Advanced Generation series
Ash caught Swellow as a Taillow in the episode You Never Can Taillow!. It was the leader of a flock of wild Taillow. It stole Max's chocolate and challenged Pikachu to a battle. It also led the entire flock on a ceaseless attack until Brock and his Forretress intervened and used Explosion to drive them away. Taillow then showed up alone and challenged Pikachu, and even though it took countless Thunderbolts and a Thunder from Pikachu, it still kept fighting. On Brock's advice, Ash captured Taillow to prevent it from getting hurt anymore. Ash then sent it out and Brock helped it recover.
Its first battle was against Nicholai and his Zigzagoon in In the Knicker of Time!. Taillow's impressive flying techniques allowed it to match Nicholai's newly caught Pokémon well. The winner of the battle is unknown. It also fought against Anthony's Pelipper You Said a Mouthful!, where it handled the battle well but lost when a Magby hidden inside Pelipper's bill used Flamethrower.
In Brave the Wave, Ash chose it to battle the Fighting-type Gym Leader Brawly's Machop. Taillow managed to land several Wing Attacks on Machop, but Brawly's Pokémon managed to weather the attacks to land a devastating Cross Chop at the vital moment, taking Taillow out in a single hit. Taillow, with Corphish, Treecko and Pikachu, took up a meditation program to prepare for the rematch. However, Taillow was substituted for Corphish in the rematch.
It battled against Drew's Roselia in Pros and Con Artists, where it managed to outfly Petal Dance, but was paralyzed by Stun Spore when it attempted to attack at close range, causing it to lose the battle.
As a newly evolved Swellow, winning the PokéRinger tournament
In That's Just Swellow, Ash entered Taillow in a PokéRinger tournament. Unbelievable determination allowed Taillow to shrug off Electric-type attacks that reigning champion Volt's Volbeat was using, and it made a counterattack to fly to victory. In the final, Taillow went up against James and Dustox, and started strongly until a sand tornado kicked up. Determined to win, Taillow flew into the storm, but was too small against the wind. Desperate, it evolved into Swellow and easily broke through, using Wing Attack to flick the ring onto the goal and win the tournament. It then single handedly sent the sore losers Team Rocket blasting off.
Defeating Winona's Swellow
Several episodes later, in Sky High Gym Battle!, Swellow fought Winona's Shiny Swellow despite Winona's using more powerful attacks such as Hyper Beam and Aerial Ace. However, Ash quickly recovered from Hyper Beam and ordered Swellow to create a dust cloud to blind and slow Winona's Swellow down. It then struck with Wing Attack and knocked out Winona's Swellow, winning the battle for Ash.
After this victory, Ash became determined to teach his Swellow Aerial Ace, beginning in Pokéblock, Stock, and Berry. Swellow initially struggled but with the help of Vladimir and his Pidgeotto, Ash was able to teach Swellow the move in Lessons in Lilycove!.
In Solid as a Solrock it teamed up with Pikachu in a Double Battle against Tate and Liza, as Ash inspected the terrain and decided an aerial fighter would be necessary. The two of them struggled to defeat Lunatone and Solrock, with Swellow only landing a Peck on Lunatone with little damage inflicted. Swellow came to Pikachu's rescue, bearing it into the air and away from the pummelling it was taking. Pikachu's Thunderbolt was sent back onto the team by Lunatone's Light Screen, but Swellow managed to shrug off most of the damage. Swellow then used a newly learned Double Team to distract the duo whilst it flew above them. Pikachu used Thunder on some thunder clouds above, the clouds spat the electricity back at Pikachu and Swellow, and what would have expectedly hurt Swellow, powered it up along with Pikachu and turned it a metallic golden color. This was called Thunder Armor. Swellow easily diverted SolarBeam and Ice Beam and then took Lunatone out with a supercharged Aerial Ace.
Swellow was used again against Juan in Eight Ain't Enough, where it battled Juan's Whiscash, where it started on the back foot after being Tickled and then hit by Hyper Beam. However, Swellow recovered and knocked Whiscash into the air with Aerial Ace. In midair it was able to dodge Hyper Beam and it then smashed Whiscash back into the water with Quick Attack. Just before Whiscash hit the water, Swellow landed an Aerial Ace and knocked Whiscash out. Juan complimented Swellow's ability to meld its attacks together and called upon his final Pokémon, Milotic. Swellow, though tired, managed to dodge Hydro Pump and strike Milotic with Aerial Ace, before using Double Team to avoid an Iron Tail. However, Milotic used Twister to sweep up all the copies and slam Swellow against the ceiling, knocking it out.
Taking out Katie's Scizor
In the Ever Grande Conference, Swellow battled Katie in A Judgment Brawl. It managed to defeat her powerful Venomoth, despite having Peck Disabled, as it smashed Venomoth towards the water with Aerial Ace, then used Quick Attack to push through its last resort Confusion and leave it unable to battle. It then fought Katie's Scizor. Disable had worn off and Swellow used Peck, only for Scizor to use a Double Team/Metal Claw combination to intimidate Swellow with numerous attackers bearing down. Swellow used its own Double Team to counter, and then used Aerial Ace. Scizor threw away the clones to use Hidden Power, but Swellow dodged the attacks and dashed forward with Quick Attack. Scizor charged forward with Metal Claw, but Swellow dodged at the last second and took Scizor out. Katie's Walrein was much more robust, easily shrugging off Swellow's attacks and then freezing Swellow's wings with Ice Beam. After dodging Swellow's attacks underwater, Walrein struck with another Ice Beam and took Swellow out of the fight.
Then in Choose It or Lose It!, it battled Morrison's Gligar, using Double Team to evade a one-hit knockout from Guillotine. Swellow was able to dodge Iron Tail, but was hit by Hidden Power and sent plummeting to the ground. Swellow recovered and after a trio of clashes between Aerial Ace and Steel Wing, both Swellow and Gligar were knocked out.
Catching Donphan at the last second
In Ash's battle with Tyson in At the End of the Fray, Swellow battled masterfully and was one of Ash's biggest assets. It first went up against Tyson's Hariyama, who had just defeated Torkoal and Corphish in quick succession. Just as it struggled against Machop at the beginning of the journey, Swellow was caught at close range and badly hurt by Seismic Toss. However, this time Swellow had Aerial Ace in its arsenal, and was able to dodge Hariyama's Focus Punch and land a powerful blow on its chest, taking it out. It also battled Donphan, managing to beat back a Sandstorm but was struck by Donphan's Rollout. Donphan managed to use Rollout to propel itself off rocks and into the air, causing damage to Swellow, until Swellow managed to catch Donphan with its feet and drop it, knocking it out. However, it was heavily disadvantaged against Metagross, who managed to halt it in midair with Psychic. Swellow blasted forward and used Quick Attack to crack Metagross's armor, but takes recoil from the impact and is finished off with a Hyper Beam.
Kanto Battle Frontier
Celebrating the win over Tucker
Swellow went with Ash through the Kanto Battle Frontier and fought well against Frontier Brains. In Tactics Theatrics!! it teamed up with Corphish to battle Dome Ace Tucker and his Swampert and Arcanine. Swellow launches a Quick Attack on Swampert but Arcanine leaps forward and collides with Swellow, knocking it away from its intended target. It kept trying to attack Swampert, but Arcanine's speed was too great and it continued to block it. Swellow managed to dodge Flamethrower with Double Team, but Arcanine powered up into a Fire Blast and hit Swellow dead on. After some maneuvering from Arcanine and Swampert, Swellow and Corphish were trapped inside a fire and water structure. Swellow managed to smash Arcanine into Swampert for a direct hit on both, whilst Corphish protected Swellow from Fire Blast with BubbleBeam. The smoke from the clash between bubbles and fire blinded both of Tucker's Pokémon and Swellow managed to hit Swampert in its blind spot. Swampert fainted just as Corphish finished off Arcanine, winning Ash the Tactics Symbol.
In Ka Boom with a View!, Swellow went up against Spenser's Venusaur, who had just flattened Ash's powerful Heracross. Changing tactics, Ash ordered Swellow to fly towards the ocean. Whilst flying it could easily dodge Venusaur's Vine Whip and Razor Leaf. On the beach, battle recommenced with Swellow dodging all of Venusaur's attacks with Double Team and striking hard with Quick Attack. A final Aerial Ace took down Venusaur and Spenser replaced it with Claydol. Swellow was noticeably worried but fought on anyway with Quick Attack. However, Claydol dodged with Teleport and then knocked Swellow out of the air with Rapid Spin. A follow-up Hyper Beam knocked Swellow out of the match.
In Pokémon Ranger - Deoxys' Crisis! (Part 2), it did its best in a fight against Deoxys, but could not possibly match the Speed Forme and was knocked to the ground.
In Slaking Kong, Swellow took on a giant robot Slaking long enough for Sceptile to rescue May. It then slammed it into the ground to permit others to attack it. In Aipom and Circumstance it took on a whole flock of mechanical Fearow despite being outnumbered and managed to cause serious damage.
Ash left Swellow at Professor Oak's lab when he set off for Sinnoh in Home is Where the Start Is!
Diamond & Pearl series
In The Semi-Final Frontier!, Swellow was brought back to Ash's team where it battled against Tobias in the semi-finals of the Lily of the Valley Conference. It used Quick Attack on Tobias's Latios; however, Latios was able to dodge it. Swellow then used Aerial Ace and flew from the sky towards Latios, coming down with great force. However, Latios used Luster Purge on Swellow, knocking it out.
Best Wishes series
It was mentioned in The Fires of a Red-Hot Reunion! by Professor Oak, who mentioned that Swellow would sometimes have air races with Staraptor at the lab, and Unfezant would make a good addition to those races.
Swellow was later reunited with Ash in The Dream Continues!, where it and the rest of Ash's Pokémon posed in a group photo with him.