Immigration can occur during any season. A migrant is a member of a wave of immigration.
Migrants will spawn in groups at the left edge of the map (near your road, if you have one) and move towards any meeting area that you have designated.
If there is no meeting zone they will simply head to the cliffside where your wagon originally arrived until their "migrant" status fades and they find a job. By default, immigration will stop when your fortress reaches 200 or more, but you can change this in the init.txt file. It will take several years to reach this number, assuming you want to reach it at all. Once your population falls below this number, immigration will begin again.
 How immigration is calculated
Immigration figures are influenced by your fortress's total "Created Wealth", probably further influenced by reporting of the yearly dwarven caravan. If, for any reason, the caravan does not arrive, or does not make it off the map alive the first year, no wealth is reported, and this will seriously hurt your chances for immigration. Later, if they fail to make it off your map, then the previous year's caravan report is used to determine the next immigration waves. The other known factor that influences immigration is that dwarf deaths in your fortress will make immigrating dwarves scared, reducing immigrant numbers possibly down to zero.
Created Wealth is visible on the Status (broker. All objects with any value (including mined-out areas, bridges, and just about every kind of created good) will be included in the total. Artifacts usually are one of the largest influences of fortress wealth. Artifacts made of precious resources and heavily decorated can potentially double fortress wealth early in the game.) screen once you get a
It is not a hard formula - identical fortresses with identical created wealth will tend to get more or less based on that wealth, but can experience some differences in the exact number arriving. The wealth seems to create a target number, and the exact number of immigrants varies around that target.
Immigration will be stopped if your fortress reaches the Population Cap set in the init file, but a given wave may cause your total to exceed that number. If your current population is even 1 less than the Population Cap, a full wave of 2 dozen (or more?) immigrants can arrive, though this is less likely to happen if you were fairly close to your population cap when the last caravan departed.
Additionally, the total size of your fortress'snits list (which consists of dwarves, invaders, merchants, and animals which either died or currently live at your fortress) is known to affect the sizes of migrant waves. As this number increases, the maximum size of migrant waves will be reduced: starting at a local population of 1000, migrant wave sizes are limited to 10, and at subsequent levels of 1300, 1600, 1800, 2000, 2200, 2400, 2600, 2800, and 2900, the limit is decreased by 1, and once you reach a local population of 3000 you will cease to get migrants at all.
 When does Immigration usually occur?
Usually you have to wait until the dwarven caravan arrives - and leaves alive - in Autumn to start the immigrants on their way. If the caravan reports a high Created Wealth, you can expect to receive maybe eight or more migrants in Winter after the Caravan, and/or more in the Spring following. If your dwarves are extremely productive, immigration can occur before the caravan arrives, in Autumn or even in Summer of your first year. Even with low created wealth, immigration does usually begin in Spring of the second year on.
Migrants seem to have a greater tendency to arrive in Spring and in greater numbers, but can arrive in any season.
 Immigrants' skills
Migrants are never very skilled - they arrive with Novice level skill in several related skills for their profession:
- Miner - Miner
- Carpenter - Carpenter, Wood cutter, and Engineer/Bowyer
- Mason - Mason and Engraver
- Trapper - Animal trainer/Animal caretaker, Ambusher/Small animal dissector, and Trapper/Marksdwarf
- Metalsmith - Weaponsmith/Armorsmith/Metalsmith, Metal crafter, and Furnace operator
- Jeweler - Gem cutter and Jeweler
- Craftsdwarf - Bone carver/Glassmaker/Weaver, Stone crafter/Wood crafter, Leatherworker/Clothes maker
- Fisherdwarf - Fisherdwarf, Fish dissector, and Fish cleaner
- Farmer - Grower, Butcher, and Milker/Cheese maker/Tanner/Dyer/Herbalist/Brewer/Thresher/Cook/Miller
- Mechanic - Mechanic
Migrants will sometimes bring their unskilled spouses, children, and even their pet animals. Children can perform some jobs, and will slowly grow to become adults, and any adult dwarf can be given any job, and eventually learn that skill while performing that labor.
 How to influence the types of dwarves that immigrate
Immigrant professions are influenced not only by the number of each profession you have at your fortress, but also the types of jobs you've been completing. Soldiers will only arrive if you have made their type of weapon; if you make one battle axe, you will only get axedwarves. If you make a crossbow and a war hammer, you will get marksdwarves and hammerdwarves. This is not affected by weapons brought to your fort; getting immigrant woodcutters won't let you get immigrant axedwarves. It is not known if the number of weapons created matters.Verify
 What are the drawbacks to immigration?
Most of the time, immigration is a Good Thing, and the more, the merrier. More dwarves means more work gets done (especially hauling, since immigrants are basically peasants), more dwarves can be given "luxury" tasks like defending the fortress or making dyed cloth, and some nobles require a certain number of dwarves before they arrive. However, more dwarves also means more CPU cycles eaten up, especially if your fortress is already taxing on your computer.
Of course, additional dwarves also require additional bedrooms, workshops, and booze before they can be properly productive, and some players prefer to increase their population slowly to allow them to add the extra infrastructure at a more sedate pace.
Useless tossers Nobles also can come with immigration waves and will require posh rooms immediately and other items eventually.
 How can I curb immigration?
The easiest method to handle immigration is to change the point when immigration stops in init.txt, under "population_cap". You can first set it low and raise it as you see fit. Note that you may consider this to be cheating, and it also requires exiting the game each time you want more migrants.
You can also make sure your fort does not produce or build much. Of course, this is not an especially interesting way to play the game. Since artifacts often account for most of your fortress' wealth, killing dwarves that enter strange moods may be a better option. This can often be accomplished by making sure all your workshops are in enclosed rooms, then locking the door on them until they go insane, then starve to death. This has a drawback in that dwarves that do manage to produce an artifact often gain a desirable legendary skill - lock the door at your own discretion. On the other hand, you can only ever get 18 artifacts in a single fortress, so you may want to save them for worthwhile skills.
You can also kill the migrants themselves, deliberately or otherwise. Large numbers of deaths will reduce immigration or prevent it entirely.