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Influence of nest position on blackbird's Turdus merla breeding success in urban habitats.

INTRODUCTION

Blackbird Turdus merula is one of the most successful species judged by its ability to adapt to a wide range of environments including woodland, farmland and urban habitat [13, 15, 17]. Various studies have showed that blackbirds are one of the easier-adaptable species to urban landscapes [13]. The species were recorded by Gurney in the 19th century as very abundant but very shy, in northeastern Algeria [11]. More recently, in natural environments in the same region, the species was found within 09 / 11 habitats of different vegetation layers [1].

Moller [19] & Dominoni et al, [5] have related the adaptation of some bird species (including Blackbird) which become very abundant in urban areas respectively to increasing food availability and night lighting in these ecosystems. One of the oldest colonizers of urban areas, the north-African Blackbird T. merula. spp constitutes the only breeding thrush species in urban ecosystems in north- eastern Algeria (pers. unpub data).

The aim of several studies in ornithology is identification of requirements and factors that determine life history traits [12] or otherwise sustainability. Many authors have discussed factors influencing Blackbird's reproductive success. Breeding success has been related to nest height [24] and [16] had marked nesting site availability as a factor that contributes as a significant way to regulate densities of nesting pairs and consequently breeding success. The selection of nest site were classed one of the most important decisions affecting breeding success in birds [26] which was in fact the case of a study confirming that breeding success was strongly influenced by microhabitat selection (i.e. nest-site characteristics) [12]. In Madrid's urban parks, the same success was bounded to the surface of the studied parks [8]. Multitudes of factors interact and the importance of each factor is not easy to assess. Adapting to urban areas has given him several advantages including, higher densities, consequently a smaller territory, higher breeding productivity (longer breeding season), less predation pressure and more food availability ....

In microhabitat use, only patterns for vegetation cover (height strata) and nest characteristics were documented [21, 12] while tree architecture and their influence on breeding biology remain little reached. Nest's microclimate influences directly the conditions experienced by the developing offspring, and experimental studies have shown that offspring raised in suboptimal nest microclimates suffer negatively in terms of their growth and development [18]. In our study, the tested parameter was based on identification of tree architecture and the nest position in used support species by urban population of the north- African Blackbird Turdus. m in order to verify their influence on breeding success.

MATERIAL AND METHODS

Selection of study sites was based on preferential usual criteria by birds in urban ecosystems. Urban birds tend to choose shrubby vegetation cover, typical formation in parks, gardens and even cemeteries in cities, or even wooded streets, for the most generalist species [8]. It was showed that the volume of woody vegetation is one of the determining parameters of total birds' abundance [20] and that the vegetation height can structure the avifaunal stands [3]. Therefore vegetation plays a key role for the maintaining of bird populations.

1st site:

The first site is an old Christian cemetery presenting an area of 6,2 ha, located in urban center (36[degrees]54'58, 36"N 7[degrees]45'45, 26"E). The site contains an important bird richness including nesting, crossing and wintering species. The vegetation cover makes the cemetery an ideal location for breeding passerines. The habitat has a tree layer composed of four species, and dominated by Cypress Cupressus sp with 91.41% of the total tree number (n=788). The high shrub layer is composed of four species namely: Ficus carica, Pistacia lentiscus, Melia sp and Acacia sp. The herbs layer is mainly composed of Poaceae and Chenopodiaceae, and which is developed seasonally. The rest of the area has a built surface (cellars and chapel) and pedestrian paths that divide the cemetery into several parcels.

2nd site:

The public garden in the Nouri Hacene instead, is a small park with an area of about 0.6 ha. It is located in the city center in latitudes (36[degrees]53'58, 48"N-7[degrees]45'14, 69"E). The site is located in front of Annaba's downtown bus station where human activities almost never ceasing. The vegetation on this site is represented by six tree species with a dominance of old Rubber trees Ficus elastica. A high shrub layer is marked by three species namely: Pearl bush Pistacia lentiscus, Wild Olive Olea europea, and Oleander Nerium oleander. The herbaceous layer is virtually absent because of the weekly maintenance of the park. This reflects the low undergrowth present in this site.

During the study period 2011-2013 we have been sampling 7.2 ha in both sites where monitoring has revealed the presence of 24-30 pairs of Blackbird. All suitable habitats were regularly searched by observers from first February through the end of July in each year of the study. Nests were considered as active once the first egg had been laid. Mapping nests in our study sites showed a territory of 0.1 pair / ha for the first site and 0.12 pair / ha for the second one. Follow-up was conducted by an exhaustive counting of active nests, a monitoring model which is favorable in small areas including urban sites [7]. For almost all birds, many broods are subject to direct action (predation, vandalism ...) or indirect (desertions, weather conditions ...) causing failure of the brood. We have noted any signs of direct predation on nests, all of them were not at least visibly destroyed. Thus, we should not determine the cause of the clutch lost and broods were classed as disturbed by unknown cause. We calculated breeding success by the number of chicks as a percentage to laid eggs. The percentages were more important for broods presented null and complete success respectively (37 and 23%) and the rest was for partial success.

Nests were disturbed as little as possible during poor weather either especially for early broods. We have presented the proportion covering the aerial parts of tree species respectively as excurrent crown and decurrent crown for Common Cypresses and old Rubber trees [9]. The support tree species and positioning type of the nest were identified to show siting (Table 01). Cause it has been accepted by most researchers that predation is the primary source of mortality during breeding season in birds, we have choose the positioning of nest as a parameter to demonstrate accessibility of predators to nest site.

We have used Microsoft excel 2013 for basic statistical analysis in order to determine mean and standard deviation for reproductive parameters and nest characteristics while we have tested nonlinear regression analysis between the reproductive success by using Statistica 10.

Results:

Our research has revealed presence of 73 active broods. The evolution of brood's number took a curve of positive growth during three years study, where 25 broods were recorded in 2011, 27 in 2012 and 32 for the last year. Breeding success average for the study period has given 45.66%. Partial success was the most represented throughout the study period with 40%. During study seasons, we have noted a large gap between nest's number and reproductive success in both study sites. This would be due to area' surface which is highly different as was mentioned previously. More important is the case of reproductive success which presented a considerable difference counter to surfaces, while the success was 44.70% in cemetery (1st site) and 51.66% in Nouri Hacene' garden (2nd site). The increased number of nest finding between years increased in same time with reproductive success which revealed a significant inter-annual effect in success rate (t=39.86, P = 0.016). Success within sites weren't discussed in relation to important difference between surfaces and broods number, from where 86% of nests were counted in first site.

Despite existence of six species belonging to the shrub layer, the studied population of blackbird have presented an exhaustive arboreal nesting, normally, because the shrub layer represents just 1.66% of the total number of tree and shrub species in study sites. Nest positioning type showed different cases on tree species in both sites. Seven nest position types were obtained and illustrated, namely: Close to the trunk, on the branch, in trunk division, between branches, in a trunk hole, on a palm and between dense branches (Fig. 03). These types demonstrate exposure and concealment forms. Those constructed on Cypress 1, 2, 3 and 5th presented different types of exposure, whereas in old Rubber trees just one type were marked 7th, presented by a great concealment case. Other types (4 and 6) were neglected in reference to their low percentage presented by those constructed on Date palm Phoenix sp and Wild olive Olea europea trees respectively 03 and 02 broods during the whole study period.

Table 01. Positioning type and breeding success.

Breeding success have demonstrated a large gap from a nest-position to another (26-73%). Nests positioned in trunk divisions presented by the higher reproductive success show a low percentage of occupation (23%). Using T and Z tests for two samples, differences between averages of the occupation and the breeding success have revealed a significant correlation according to Satterthwaite (t = -2.7, P = 0.024). All broods in which nest constructed in the trunk division were distributed during the entire breeding season, no date effect was present. The successes (hatching, fledging and breeding) of different positioning types have presented a significant correlation by Bartlett test (ACP) (Khi2 =28.57, P < 0.0001).

Only 7.26% of the trees number has been occupied, this reflects the presence of a "forest cover" with a high density of vegetation in urban areas, including tree layer (exclusively occupied one). The most commonly tree species used by our model was Cypress Cupressus sp and Rubber trees Ficus elastica.

Success recorded by broods built on the two dominant species was respectively 44.70% and 51.66% for Cypress and Rubber trees (Table 02). Despite high occupancy rate on Cypress, reproductive success was best represented for nests built on old Rubber trees, which were occupied by just 13%. The most occupied tree species present different shape cover. Rubber's tree shape is a canopy spreading habit: horizontal tree cover (decurrent crown) while that of Cypress is slender canopy: vertical tree cover (Excurrent crown).

Discussion:

Because few literature reports works on the North African Blackbirds, most of results in this paper were compared with those of European urban Blackbirds. The species built their nests in several plant species [10] the same was presented by our blackbirds with almost an absolute preference for trees, which leads to say that is a wooded breeding population. In accordance with [2] the seasonal pattern of breeding success in our population presented a height percentage in early breeding season, although in our sites, breeding season began during second half of February. This would be explained by predator' behavior, which begin plunder nests later than the appearance of nest [23,6].

Two important of many factors provoking failure or save the brood from a predation' act, are vegetation cover and nest height in open nests [22]. Ecologists have long recognized that vegetation physiognomy influence refuge from predation [25]. Parental care behavior can attract predators to nest site [4]. The architecture and position type in Cypress may give opportunity to aerial predators. This, result a higher predation rate for nests construct on Cypress (Fig. 03: 1, 2, 3 and 5th), whereas in old Rubber, dense leafing reduce the predator effect on active broods cause all nests were construct between branches. Most occupied species (Cypress and Rubber) in our sites are both evergreen trees, which exclude the seasonally variation in nest site selection resulting by growth of leafy canopies during the second half of breeding period according to [26].

As was mentioned previously nests built on Rubber trees was more hidden then those built on cypress, and nest height added a more safe position against terrestrial predators. Despite the fact that average height of Cypress exceeds twice of Rubber tree's average height. Therefore, nest height would be an important factor reducing by the way the terrestrial predator pressure (lizards, rats and rarely cats) beside aerial ones by using sites between dense branching.

Tree species which leafed out earlier offered much better shelter for nests than trees with later foliage development [26]. Whereas overlap volume and tree architecture have been neglected. Most trees can be classified as showing one of two contrasting types of branching pattern. In orthotropic branching, the axes are upwardly inclined and the leaves are arranged around them in more or less radial symmetry, whereas in plagiotropic branching, planes of foliage are formed from leaves arranged along the sides of nearly horizontal twigs [14]. The vegetation architecture of tree species necessary for nesting and maintaining bird populations are trunk, branches, foliage, tree density and undergrowth, or the horizontal distribution and vegetation height in water (water birds) in a word the physiognomy of the habitat [2]. The three- dimensional distribution of morphological elements (horizontal projection of the aerial organs of support plant) would be a very important factor favoring dissimulation of broods in Blackbird, particularly because the species present an arboreal nesting (building nests on trees only) in our study. Trees with dense vegetation cover would assure a good shelter for altricial birds including Blackbird.

Conclusion:

The horizontal overlap can create a more protected living space around the nest for a species such as Blackbird. The tree architecture provides an enabling environment for chicks fed by its parents until development (flight and feed independently). Without diminishing the value of undergrowth relatively important for the survival of chicks in thrushes, the aerial part of the support species can be safer for chicks after leaving the nest. This is clearly the case for monitoring in urban parks, which present usually low undergrowth favorable to the survival of chicks.

ARTICLE INFO

Article History:

Received 5 August 2015

Accepted 28 August 2015

Available online 15 September 2015

REFERENCES

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Imed Djemadi, Selma Bouzid, Zihad Bouslama

Laboratory of terrestrial and aquatic systems. Faculty of Science. Badji Mokhtar University, B.P. 12., Annaba, Algeria.

Corresponding Author: Imed Djemadi, Laboratory of terrestrial and aquatic systems. Faculty of Science. Badji Mokhtar University, B.P. 12., Annaba, Algeria.

Table 02: Tree support and percentage of occupation.

Supports                        nnest     %     Succes repro %

Cypress Cupressus spp            58     79.45       46.83
Old Rubber Ficus elastica        10     13.69       51.66
Date palm Phoenix dactylifera    03     04.10       33.33
Wild olive Olea europea          02     02.73       00.00
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Author:Djemadi, Imed; Bouzid, Selma; Bouslama, Zihad
Publication:Advances in Environmental Biology
Article Type:Report
Geographic Code:6ALGE
Date:Aug 1, 2015
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