sounds like a single owl hooting [13]. Both sexes may call at any time, the male usually around dusk and dawn and female in the early hours of the night [7]. In Malawi breeding season August–October [7].

From the studied 12 species, the far most vocal were Barn Owl and the Pearlspotted Owl. The Southern White-faced Owl can also be audible most of the year, and in the captivity in Mozambique 1992, it was the most vocal of any of those owl species we have recuperated at home. Also, the less studied owl species can be more vocal than anticipated but this material is not enough to prove that.

The similarity index was low when comparing the calling hours and calling months between The Gambia and Malavi, and there was no similarity at all between two closely related White-faced Owls in The Gambia and Malawi. This may support further the separation of these owls.

Several factors are known to influence the patterns of vocal activity of nocturnal birds. One of the most obvious is the time of year, with the calling rate varying within the breeding cycle [25]. Owls are normally calling most actively especially just before breeding [4], but in this material, the peak months coincided very little with the given breeding times in Malawi and The Gambia. It is possible that the validity of the breeding times is not enough, but it is also clear that little seasonality was noted in the calling activity of the most owls studied throughout the year. This is something particular to the tropical weather conditions. The territorial calling of the Northern owls is more limited to the breeding cycle, like that of the Eurasian Pygmy Owl *Glaucidium passerinum* from March to May [1].

The overall activity pattern governs the calling as well, that is, strictly night-active species are mainly vocal in the dark while at least partly day-active species can call all around the clock. Well studied Malawi Pearl-spotted Owl is a good example of that as there are only two hours in 24 h, without any recorded calls. Month-wise the Barn Owl call all year round but has a clear break in the calling during the daytime. Similarly, White-faced Owls and Spotted Eagle Owls were not heard during the daytime.

*The Vocal Activity of Twelve African Owl Species DOI: http://dx.doi.org/10.5772/intechopen.103887*

This chapter wanted to give particulars species by species also commenting how well limited calling activity surveys are likely to reveal the actual number of existing owls.

Differences in calling rates among owls suggest that not all owls will be equally detectable using calling surveys [cf. 26]. It seems that in Africa, the Pearl-spotted Owl and Southern White-faced Owl populations and distribution should be possible to study by using the vocal surveys. They both are laud and calling actively in the evening hours. The nocturnal calling survey is not as good to map the distribution of the Barn Owls in the area as they seem to call erratically and between long intervals. Malawian data for Spotted Eagle Owl are limited but give the impression that vocal studies can serve to map the population size and distribution as the call takes place at least during the breeding times. In Kruger National Park, South Africa, it has been determined that individual African Wood Owls can be identified reliably by their vocalizations [27]. Identification of individuals by their calls has the potential for censusing, long-term population monitoring and is a valuable aid for planning the conservation of this species in Africa [27].

The influence of the weather was also studied. Heavy rain and wind are silencing the owls or at least make it impossible to hear their voices due to the background noise. Barn Owl was often calling immediately when the heavy rain and windy storm stopped. There are some examples that the temperature is not so important if the other conditions are suitable for calling, Barn Owl has been heard in +36°C as well as in +15°C; similarly, Pearl-spotted Owl records cover a similar temperature range from +37°C to +16°C.

The effect of moon luminosity on owls was also studied but with somewhat contradictory results. It seems that some owl species may increase vocal displays during full moonlight (such as the African Barred Owlet, Pearl-spotted Owl, and Southern White-faced Owl in this material) but others call less or not at all during the full moon (Barn Owl in this material). The impact of the full moon was not that obvious as the bright sky can also activate the Pearl-spotted Owl. Barn Owl started to call actively again when the moon was diminishing to 60% of its full size and its luminosity. It has been noted with other owls that they call more in the last quarter and the new moon phase of the lunar cycle [26]. In the classic Tawny Owl *Strix aluco* study in Denmark the owl called less when the moon was up than when the night was cloudy and overcast [28]. And recently studied Long-eared Owl *Asio otus* in Russia was calling both during the rising and waning phases of the moon but again no calls were recorded during the full moon [29]. It has been suggested that small mammals and even some small birds are more active on moonlight nights, with the result that owls then hunt more and call less [16].

The pitch at which an owl calls is related roughly to its body size. Small owls usually utter higher calls than their larger relatives, but sufficient for the smaller area of the territory they defend and the shorter distances over which they must communicate. Furthermore, in owls, as in other predatory birds, females are larger than males and so their calls are usually, but not always, pitched slightly lower than those of their mates (e.g., Milky Eagle Owl in this material).

Owls seem to call more frequently on still nights when there is little interference with sound transmission. The larger owls with deep voices are especially wont to call in the still hours before dawn. They may be taking advantage of layers of air of different densities—the cool dense air of the pre-dawn chill has warmer air above—that bounce back some sound from their interface and enable calls to carry over greater distances. This also ensures that they have their say before being drowned out by the dawn chorus of diurnal birds [4].

Notably, owls usually fly to prominent perches (like our garden Spotted Eagle Owl in Malawi came often on the rooftop) before they call, in this way avoiding the absorption of sound by the ground. This could entail a risk for smaller owls, whose calls might attract larger species to prey on them, but the advantage of successful communication would have to be weighed against this threat. Such interactions may explain the choice of some of the sites from which owls call, as well as the ventriloqual nature of some of their calls [4].

The predation risk, indeed, is among the most principal factors that will influence the patterns of vocal activity in owls [30, 31]. In this study, it was noted that the obvious predation risk and interference competition was altering the vocal activity of the African Scops Owl which stopped calling when the Barn Owl was active. A larger Barn Owl can be a predator that eats the smaller African Scops Owl [31]. In Malawi, it was also suspected that Spotted Eagle Owl calling silenced the Pearl-spotted and Southern White-faced Owls. And there are clear indications that Spotted Eagle Owl could prey on these smaller owls [13].
