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Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)13749-8
10.1016/j.heliyon.2024.e37718
e37718
Research Article
Breeding biology of Great White Pelican Pelecanus onocrotalus at Lake Tana, Ethiopia
Yimer Ahmed Yesuf ahmedyesuf2012@gmail.com
a⁎
Zelelew Shimelis Aynalem b
Ashagrie Mezgebu b
Alemkere Azmeraw b
Alelign Aschalew b
a Dilla University Botanical and Ecotourism Research Center, Dilla University Ethiopia, Ethiopia
b Department of Wildlife and Ecotourism Management, College of Agriculture and Environmental Science, Bahir Dar University Ethiopia, Ethiopia
⁎ Corresponding author. ahmedyesuf2012@gmail.com
10 9 2024
30 9 2024
10 9 2024
10 18 e3771814 5 2024
3 9 2024
9 9 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Great White Pelicans, once thriving at Lake Shala, have faced a decline in breeding pairs at Lake Abijata due to increased salinity and overfishing. A study at Lake Tana Biosphere, from December 2020 to December 2021, aimed to understand their breeding biology, which had not been previously documented. We conducted intensive searches for nests and monitored them during incubation and post-hatching to assess reproductive success. The study found pelicans breed year-round, with peaks from late December to March and June to July. The average clutch size was 2.14, with egg dimensions averaging 90.33 mm in length and 58.70 mm in breadth and a mean weight of 173.68 g. No significant differences were observed in these measurements, but a positive correlation was found between egg length, weight, and breadth and weight. However, no correlation existed between length and breadth. Out of 98 eggs, 84 hatched, resulting in 80 and 91.5 % hatchability rates across two breeding seasons, averaging 85.71 %. The hatching success rate did not differ significantly between seasons. Unfortunately, only 37 pelicans reached the pre-fledging stage, indicating a high mortality rate of 63 %. The study concluded that the breeding performance of Great White Pelicans at Lake Tana is poor, with human disturbance, overfishing, and predation on nestlings posing significant threats to their nesting success. This highlights the need for conservation efforts to address these challenges and protect the species.

Keywords

Breeding
Egg morphometry
Great white pelican
Hatching and breeding success
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pmc1 Introduction

The genus Pelecanus was first described by Linnaeus in 1758 in his Systema Naturae tenth edition [1]. Today, there are seven species of the genus Pelecanus in the world [2]. However, only the Great White Pelican (Pelecanus onocrotalus) and the Pink-baked (P. rufescens) are found in Africa [3]. The Great White Pelican is a large, conspicuous white bird easily seen and identified. The Pink-backed Pelican is generally greyer overall than the Great White Pelican.

The Great White Pelican is listed in the IUCN Red List as 'Least Concern' (L.C.) [4] and extensively distributed from Southeast Europe through Asia and Africa. In the Palearctic, Great White Pelicans are long-distance migrants from Southeast Europe to East Africa [5].

The Great White Pelican is listed on the IUCN Red List as ‘Least Concern’ (L.C.) [4,6]. Even though the population trend is unknown, the Great White Pelican population size is estimated at 260,000–300,000 individuals globally [6]. The species also has a broader distribution range extending from Southeast Europe through Asia and Africa. Among the global population of Great White Pelicans, about 80 % are in Africa [[7], [8], [9]]. They are highly mobile birds and migrate longer distances [10]. In the Palearctic, the Great White Pelicans are long-distance migrants from Southeast Europe to East Africa [11].

In the biogeographic range of the species, great white pelicans dwell as residents, breeding, non-breeding, and migration passage in different countries [6]. Ethiopia is one of the resident areas of Great White Pelicans, and they are found in Lakes Shalla, Hawassa, and Tana [9,12,13] in the list of countries of occurrence of the species. Great White Pelicans have been known to breed at Lake Shala in the Rift Valley area of Ethiopia [9]. However, due to salinity and overfishing activities in the Abijata Lake, the breeding colonies decreased from 6000 to 8000 pairs in the 1960s to a few hundred pairs [9]. Besides the Rift Valley areas, the Great White Pelicans are also found in Lake Tana [14]. During the Lake Tana Biosphere designation, the Great White Pelicans were considered a key species indicator to monitor the Lake Ecosystem. Their distribution and population were estimated recently, but their breeding biology was unknown.

This study aims to investigate the breeding biology of the Great White Pelican (Pelecanus onocrotalus) at Lake Tana, Ethiopia. Studying the breeding biology and reproductive success of pelicans in Lake Tana will provide valuable information to understand the population growth status, which is important for conserving the species in Lake Tana.

2 Methods

2.1 Study area

The study sites were located in northwestern Ethiopia at Lake Tana, specifically Fikir Mefja Islands (Fig. 1). The area receives an average of 1428.9 ± 35.36 mm rainfall annually (2000–2020). During the same period, the minimum and maximum temperatures were 10.33–13.80 °C and 26.75–28.77 °C, respectively [15].Fig. 1 Map of the study area, Lake Tana area.

Fig. 1

2.2 Field methods

The study was conducted from December 2020 to September 2021. An intensive field survey was conducted in various areas of the Lake to find nests of Great White Pelicans. Those sites such as Tana Kirkos Island, Mistily Fasiledes Island, Daga Estifanos Island, Fikir Mefja Twins Rocky Island (615 m apart), Fikir Mecheresha Rocky Island and Dek Island were the main ones. During the survey, local knowledge of the possible nesting sites was gathered. However, only a pair of Islands was identified as the nesting sites of Great White Pelican (Fig. 2).Fig. 2 Great White Pelican Breeding Colony at Fikir Mefja nesting site, Lake Tana (©Ahmed Yesuf, 2021).

Fig. 2

Nest dimensions such as the nest breadth (width) and inter-nest distance (IND) of nests from the nearest neighboring nest were also measured using a measuring tape. The egg's length, breadth, and weight found in the nest were measured using a Carbon Fiber Composite Vernier Digital Electronic Caliper Ruler U.S. and Digital Pocket Scale [13]. For each egg, the egg shape index (ESI) and volume (V) were calculated according to Hoyt [16,17].ESI=B/L×100

Where ESI = Egg Shape Index, B breadth of egg, and L = Length of the eggV = 0.51 × LB2

Where V = egg volume, 0.51 = scaling constant was calculated by Ref. [16] where most bird species have values very close to this average, egg weight measurement was adjusted with a correction factor of 199.7/200, in which every egg weighed in the field was multiplied by 0.9985 to get the correct weight of the egg [13].

The egg stages were determined as first laid, second laid, third laid, and fourth laid based on the eggs' time [13]. Independent egg measurements of each egg were taken; each clutch had separate measurements for comparison. The egg's color condition (white or red-brown), weight, and length tell the different stages. The dirtier the egg was assigned old/older/oldest depending on the clutch size, and the whiter was assigned newer laid egg [13].

Active nests were monitored by visiting four times during the incubation and post-hatching to determine the clutch size and reproductive success (hatching and breeding success). Hatching or nesting success was determined by examining the fate of the eggs laid by the pair at the end of the incubation period or early hatching time. Those nests considered successful were those that have hatched ≥1 egg [18]. In addition, the fledging or breeding success of the P.onocrotalus was calculated by the number of young birds fledged per total number of eggs hatched*100. Young birds reaching the height of their parents and starting to flee were considered as fledged [19].

Nest or clutch initiation was defined as the date the first egg was laid in a nest. This was determined through a series of weekly checks [20,21]. For those nests found with incubating pairs, nest, and clutch initiation dates were extrapolated and deducted from hatching dates to determine the peak breeding season [22].

2.3 Data analysis

Data normality was checked using Shapiro-Wilk's test [23]. Following this, those data that satisfied the standard normal distribution criteria were subjected to parametric tests ANOVA, t-test, and Pearson correlation (r). One-way ANOVA was employed to see the significant difference between all measurements within and between eggs-laid stages. A Post Hoc Test-Tukey HSD (Honest Significant Difference) was used to conduct multiple mean comparisons for variables whose F values showed a significant difference.

However, the Independent Sample Mann-Whitney U test was used for data that was not normally distributed. Moreover, other analyses, such as the Egg Shape Index (ESI) and Egg volume (V), were used. SPSS version 26 and Stata software version 16.0 were used during data analysis. All the statistical tests were two-tailed, and differences were considered statistically significant at the 5 % level.

3 Results

The Great White Pelicans breed in the twins' rocky islands known as Fikir Mefja. The total area of the breeding sites Fikir Mefja 1 and Fikir Mefja 2 were estimated at 8084 and 4083 m2, respectively, during the dry season when the water level subsides. These areas are 2.53 km and 3.22 km from the nearest lakeshore mainland.

These two islands are the only nesting sites of Great White Pelican in the area. The sites seem rich in fish sources and inaccessible to human disturbances, though few fishermen were observed in our field surveys. The nests were built on bare rock areas. No nests have been seen in adjacent lakeshore areas, wetlands, floodplains, or water reservoirs. The first clutch was started along with nest construction. The nests were rudimentary and almost flat.

The nest characteristics of Great White Pelicans were almost similar. During the intervals between copulations, the newly mated pair would perform sideways shoveling actions with the bill, the head and neck being stretched out nearly level with the ground. There was an earth or sand, and the flexible lower mandible bent in response to resistance, and, on an aloft substratum, a few such sideways strokes would result in a hollow large enough for a nest. A sitting bird might reach out from time to time and pull small stones or gravel towards itself, but often, nests were simply depressions in the earth, just large enough to contain the eggs. Birds, perhaps males near displaying flocks, were sometimes seen to pick up a piece of grass, walk about with it, and then drop it. However, more elaborate nest building was done sometimes, especially in the wet season. Males collected all nesting material, while the females remained where they would later lay.

During the breeding season, 49 nests with 98 breeding individuals were recorded. The active breeding times of Great White Pelicans were throughout the year in dry and wet seasons. However, peak breeding months were from late December to March (dry season) and June to July (wet season) (Fig. 4).

3.1 Nest morphometry

A total of 49 nests were used to analyze the morphometry of Great White Pelican nests, and a Shapiro-Wilk Test of Normality showed that the nest diameter was approximately normally distributed. But, the Inter-Nest-Distance between nests was not normally distributed. The nest diameter ranges from 37.5 to 68 cm (N = 49, mean = 51.28 ± 0.81 cm), and the Inter-Nest-Distance range was 41.80–118.36 cm (N = 49, mean = 76.73 ± 3.72 cm).

Independent Sample Mann-Whitney U indicated that there was no significant difference in Inter-Nest-Distance between nests of the dry season (Median = 62, N = 26) and wet season (Median = 95, N = 23), U = 257, z = −0.84, (P > .05). The criteria 0.1 is a small effect, 0.3 is a medium effect, and 0.5 is a large effect [24]. Hence, 12 % (small effect) of the variance in the inter-distance between nests was explained by the seasons of the year. Similarly, the nest diameter in both seasons did not show a significant difference (t (47) = 0.316, P > .05).

3.2 Nest and nesting density

Nests are small, ranging from 37.5 to 68.0 cm across, and close together (421 nests averaged 3.46/100 m2). Forty-nine breeding pairs with eggs were recorded, 26 in the dry and 23 in the wet seasons. The nesting density of Great White Pelican was 3.64 pairs/100 m2 in the breeding site of Lake Tana. Great White Pelicans were observed to use similar nesting areas during both seasons, though it is not sure whether identical breeding pairs were using the same location since birds had no marks. The mean inter-nest distance (cm) in the study sites was 74.62 ± 5.10 in site one and 78.92 ± 5.52 in site two. Statistically, they had no significant difference (t (47) = −0.573, P = .569).

3.3 Eggs and morphometric measurements

A total of 98 eggs were recorded from the surveyed 49 nests. The average clutch size was 2.14. The mean average weight of clutch one was 173.47 ± 16.39g, clutch two 174.61 ± 17.70g, clutch three 173.5 ± 23.62g, and clutch four 156.84 ± 16.47 g. Egg measurements showed that the mean length of 98 eggs was 90.33 ± 4.39 mm, with a minimum length of 78.80 mm and a maximum of 102.7 mm. The mean breadth was 58.70 ± 2.41 mm, with a minimum length of 51.40 mm and a maximum of 63.40 mm, and the mean weight of all eggs was 173.68 ± 18.44 g, with a minimum of 139.19g and a maximum of 212.78g.

There was no significant difference in all measurements at 5 % level of significance (length, N = 98, df = 96, t = 0.643, P > .05; breadth, N = 98, df = 96, t = 0.647, P > 0 0.05; weight, N = 98, df = 96, t = 0.193, P > .05).

However, this analysis did not consider the ages/stages of all eggs. The first clutch size, second, and third clutch had different measurements. To see whether there is a statistical variation in eggs' mean length, breadth, and weight, all eggs were categorized into 'new,' 'old,' 'Older,' and 'oldest' stages. The mean length of the first laid egg (new), second laid egg (old), third laid egg (older), and fourth laid (Oldest) Table 1 shows the detailed summary statistics.Table 1 Mean length, breadth, and weight of 1st, 2nd, 3rd, and 4th laid eggs.

Table 1Measurements	Egg Stage/age	N	Egg measurements Mean (±S.E)	
Egg Length	New	44	91.44 ± 0.49	
Old	48	88.73 ± 0.45	
Older	5	85.84 ± 0.90	
Oldest	1	84.8	
Egg Breadth	New	44	60.23 ± 0.29	
Old	48	58.17 ± 0.29	
Older	5	55.58 ± 0.29	
Oldest	1	54.9	
Egg Weight	New	44	188.03 ± 1.75	
Old	48	161.30 ± 1.43	
Older	5	147.64 ± 3.33	
Oldest	1	140.29	

However, the difference between the egg stages was highly significant. Table 2 shows the summary of the findings.Table 2 One-way ANOVA for egg morphometry based on egg stages (1st, 2nd, 3rd and 4th laid).

Table 2	Sum of Squares	df	Mean Square	F	Sig.	
Egg Length	Between egg stages	529.742	3	176.581	12.385	0.000	
Egg Breadth	Between egg stages	69.785	3	23.262	4.433	0.005	
Egg Weight	Between egg stages	11637.23	3	3879.077	17.072	0.000	

Multiple comparisons to see the mean differences in egg length, breadth, and weight were analyzed using a Post Hock test. However, egg stage "oldest" for clutch size four was excluded from multiple comparisons of the Post Hock test due to a smaller number of groups, as shown in Table 3.Table 3 Multiple comparisons of egg stages/ages for egg morphometry using Tukey HSD test.

Table 3Dependent Variable	(I) Egg stages	(J) Egg stages	Mean Difference (I-J)	Std. Error	Sig.	
Egg Length	New	Old	−4.70360a	0.78969	0.000	
	Older	−1.65208a	1.63837	0.000	
Old	Older	3.05152	1.64662	0.158	
Egg Breadth	New	Old	−1.58979a	0.4807	0.004	
	Older	−1.19896	0.9973	0.455	
Old	Older	0.39083	1.00232	0.920	
Egg Weight	New	Old	−22.35647a	3.12967	0.000	
	Older	−16.03388a	6.49311	0.040	
Old	Older	6.32259	6.52582	0.598	
a The mean difference is significant at the 0.05 level.

There was a strong positive correlation between egg length and weight, which was statistically significant (r = 0.71, P ≤ 0.001, P < .001, N = 98). There was also a strong positive correlation between egg breadth and weight, which was statistically significant (r = 0.58, P = .000, P < .001). However, the correlation between egg length and breadth was insignificant (r = 0.166, P = .103, P > .05, N = 98). Egg stages and the morphometric measurements of eggs (length, breadth, and weight) have significant intermediate positive (Egg Length and Egg Breadth) and strong negative correlation, length (r = 0.396, P < .001, N = 98), breadth (r = 0.283, P = .005, N = 98) and weight (r = −0.513, P < .001, N = 98), respectively.

The measurements of egg morphology and inter-nest distance are summarized in Table 4.Table 4 The mean length (mm), breadth (mm), weight (g), Nest Size (cm), and Inter nest Distance (cm) of the nesting sites in the study area.

Table 4Nesting Sites	Variables	Sample Size (N)	Mean Measurements (±SD)	
Fikir Mefja 1	Egg Length	25	89.17 ± 4.23	
Egg Breadth	25	58.22 ± 1.76	
Egg Weight	25	168.06 ± 14.55	
Nest Size	25	51.84 ± 6.72	
Inter Nest Distance	25	74.62 ± 25.47	
Fikir Mefja 2	Egg Length	24	90.03 ± 3.78	
Egg Breadth	24	59.43 ± 2.21	
Egg Weight	24	173.71 ± 15.60	
Nest Size	24	50.70 ± 4.41	
Inter Nest Distance	24	78.92 ± 27.04	

On the site, based on a comparison of the egg morphology, differences in all measurements were not a statistically significant difference between the nesting sites, except egg breadth (t = −2.115, df = 47, P = .04) (Table 5).Table 5 Independent Sample T-Test for egg morphometry based on nesting sites.

Table 5Variables	t	df	Sig.	Mean Difference	Std. Error Difference	95 % CI of the Difference	
Lower	Upper	
Egg Length	−0.754	47	0.455	−0.86533	1.14762	−3.17405	1.44338	
Egg Breadth	−2.115	47	0.040	−1.20642	0.57045	−2.35401	−0.05882	
Egg Weight	−1.313	47	0.196	−5.65549	4.30759	−14.32124	3.01027	
Nest Size	0.698	47	0.489	1.13767	1.63074	−2.14296	4.41829	
Inter Nest Distance	−0.573	47	0.569	−4.30117	7.50205	−19.39335	10.79102	

The egg shape index and egg volume were analyzed using freshly laid eggs. The mean difference of freshly laid egg length (mm), egg breadth (mm), ESI, and egg volume in the dry and wet season were length: 2.05 ± 1.05; breadth: −0.42 ± 0.52; volume: 1.29 ± 3.21 and ESI: −2.07 ± 0.97. This mean difference between the egg parameters of the freshly laid eggs, egg length, breadth, and egg volume was not statistically significantly different at 5 %, df = 46, P > .05 when t = 1.96, −0.81, 0.40, respectively. However, the Egg Shape Index significantly differed between dry and wet seasons, (t = −2.15, df = 46, P < .05 (N = 48). Table 6 shows the summary of the findings.Table 6 Independent Sample t-test for equality of means between dry and wet seasons fresh laid eggs.

Table 6Variables	t	df	Sig. (2-tailed)	Mean Difference	Std. Error Difference	95 % CI of the Difference	
Lower	Upper	
Egg Length	1.956	46	0.057	2.0458	1.04581	−0.05931	4.15092	
Egg Breadth	−0.811	46	0.422	−0.4229	0.52171	−1.47305	0.62725	
Egg Volume	0.404	46	0.688	1.29354	3.20525	−5.1583	7.74538	
Egg Shape Index	−2.146	46	0.037	−2.07453	0.96669	−4.02037	−0.12869	

3.4 Breeding performance

Of the 49 nests recorded, only 43 (87.76 %) have hatched chicks, while the other six (12.24 %) were destroyed due to human disturbance (Fig. 3). In the field study, we observed that fishermen used the nesting site of great white pelicans for drying fish meat (Fig. 5). This human activity at the site destroyed some of the pelicans’ nests. Re-nesting of the destructed nests was not detected.Fig. 3 Breeding performance of Great White Pelican at FMRI nesting site, Lake Tana (©Ahmed Yesuf, 2021).

Fig. 3

Fig. 4 The peak breeding season of the Great White Pelican in the Lake Tana area.

Fig. 4

Fig. 5 Fishermen disturbance at Fikir Mefja 1 nesting site (©Ahmed Yesuf, 2021).

Fig. 5

Similarly, out of the 98 eggs in 49 nests recorded, only 84 eggs (85.71 % (5 eggs from clutch size one, 58 from clutch size two and 21 from clutch size three, none from clutch size four) were hatched. The hatching success per clutch size was 83.33 %, 90.63 %, 87.50 %, and 0 % for clutch sizes one, two, three, and four, respectively. The hatching success and failure of Great White Pelican nests in Lake Tana were 0.8776 and 0.1224 chicks per nest, respectively. The hatching success was positively correlated to clutch sizes (r = 0.305, P < .05, N = 49).

Also, examinations of the history of 49 nests with successive visits confirmed that the hatching success was significantly different between nests with one, two, three, and four eggs. However, the lowest value was observed in clutch size four.

The one-way ANOVA was used to evaluate the mean differences across the four clutch size levels over the Great White Pelican hatching success. The test revealed a highly significant difference (F = 7.97, df = 3, P < .001) in the hatching success of clutch sizes (clutch size one, N = 6; clutch size two, N = 33; clutch size three, N = 9; clutch size four, N = 1) (Table 7).Table 7 One-way ANOVA for hatching success based on clutch sizes.

Table 7Source	S.S.	df	M.S.	F	Prob > F	
Between Clutch Size	11.106061	3	3.70202	7.97	0.0002	
Within Clutch Size	20.893939	45	0.46431			
Total	32	48	0.66667			

4 Discussion

Great White Pelicans have selected inaccessible nesting sites in Lake Tana, a similar scenario reported by Ref. [9]. They have reported that the inaccessibility of a breeding site overrides climate and food supply factors. At Lake Shala, St. Lucia Bay, northern Chad, and northern Nigeria, the breeding sites are on islands or mountain tops inaccessible to mammalian predators and humans. The first nest of the Great White Pelican in Lake Tana (Fikir Mefja) was discovered on January 2, 2021, with mass gatherings observed in February. This discovery marks the first documentation of this species' nesting site and breeding biology in Lake Tana region.

The breeding season of the Great White Pelican varies by region: it starts in April or May in temperate zones, can occur year-round in Africa, and spans from February to April in India [5]. However, in Africa, the Great White Pelican breeding season occurs throughout the year. The same pattern of the breeding season is reported in Lake Shala, Ethiopia; breeding happens throughout the year, with a peak of late December to March [9]. Similarly, Great White Pelican in Lake Tana breeds in all seasons, with peak breeding times from January to March and July to August.

The species occurrence or its nesting site is associated with relatively large, warm, shallow fresh, brackish, alkaline, or saline lakes, lagoons [3,25], marshes [3], broad rivers [25], deltas [3,25], estuaries and coasts of landlocked seas [26]. In Lake Tana, they nest on rocky islands that provide ample fish and are inaccessible to avoid disturbance. This is consistent with findings from other regions where inaccessibility is a critical factor in site selection, overriding climate, and food availability.

The nesting behavior of Great White Pelicans in Lake Tana reflects their preference for sites with little to no vegetation, creating depressions in the ground to lay their eggs. Some nests are constructed from available materials such as soil, plastic, leaves, stems, and papyrus. The nests are elliptical, almost flat, and vary in size from 37.5 to 68.0 cm across, with a high density of 3.46 nests per 100 m2. This density is much higher than that recorded at Lake Shala, which averages 1.55 nests per m2.

Great White Pelican is a colonial ground-nesting bird that forms large breeding colonies. In Lake Tana, the uncontrolled exploitation of fish resources and habitat destruction have led to fewer breeding pairs, with nests concentrated in specific, human-inaccessible locations. The mean inter-nest distance indicates localized nesting, influenced by high fishing activity and disturbances.

African Great White Pelican colonies usually do not build nests since the nesting sites lack vegetation. They create a slight depression in the ground with their beaks to lay their eggs [9]. Similarly, most of the nests of Great White Pelicans in the Lake Tana area, especially in the dry season, were slightly depressed at places with almost no vegetation cover, and some nests were built with available materials. They use whatever material, especially scraps and rubbish, and fresh green material, like soil, plastic, dry and wet leaves, stems, and chopped papyrus.

The study revealed that Great White Pelican nests are relatively small, with diameters ranging from 37.5 to 68.0 cm. These nests were densely packed, with an average of 3.46 nests per 100 square meters across (n = 421 nests). These results were far from the nesting densities of Great White Pelican recorded in Lake Shala (364 nests averaged 1.55/m2). The nesting density at Lake Tana was calculated to be 3.64 pairs per 100 square meters. Despite using similar nesting areas during both seasons, it remains uncertain if identical breeding pairs returned to the same locations due to the lack of marked individuals. These findings suggest a potential preference or suitability of the nesting areas for Great White Pelicans, as indicated by their use of similar sites across seasons. However, the uniformity in inter-nest distances across different sites may reflect consistent spatial requirements for nesting of the species. To gain further insight into nesting site fidelity and selection factors, future studies could benefit from marking individual birds to track their nesting patterns more accurately.

The uncontrolled use of fish resources in Lake Tana and the destruction of nests and nesting habitats could account for lower nesting density than in Lake Shalla. Although the breeding pairs at the Lake Tana were larger and the nests were concentrated at a specific location. The mean inter-nest distance (74.62–78.92 in two breeding sites) shows how nests were localized in specific sites. And these results were similar to the results recorded in Lake Shalla, which was 77 cm [9]. However, variation of inter-nest distances at Lake Tana is associated with the ununiformed nature of the nesting place, making high fishing practices and disturbances.

The clutch size of the Great White Pelican in Lake Tana (2.14 eggs per nest) is far from the results reported by various scholars [2] in Africa [9,27]. However [28], reported only 1.62 eggs per nest. The highest number of eggs observed in a Great White Pelican nest was 4, but this nest failed to hatch for unknown reasons. In similar cases, many eggs (4.5 or 6) were in the same nest [9].

In addition to hereditary factors, clutch size can be affected by the abundance of food [29,30] and age. The younger individuals lay fewer and smaller eggs than older and more experienced ones [30]. Egg size also affects clutch size because more energy is required to produce larger eggs than small eggs. The size of bird eggs may be necessary in determining the body size and condition of fledglings, their probability of survival, and, ultimately, their reproductive success [31]. In addition, it was reported that egg dimensions affect the size of nestlings and thus may influence their survival [32]. The egg measurements of Great White Pelicans showed lower variations in egg breadth than length and weight. This variation might be due to differences in the relative proportions of the egg components [33]. Variation in egg dimension results from genetic determination and environmental conditions' impact on these features [34]. [33] shows the mean egg weight was 181.6 ± 19.1 g (range 152–226 g), and the average egg weight (173.68 ± 18.44g with a range 139.19–212.78g), measured at Lake Tana is closer to it.

Egg measurements can differ in their within-clutch variation [34]. In the Great White Pelicans, the variation was observed in egg weight (new with old stage, and new with older eggs stage). In addition, egg breadth variation existed between the new and old egg stages, and egg length variation was observed between the new and old egg stages. The variations in egg weight may be attributed to the observation date after egg laying.

Although the most oversized clutches produce the youngest, these chicks might be relatively malnourished and have lower survival rates than young fledging from smaller broods. Some studies have supported this contention [35]. Still, more often, a positive association between clutch size and clutch productivity is observed when productivity is measured in the number of young surviving to independence or maturity. Similarly, the Great White Pelican's hatching success in Lake Tana correlates with the size of the clutches.

More than 85 % hatching success and 15 % loss of Great White Pelicans were recorded in Lake Tana, greater than a study of Great White Pelican in Lake Shala, Ethiopia, which reported 80 % hatching success and 20 % loss. However, pelicans had different hatching success stories in different areas and times. The lowest hatching success rate of the Great White Pelican was reported to be 65 % [5]. Similarly, the hatching success rate of the Brown pelican was reported to be about 71 % [36]. However, the American White Pelican had 64–90 % success in colonies without disturbance [37]. The destroyed eggs in Lake Tana resulted in a 15 % loss of hatching success rate, mainly because of human disturbance, egg displacement, and predators like the African fish eagle (Haliaeetus vocifer). Fishermen displaced breeding pelicans and destroyed their eggs intentionally might be to minimize competition for fish and space. The greatest conflict exists between breeding pelicans and fishermen on Fikir Mefja Island 1. At this place, the fishermen used the island to dry the harvested fish in open space, sun drying. The breeding success of the Great White Pelican population in Lake Tana was 76 young per 100 nests. The proportion of young to adults was about 1:4 at breeding Islands; however, the result was less than in Lake Shala, which was eight to nine young per 10 nests or about 50 % of the total number of eggs laid.

5 Conclusion

Lake Tana is the second most significant breeding site for the Great White Pelican in Ethiopia, following Lake Shala. Despite the potential for nesting on various islands within Lake Tana, only the two inaccessible Fikir Mefja islands in the eastern part of the Lake were identified as active breeding sites for this species. Interestingly, no nests were detected in adjacent lakeshore areas, wetlands, floodplains, or water reservoirs surrounding Lake Tana, suggesting the pelicans have a strong preference for nesting on the isolated Fikir Mefja islands.

The breeding patterns of Great White Pelicans at Lake Tana closely resemble those at Lake Shala, characterized by year-round breeding with a peak during the dry season. The nests are simple, flat structures built on the ground. Nest dimensions, such as diameter, remained stable in both seasons, and the distances between nests showed no significant variation between the dry and wet seasons. This suggests that factors other than seasonal changes may affect nest spacing. Egg characteristics showed no significant differences when all eggs were considered collectively. However, there were variations in the morphological measurements of eggs at different developmental stages, with the 'old' egg stage exhibiting the highest measurements. Additionally, a significant difference in egg breadth was noted between the two nesting locations. The Egg Shape Index varied significantly between seasons.

Regarding breeding success, 12.24 % of nests were destroyed by human disturbance, while 87.76 % hatched chicks, resulting in a high hatching success rate. However, the reproductive success rate at the pre-fledging stage was low, indicating a high mortality rate after hatching. Interestingly, the hatching success was positively correlated with clutch size, indicating that larger clutches had a higher probability of successful hatching. To conserve pelicans sustainably at Lake Tana, it is essential to develop greenhouse technology for drying harvested fish to reduce human-pelican spatial competition. Engaging the local community through awareness campaigns and conducting long-term monitoring and research are crucial for improving habitat quality and fostering conservation efforts.

Data availability statement

The corresponding author holds the data supporting the findings of this study and can be made available upon reasonable request.

CRediT authorship contribution statement

Ahmed Yesuf Yimer: Writing – review & editing, Writing – original draft, Visualization, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Shimelis Aynalem Zelelew: Supervision. Mezgebu Ashagrie: Formal analysis. Azmeraw Alemkere: Data curation. Aschalew Alelign: Project administration.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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