This bird survey showed that the Woodpigeon, originally a forest bird species, is currently pursuing in Algeria a similar trend that had made of it a bird adapted to sub-urban parks and gardens of large cities in most European countries. In some cases, this observation is old; for example, it dates at least of the eighteenth century in Paris [27–29]. However, outside the cities, the Woodpigeon still has non-synanthropic behaviour and does not occur on farmland except if there is no disturbance source or any danger in sight.
The increase of Woodpigeon numbers in Algiers region seems to be related to an adaptation and change in species diet. Before 1990, the species fed mainly in forests of Mitidja Atlas mountains, where they found abundance food resources, including berries of Phillyrea
angustifolia in July–August, Bay Noble (Laurus
nobilis) in August–September, Zeen Oak acorns in October, and fruits of Olive (Olea
europaea) in November–January [16]. Since 1990, repeated violent forest fires occurred annually in the region as in the rest of Algeria [30], which have significantly reduced food resources of the species. Deprived of very large part of its usual food, the Woodpigeon has to rely on farmland resources, which the species used already in the past with small flocks mainly coming from Tellian Atlas forests for foraging in the countryside around Algiers [31]. This decline in forest food resources has thus merely precipitated a process already well underway.
The food of Woodpigeon in Algiers plains is quite diverse. Our observations are quite similar to those reported in diet analysis of the species (e.g. [32]. Moreover, according to Merabet et al. [33], in early spring woodpigeons consume young leaves of ash trees that grow along Wadis of El Harrach, Boudouaou, and Hamiz; where it also ingests Loquat fruits (Eriobotrya
japonica), young leaves of White Mulberry (Morus
alba) and Elm (Ulmus
campestris). In summer, Woodpigeons feed on green pods of Kurrajongs (Brachychiton
populneum), Beans (Bauhinia
purpurea), Figs and fruit of Mediterranean Hackberry (Celtis
australis). In winter, they also eat dates of the Canary Palm (Phoenix
canariensis), olives (Olea europaea), berries of Mediterranean Buckthorn (Rhamnus
alaternus) and Japanese Privet (Ligustrum
japonicum).
In our study area, during cool and wet periods, pigeons fled to sleep in the canopy of pine trees, which provide good shelter against rain and wind. In Bejaia, the species occupied crowns of Aleppo Pine (Pinus
halepensis) located downtown, which are left in the morning to reach the Soummam Valley located at more than 10 km from the city [19]. During the period of nestling feeding (April–May), pigeons make flights of short range to the countryside around Algiers in order to supply their chicks.
Reasons of Woodpigeon’s population increase in our study area during these nineteen years is based on the assumption that we are dealing with increasing sedentary populations; contrary to recruitments of few individual migrants reaching the area from northern Morocco and middle Europe during postnuptial migration [29, 34]. Moreover, we are uncertain if these migration routes may reach up central Algeria and thus lead to population increase. However, we hypothesize that the diminution in hunting pressure, reduced almost to zero, for over a decade in Algeria. Now, hunting is prohibited all times, except for some species such as wild boar (Sus
scrofa). Generally, this factor was favourable for a wide range of game birds [35]. In addition, poaching of woodpigeons, although there is still, remains insignificant. Another high-probable hypothesis, but not exclusive as the one of hunting banning, is that the woodpigeons in Algiers region like in Europe, have developed adaptation to new and abundant food resources provided by the extensive agriculture. Indeed, woodpigeons still are predominantly occurring in natural woodland in Morocco, but they frequently forage in cultivated areas [27, 36–38]. In Europe, large cereal crop areas certainly played a key-role in increasing Woodpigeon populations [37, 39]. However, this factor seems insufficient to explain the same phenomenon in Algiers region, as cereals are intended for livestock feeding on one hand and they are seasonally produced and only on about 9300 ha (about 15 % of all cropland areas) on the other hand. Under these conditions, even if the cereal production has favoured population growth of the Woodpigeon at its beginning, it will quickly become a limiting factor, if it is not already done so, unless the species is boarding its feeding niche by diversifying its diet and foraging on other plain habitats.
The first observation of the Eurasian Collared Dove in Algeria was carried out in a residential area of Annaba city in East Algeria, where 40 individuals were observed in March 1994 and then 115 individuals in December 1997 [17]. Later in 1999, the species was reported in Bejaia (Further west of Annaba) [40]. Its spread over the northern Algeria seems to have been slow because it took until March 2000 for the observation of first individual in coastal areas of Algiers [3].
Contrary, the establishment of the Eurasian Collared Dove in Tunisia is certainly old, because a small breeding population existed in 1963–1967 in Thibar [41]. It would probably be the same in Morocco, where it was considered as common in most cities particularly in coastal zones [34, 42]. These facts suggest that the Collared Dove dispersed towards Algiers area from the western or eastern extremity of the Maghreb, and most probably from the east, because no study reported its presence in western Algeria until 2001 [19]. In the Sahara Desert (South Algeria), it was reported in 2003 in Biskra [43] and in 2007 in Tamanrasset [18]. Now, it is considered among the common bird species in several arid regions in Algeria [44, 45]. These statements are naturally comparable with the well-known worldwide expansion patterns of the species, which has now colonized most of Europe and America, where its numbers are still growing even in areas where it had existed long time ago [46–48].
Well adapted to local conditions, the Eurasian Collared Dove sings and courtship displays anytime during the year, even out the breeding season and even more during rainy and snowy times, as was the case in late January 2005. It breeds from March and continues until mid-July, but in autumn (October–November), it is just displaying [49]. In this respect, its behaviour in Algeria is not different from that of populations living in Europe [34].
The fragmentation of lands and habitats of the Mitidja Plain and its surroundings into small areas with expansion of urbanization has indirectly led rapid expansion of the Eurasian Collared Dove and Woodpigeon. Factors that currently control numbers of these Columbidae species are predation, disturbance due to human activities and food availability, which has changed little in the past two decades. One of the main factors behind the current increase in numbers of Columbidae in Algiers region is the reduction of hunting pressure since 1990s, which has mainly benefited the Woodpigeon and the Eurasian collared dove. Adaptation to urban areas and the acclimatization to new food sources, for both surveyed species, have contributed to the increase of their numbers that was very clear in the Woodpigeon. In lights of these conditions, we are expecting further increases in both range and densities of species we surveyed. Therefore, further monitoring of such opportunistic species with large ecological valence is of key importance to understand the environment status and to assess possible ramifications for native avifauna.