diff --git a/src/MediaTypeDefaultFieldTrait.php b/src/MediaTypeDefaultFieldTrait.php
new file mode 100644
index 0000000..83d40be
--- /dev/null
+++ b/src/MediaTypeDefaultFieldTrait.php
@@ -0,0 +1,161 @@
+<?php
+namespace Drupal\media_entity;
+
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\Core\Entity\Entity\EntityFormDisplay;
+use Drupal\Core\Entity\Entity\EntityViewDisplay;
+use Drupal\field\Entity\FieldConfig;
+use Drupal\field\Entity\FieldStorageConfig;
+
+
+/**
+ * To exhibit by derivatives of Drupal\media_entity\MediaTypeBase.
+ * 
+ * Implements most of the behaviour required for a single source field that may
+ * be populated with a default field upon creation of a bundle.
+ */
+trait MediaTypeDefaultFieldTrait {
+  use StringTranslationTrait;
+
+  /**
+   * Produce an array of allowed field types for the source field.
+   *
+   * @return array[string]
+   *   An array containing field types that the media type allows as its source
+   *   field.
+   */
+  abstract public function allowedSourceFieldTypes();
+
+  /**
+   * Return the default source field's name.
+   */
+  abstract public function defaultSourceFieldName();
+
+  /**
+   * Return the default source field's type.
+   */
+  abstract public function defaultSourceFieldType();
+
+  /**
+   * Return the default source field's label.
+   */
+  abstract public function defaultSourceFieldLabel();
+
+  /**
+   * Return the default source field's formatter.
+   */
+  abstract public function defaultSourceFieldFormatter();
+
+  /**
+   * Return the default source field's form widget.
+   */
+  abstract public function defaultSourceFieldWidget();
+
+  /**
+   * Produce a form fragment to configure the source field.
+   * 
+   * To be called from a media type's buildConfigurationForm() method.
+   * 
+   * @param array $form
+   *   The $form array as passed to the buildConfigurationForm().
+   * 
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   * @return array
+   */
+  protected function defaultFieldConfigurationForm(array $form, FormStateInterface $form_state) {
+    $options = ['' => $this->t('- None -')];
+    $allowed_field_types = $this->allowedSourceFieldTypes();
+    /** @var \Drupal\media_entity\MediaBundleInterface $bundle */
+    $bundle = $form_state->getFormObject()->getEntity();
+    foreach ($this->entityFieldManager->getFieldDefinitions('media', $bundle->id()) as $field_name => $field) {
+      if (in_array($field->getType(), $allowed_field_types) && !$field->getFieldStorageDefinition()
+          ->isBaseField()
+      ) {
+        $options[$field_name] = $field->getLabel();
+      }
+    }
+
+    // Select the source field. Only show when the bundle is not new, so there
+    // will potentially be fields to select.
+    $form['source_field'] = array(
+      '#type' => 'select',
+      '#title' => $this->t('Field with source information'),
+      '#default_value' => empty($this->configuration['source_field']) ? $this->defaultSourceFieldName() : $this->configuration['source_field'],
+      '#options' => $options,
+      '#access' => !$bundle->isNew(),
+    );
+
+    // Add a checkbox to allow the field being created automatically on save.
+    $form['create_source_field'] = [
+      '#type' => 'checkbox',
+      '#title' => $this->t('Create a source field automatically when saving this form.'),
+      '#description' => $this->t('If checked, a default field will be created and used as a source field. If you uncheck the field, you will need to create a field and revisit this form later to select it.'),
+      '#default_value' => TRUE,
+      '#access' => $bundle->isNew(),
+    ];
+
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function reactOnBundleCreated($bundle_name, $entity_type_id) {
+    if (!empty($this->configuration['create_source_field'])) {
+      $this->createDefaultSourceField($bundle_name);
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function createDefaultSourceField($bundle_name) {
+
+    // @todo Sanity-check: defaultSourceFieldType is actually allowed.
+
+    // Create / load the field storage.
+    if (!$storage = FieldStorageConfig::loadByName('media', $this->defaultSourceFieldName())) {
+      $storage = FieldStorageConfig::create([
+        'field_name' => $this->defaultSourceFieldName(),
+        'entity_type' => 'media',
+        'type' => $this->defaultSourceFieldType(),
+      ]);
+      $storage->save();
+    }
+
+    // Create the field instance.
+    FieldConfig::create([
+      'entity_type' => 'media',
+      'field_name' => $this->defaultSourceFieldName(),
+      'label' => $this->defaultSourceFieldLabel(),
+      'required' => TRUE,
+      'bundle' => $bundle_name,
+    ])->save();
+
+    // Make the field visible on the form display.
+    /** @var \Drupal\Core\Entity\Display\EntityFormDisplayInterface $form_display */
+    $form_display = EntityFormDisplay::create([
+      'targetEntityType' => 'media',
+      'bundle' => $bundle_name,
+      'mode' => 'default',
+      'status' => TRUE,
+    ]);
+    $form_display->setComponent($this->defaultSourceFieldName(), [
+      'type' => $this->defaultSourceFieldWidget(),
+    ])->save();
+
+    // Make the field visible on the media entity itself.
+    /** @var \Drupal\Core\Entity\Display\EntityViewDisplayInterface $display */
+    $display = EntityViewDisplay::create([
+      'targetEntityType' => 'media',
+      'bundle' => $bundle_name,
+      'mode' => 'default',
+      'status' => TRUE,
+    ]);
+    $display->setComponent($this->defaultSourceFieldName(), [
+      'type' => $this->defaultSourceFieldFormatter(),
+    ])->save();
+  }
+
+}