The results of the current study show that the digestive canal in both birds is an uneven straight twisted channel that started with the mouth and ended in the cloaca. The average length of the digestive canal in the zebra finch is 17 ± 0.12 cm, whereas the average length of the digestive canal in the starling is 31 ± 0.23 cm. The digestive canal in the two birds composed of several parts that differ in length and thickness between the two birds on one side and in the digestive canal of one bird on the other (Figs.1 and 2).
The mouth represents the beginning of the gastrointestinal canal in research birds. It is composed of the oropharyngeal cavity, surrounded on the outside by the horny beak (B) covering the jawbones as it consists of an upper beak (UB) covering the upper jaw and a lower beak (LB) covering the lower jaw of both birds (Figs. 3 and 4). The zebra finch beak appeared orange, triangular, and curved from its central part. The lower edge of the upper beak corresponds to the top edge of the lower beak completely, giving the beak a pyramidal shape when closed (Fig. 3). The upper beak appeared slightly taller than the lower beak. The average length of the upper beak was 1 ± 0.02 cm and width 3–4 mm ± 0.04, while the length of the lower beak was (8 mm) and its width 3–4 mm ± 0.12. Starlings’ beak appeared elongated and with a notably tapered end. Its color is black in most of its parts and white in other parts, the upper jaw was longer than the lower jaw, and the middle part of the upper jaw appeared prominent and broader than the rest of its parts. They are at the base of two openings of the nostril (N) that are large and elongated in shape; the average length of the upper beak is 3.5 ± 0.32 cm while the average width is 4 ± 0.012 mm. The lower beak appeared shorter and thicker than the upper beak; its average length is 3 ± 0.22 cm, while the average width is 3.5 ± 0.36 mm. And its lower part from its base to the middle of it is in the form of a groove towards the inside with a triangular shape, as they are at the bottom of the lower beak, oval grooves are identical to the nairs in the upper beak. Still, they are not open (Fig. 4).
The roof of the mouth (MR) appeared in both birds containing a longitudinal slit located within the hard palate that represents the choanal slit (NF). Its average length in the zebra finch is 5–6 ± 0.32 mm and the average width is 2 ± 0.03 mm, while the length in the starling is 1.5 ± 0.43 cm and the average width is 4 ± 0.04 mm (Figs. 4 and 5). Zebra finch contain many longitudinal papillae (PAP) distributed randomly in all parts of the roof of the mouth. Still, it becomes more uniform around the sides of the rostral nostril. These papillae interfere with each other at the beginning of the choanal slit, but they diverge from each other as we head toward the base of the nostril (Fig. 5), while in starlings, longitudinal papillae appeared longer compared with zebra finch and also appeared randomly distributed in all parts of the roof of the mouth, but the longest papillae appeared on the sides of the rostral cavity. The top of the oropharyngeal cavity has two at the base and one at the top. There are about ten longitudinal papillae intertwined with each other towards the base of the nostril as in the zebra finch (Fig. 6).
The tongue (T) resides in both birds at the bottom of the oropharyngeal cavity and between the edges of the lower beak. It appeared in the zebra finch elongated with a triangular end like a spear, and its top does not reach the end of the lower jaw. It reached an average length of 7 ± 0.042 mm and an average width of 1 ± 0.002 mm. Its color appeared white with a light black end. At its broad base, there are two rows separated by filamentous papillae, which are known as lingual papillae, a row on each side, and the color of the tongue in front of these papillae on the side is black (Fig. 7). Starling’s tongue appeared in a lengthy leafy shape with a slotted end (LE) that is in the form of bristles numbering 3 capillaries at the end of the tongue, and the top of the tongue does not reach the end of the lower jaw as in zebra finch, but it is more pronounced than in it. The side edges of the tongue are elevated up from the center of the tongue, giving the tongue the shape of the calf. The tongue also appeared in a highly transparent white form, so that its internal structures were visible. As in zebra finch, two rows of lingual papillae appeared on either side of the tongue. The tongue seemed longer than it is in the zebra finch with an average length of 1.5 ± 0.22 cm and a width of 2 ± 0.02 mm (Fig. 8). In both birds, the tongue is attached to the two hyoid bones (HB), which take the shape of the number (λ in Arabic) around the esophagus (Figs. 7 and 8).
The esophagus appeared in the research birds in the form of a muscular tube that connects the oral pharyngeal cavity from the upper side and the proventriculus (PRO) from the lower side (Figs. 1 and 2). In the zebra finch, the esophagus appeared in a large tube with relatively transparent walls that continue to exist along the esophagus. The average length of the esophagus from the oropharyngeal cavity to the proventriculus is 3 ± 0.22 cm, while the average diameter is 1 ± 0.01 cm. The esophagus expands in the middle of it to be a crop (C) that shows two-lobed pear-shaped pinnacles and its top facing up. Its base towards the bottom is clearly distinguished into two rooms and pink color, but it is darker than the rest of the esophagus and has thick walls which are more pronounced than in the starlings. It also contains internal longitudinal folds. The average length of the crop was 5 ± 0.02 mm and the average width was 3 ± 0.001 mm (Fig. 1). The esophagus is divided into two parts; the upper part, or what is known as the cervical esophagus (CE), connects the oral pharyngeal cavity and the crop, with an average length of 2 ± 0.1 cm. In contrast, the lower part, or what is known as the thoracic esophagus (TE), connects the crop and proventriculus with an average length of 1 ± 0.12 cm. The thoracic esophagus is darker in color and more equipped with blood compared to the cervical esophagus (Fig. 1).
Starling’s esophagus appeared in a more pronounced muscle tube than in the zebra finch, and its walls are of a high blood supply compared to its counterpart in the zebra finch. Its walls appear pink in color, and its inner walls contain continuous longitudinal folds along the esophagus as in the zebra finch. The average length of the esophagus is 6 ± 0.31 cm while the average diameter is 2 ± 0.03 mm, as in the zebra finch. The crop appeared almost spindle and whose walls are similar to the walls of the esophagus, and therefore, it is difficult to distinguish them from the rest of the esophagus. The inner lining of the crop contains longitudinal folds that increase its area, and its average length of the gland is 6 ± 0.06 mm and its average diameter is 4 ± 0.022 mm (Fig. 2). As in the zebra finch, the crop in the starlings divided the esophagus into an upper or cervical esophagus, with an average length of 4 ± 0.12 cm, and a lower or thoracic esophagus with an average length of 2 ± 0.09 cm. The cervical esophagus, as in the zebra finch, is lighter in color and has less vascular equipment compared to the thoracic esophagus (Fig. 2).
The stomach appeared in the two birds divided into two parts, a front part known as the glandular stomach or proventriculus and a posterior part known as the muscular stomach or gizzard (GIZ), separated by a small transition region known as the isthmus (IS). The proventriculus of zebra finch appeared in the sperm, conical shaped, its top towards the esophagus, and its base towards the gizzard, with walls similar to the thoracic esophagus but with a higher blood supply (Figs.1 and 9). The average length of the proventriculus is 5 ± 0.012 mm and its diameter is 1.5 ± 0.002 mm, and lined from the inside are many grooves and shallow folds, while the proventriculus in the starlings appeared in a conical shape, like the zebra finch, but their walls are thicker, brighter, and have a very high blood supply compared to the zebra finch (Figs. 2 and 10). The inner lining contains folds and grooves and is more pronounced than in the zebra finch; the average length of the proventriculus is 1.5 ± 0.3 cm, and the average diameter is 5 ± 0.1 mm. The isthmus region appeared in the form of a transparent part between the two stomachs in the zebra finch (Figs. 1 and 9). In contrast, in the starlings, this area did not appear clearly, as it was difficult to distinguish this region between the two stomachs (Figs. 2 and 10).
While the gizzard appeared as a solid muscular structure in both birds, in the zebra finch, the gizzard has a disc or circular shape and is pink in color, and the contact area of the duodenum with the gizzard is close to the glandular gastric region of the gizzard from the ventral side of the gizzard, and their walls have a high blood supply and a diameter of 1 ± 0.05 cm (Figs. 1 and 9). The gizzard lined on the inside with a thick, yellowish-green layer known as koillin or cuticle; this layer contains many longitudinal folds and is parallel to each other. Also found inside the gizzard are many small pebbles. Starlings’ gizzard was a disc, or spherical shape appeared pink as it is in the zebra finch. Still, it has the advantage that it has a very high vascular preparation higher than that in the zebra finch as well as in other parts of the digestive tract of the starling with an average diameter of 1.3 ± 0.42 cm. Also, the gizzard differs from in the zebra finch in the areas of glandular and duodenum contact in the gizzard; the proventriculus contacted in the middle of the upper end of the gizzard while the duodenum contacted the middle of the lower end of the gizzard (Figs. 2 and 10). As in the zebra finch gizzard, it is a line in the starlings with a layer of koillin or cuticle, which also appeared in a yellowish-green color with long parallel folds, but it was more profound than that in the zebra finch and also contained small pebbles.
The small intestine appeared highly developed for both birds and divided into three regions, the duodenum (D), jejunum (J), and ileum (I). The length and diameter of each part vary between the two birds on one side and the digestive tract of one bird on the other (Figs. 1 and 2). In the zebra finch, the average length of the small intestine is 10.5 ± 0.6 cm. The first areas of the small intestine which connect to the gizzard are the duodenum that appeared in the shape of a letter U confined between its arms the pancreas (P). It is light pink, and a high blood supply has reached an average length of 2.5 ± 0.32 cm and an average diameter of 2 ± 0.01 mm. For jejunum and ileum, they are in the form of laps numbering (2.5 laps). The jejunum and ileum appeared in yellowish-pink color and had less blood supply than in the duodenum (Fig. 1). The average length of the jejunum is 1 ± 0.1 cm and the average diameter is 2 ± 0.2 mm, while the average length of the ileum is 7 ± 0.7 cm and the average diameter is 1.8 ± 0.02 mm. The inner lining of the small intestine contains many villi that spread along the small intestine.
In the starlings, the small intestine appeared longer and thicker than the zebra finch, as it reached an average length of 20 ± 0.6 cm and is also composed of parts (Fig. 2). As in the zebra finch, the duodenum appeared as a letter U in white color, and reddish and bloody equipment is high and contains between its arms the pancreas; it has an average length of 5 ± 0.15 cm and an average diameter of 3–4 ± 0.02 mm. As for the jejunum and ileum, it will be 3 laps or rings with color and bloody preparation almost identical to that in the duodenum. The average length of the jejunum is 4 ± 0.42 cm and the average diameter is 3–4 ± 0.01 mm, while the average length of the ileum is 11 ± 0.8 cm and the average diameter is 3 ± 0.02 mm. The inner lining of the starling also contains a large number of villi protruding from the mucous layer (Fig. 2).
The small intestine in the two birds is followed by the large intestine that appeared in both research birds in the form of a non-branched or twisted tubular structure known as the rectum (R), which appeared in the zebra finch with a pink to yellow color with a wider diameter than the ileum. But its bloody preparation is almost identical to that of the ileum. Its average length is 7 ± 0.12 mm and the diameter is 2 ± 0.2 mm. The area of contact with the rectum is a pair of a very small cecum (CM). They are difficult to see with the naked eye, and these ceca were attached to the outer layer of the rectum (Fig. 1). The rectum in the starling has appeared longer and thicker than it is in the zebra finch, and it was dark pink and with high blood supply; the average length is 1 ± 0.3 cm, and the average diameter 3 ± 0.02 mm. Also, it was found that a pair of cecum appears in the contact area with the rectum and that is distinguished from them in the zebra finch as they are larger and easily identifiable and have a white color with a tubular shape and attached to the outer wall of the rectum. The average length of the cecum reached 2–3 ± 0.01 mm in diameter (0.5 ± 0.002 mm) (Fig. 2). The inner lining of the rectum appeared containing longitudinal folds of a villus-like shape for both search birds.
The digestive tract of the two birds ends with the cloaca (CL), which is the exit site for the products of the digestive, reproductive, and urinary systems. It appears in both birds in the form of an expanded structure of a diameter more significant than the diameter of the rectum and opens to the outside by a transverse incision (Figs. 1 and 2).