|This article is about the current version of DF.|
Immigration can occur at any time once per season. Smaller migrant waves of 2 to 10 arrive in early seasons, followed by a large wave in the low double digits in the second spring, one year after embark (the maximum wave size reported to date is 77). The first 2 migrant waves are hardcoded to appear, unaffected by fortress wealth or even the extinction of their home civilization. Each group of migrants will often include domestic animals, including both pets and stray livestock. Many also include children. Be prepared with adequate food, drink, and beds, among other things.
Migrants will often have skills that match your fortress' needs — migrants with skills your fortress uses a lot or skills that your fortress doesn't have at all are more likely to show up at your gates. Important skills (mining, food production, and basic crafting, according to Toady) are weighed more heavily than other skills.Source · MP3
Migration waves are generally a good thing — if you're prepared for them.
 Labor preferences
Each migrant can arrive with a wide collection of often unrelated skills, far greater than possible with one of the starting 7 dwarves, and experience levels as high as Legendary. Any and all skills might be represented, including obscure military skills (like blowgunner), high levels of one or more social skills, crutch walker, concentration and others. It's even possible to have dwarves with skills that may not be obtainable in fortress mode like military tactics or tracking.
As of v0.34.03, migrants may arrive with all labors except hauling, cleaning, recovering wounded, and caring for wounded disabled, depending on the settings one has entered into d_init.txt.
 Historical migrants
Some immigrants are historical figures. These immigrants come to your fortress with skills representing their history, and may come to your fortress with wounds they have suffered during world generation. Immigrants may even be vampires.
 Limiting/preventing immigration
In v0.40.05 and above, the d_init.txt POPULATION_CAP setting immediately prevents further immigration once the desired number is achieved. There is also a STRICT_POPULATION_CAP setting, which prevents both immigration and babies when reached (although both can be violated by a few special cases, such as the arrival of a monarch).
The number of migrants depends on the created wealth of your fortress and so is affected by your dwarves' activities. Note that if your fortress should ever become a mountainhome, you will receive an additional migration wave with the promotion, regardless of your population cap. The number of migrants is affected by how far below the population cap your fortress is. If your fortress is one dwarf short of the cap, you will receive a single migrant (if any).
It is worth noting that you need a certain minimum population size before any of your dwarves will experience strange moods. Additionally, POPULATION_CAP affects only migration, it has no effect on pregnancies. You will need to alter STRICT_POPULATION_CAP in order to limit births.
To reiterate, the population cap is a (mostly) hard limit on the number of dwarves in your fortress. If you want a fortress with 50 dwarves, simply set the POPULATION_CAP and STRICT_POPULATION_CAP to 50.
 Immigration mechanics
The date on which immigrants appear in a season seems to be fixed at the start of that season, but the number of immigrants and their skills are determined when the migrant wave arrives.
There is never a migration in the first winter. There won't even be a The fortress attracted no migrants this season message.
Migrant skill levels seem to depend on the size of the home civilization; a difference will be noticed if you picked a dwarven civilization that was not well established (few towns or none) compared to a well established one.
 Migrant wave sizes
The first two migrant waves have a minimum size of 1, if it has a relative in your group already, and a maximum size of 10. The size of these waves are unaffected by fortress wealth.
The third migrant wave and on are influenced by the created wealth of the fortress, with more wealth attracting more immigrants (more research is needed to determine specifics). Specifically, they're influenced by the fortress wealth as reported by the last outgoing dwarven caravan. Wealth created after the caravan leaves has no influence until the next year's caravan leaves. If the caravan fails to make it out then the fortress' wealth is not reported. The dwarven liaison does not report on fortress wealth, in those circumstances where the liaison makes it out but the caravan doesn't.
Imported wealth, caravan sales figures, absolute caravan profit and caravan profit margin either have no effect on migration numbers, or only have an effect by applying a percent modification to the numbers driven by created wealth. If a fortress manages to trade (not offer) away 100% of its created wealth then no immigrants will come the next season. More research is needed to determine if the aforementioned statistics have any influence on migration numbers.
One factor which is known to affect migrant wave size is the total size of your fortress'snits list (all 4 categories), which consists of dwarves, invaders, merchants, and animals which either died or currently live at your fortress. 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.
Kurik Amudnil created a DFHack script to prevent the latter from happening, by clearing (and storing, so that it can be restored as wanted) the dead units list of uninteresting creatures. It is available here and is also included in the Lazy Newb Pack.
 Adventure mode
In certain locations in adventure mode, you may come across a Migrating Group. One such location is near a recently abandoned fortress; choosing to travel to the group will allow you to talk to the members of your former fortress as they travel back to dwarven civilization.
 Fortress Failure Migration
Also, some migrants will be incorrectly listed as babies or children, when they are not in the expected age range for those categories. This will automatically fix itself when they have their next birthday. Some baby migrants may have future birth dates. Bug:3945
You might have a situation where a single migrant can't find the fort and just stands at the edge of the map, not moving at all.