Tetsuwan Birdy Decode

Both anime's feature a guy are in girls bodies(although they both can switch between when they please) They both gain new abilities and use them to solve something. 

Aliens, cute heroine, good storytelling. Similar warm fuzzys. 

This two anime have some similarity like when an Alien use your body to fight. In Birdy it was an accident and in Parasyte it was an invasion but both very similar in story i recommend both of this anime 

Birdy and FLCL both share a few characteristic, such as the similar bright art and animation style, and the fast-paced crazy action. They both also involve a fairly cynical male protagonist meeting a hot-blooded alien woman. 

Both are about a guy who lost his humanity and have to deal with some sort of Alien-Human conspiracy.  

both main character's friends are hyperactive busy bodies explosions powers skimpy outfits new worlds 

Both are sci-fi anime with a transexual alien. 

From space they arrive and protect a younger opposite gender in building destroying fights. Thats about all i could think of and in birdy the main male character is interested mostly in humans. 

Both are about a guy who lost his humanity and have to deal with some sort of Alien-Human conspiracy.  

The character designs, color palette, and action sequences in these two anime showcase Ryo-timo's signature style of bright colors and simple but flashy animations. If you're a fan of the art style and animation of one of these shows, then I'd definitely recommend you watch the other.  

Both protagonist duos meet each other by having a near-death experience and now they're both forced to be together. There's also action and aliens so you might enjoy the other series. 

Both are sci-fi/action series that feature an average high school boy who inadvertently comes into contact with aliens/a secret organization, resulting in permanent super-powered changes to his body. Both series also have a good dynamic between the main characters. 

Both are sci-fi series with a male and female duo that take place (besides some of Birdy season 2) in a town with a larger intergalactic backdrop causing many of the events in the shows to occur. The duos consist of a normal teenage boy and a female, nonhuman warrior whose origins come from that intergalactic background (there is also some interesting worldbuilding with the settings in both that makes them unique as well as similar). Mahoro and Birdy are the most similar thing between these two, as they both have important mysteries in their pasts, they often give an air of being friendly and outgoing (and sometimes just like a normal teenage girl), but when things get serious they don't hesitate to channel their 'War God' modes, their fight scenes reminded me of each other and overall they give me very similar vibes in terms of their characters. Both also balance comedy, action, and romance, with varying amounts of each throughout the stories as the characters, arcs and tones develop and change. All in all, they have similar settings, their genres and tones compliment each other, and the main duo is very similar (especially Birdy and Mahoro in terms of personality, attributes and vibes), so if you liked one you may like the other. read more 

The concept of a sharing a body is present in both series from the very beginning thus bringing forth a supernatural element. In both series, this causes some problems for the main and female protagonist who does not get along at first. However, they eventually forms a special bond. Both series has comedy and drama although Tetsuwan Birdy Decode leans towards more of the action. However, both of these series have emotional dialogues later on that deals with the main characters and how they develop throughout each episode. 

Super natural science. 

Action, Comedy, short interesting series 

They both have two main protagonists, one of them lost his body and gain afterward a huge strength. Parents are nowhere to be found in both. 

Similar setting for the main female lead. Instead of a robot, it's an alien. 

Both have the similar plot of a male to female sharing of bodies/souls. If you like either one, then you should give the other a go! 

Starts off very similar guy gets killed by alien and now has to share a body with a female. 

Even tho there's no "body sharing" in Busou Renkin, the heroine of the series has some sort of responsibility in the main guy's death, right at the start of the story. 

One is a bounty hunter, the other is a federal agent, but they both hunt alien. Action packed series with old school vibes, strong SF pulp influences, groovy soundtrack and really stylish animation. 

Something similar happens in Cinderella Boy, the two main characters (Ranma and his partner Rera) end up in a car accident and, to be able to survive, both of them have to live in a single body; one during the day and the other one during the night. 

They both have that certain magical flair about them. Both are of peculiar heroines and are both great to watch! 

Similar plot elements - humans interacting with more advanced beings, people changing thier physical bodies and a some what clueless boy with romantic feelings for a girl that happens to have the potential to change the world hidden in her. Also some visual similarities. 

Sci-fi anime, both male protagonist gain the power to save the world after an encounter with a girl with strange hair, and end up making sacrifices. 

The entire Birdy franchise and the Nanoha franchise are very similar. Comparison can be made between the Time-Space Bureau and the Federation Police. One deals with interdimentional crime related to magic and lost technology. The other deals with galactic crimes with aliens and lost technology. Both involves these dangerous technology being implemented on the characters. 

They both have two main protagonists, one of them lost his body and gain afterward a huge strength. Parents are nowhere to be found in both. 

Both titles have females lead characters with superhuman powers that can kick ass. Both shows deal with a young boy suddenly meeting a girl and eventually make a contract/bound with her. While Birdy the Mighty Decode has more of a sci-fi nature and Sekirei is more ecchi, they are both very action pack shows. If you like one, you may probably like the other. 

Corpse Princess is basically the horror version of Birdy. You've got a butt-kicking, scantily clad headstrong female lead who works with a powerful but mysterious group who are working against malicious forces, who gets involved a somewhat boring normal teenage male protagonist with annoying friends. But hey, the action scenes are awesome.  

Similar alien comes from outer space and changes main character's life. Both shows have a bit of a similar light feel but at times have action. First thing watching this it reminded me of the other show perhaps its something in the animation or music that is great.