Mapping a function to rename bands across an image collection in google earth engine

I have a function to visualize imagery of different dates. However, I need to use both landsat 7 and 8, since my the dates I want start earlier than 2013. The issue arises that landsat 7 and 8 order their bands differently, so visualizing the images in the same way would require different code. To fix this, I am trying to rename the red, green, blue, and NIR bands in the landsat 7 image collection to match landsat 8. This way I can write a universal code that will apply to both images from landsat 7 and 8. Below is the code I have written (assume landsat 7 has been imported and called 'landsat7', and a geometry that encloses the area of interest called 'bounds' is also imported).

// Function to rename landsat 7 bands to match landsat 8 bands
var adjustBands = function(landsat7){
   var adjust = ee.ImageCollection(landsat7)
   .filterDate('2010-01-01', '2010-05-01')
   .filterBounds(bounds).first();
   return adjust.select(['B4', 'B3', 'B2', 'B1'],['B5', 'B4', 'B3','B2']);
    }

print('adjust', adjustBands(landsat7));

//apply function to entire image collection
var l7a = landsat7.map(adjustBands);

However, when I run the code I get an error: Error in map(ID=LE07_001004_20000610): Image.select: Parameter 'input' is required. Any advice?

It looks like you are mapping a function onto a collection, but inside the function you are reading in the same collection. Make sure when mapping functions onto collections, that the function itself only takes a single image as its argument and returns a single image. Here's how I handle the band renaming if I want to combine Landsat collections:

function renameBandsETM(image) {
    var bands = ['B1', 'B2', 'B3', 'B4', 'B5', 'B7', 'pixel_qa'];
    var new_bands = ['B', 'G', 'R', 'NIR', 'SWIR1', 'SWIR2', 'pixel_qa'];
    return image.select(bands).rename(new_bands);
}
var etm = ee.ImageCollection('LANDSAT/LE07/C01/T1_SR')
  .map(renameBandsETM)

I then do the same to the LC8 imagery, taking into account the different band naming system:

function renameBandsOLI(image) {
    var bands = ['B2', 'B3', 'B4', 'B5', 'B6', 'B7', 'pixel_qa'];
    var new_bands = ['B', 'G', 'R', 'NIR', 'SWIR1', 'SWIR2', 'pixel_qa'];
    return image.select(bands).rename(new_bands);
}
var oli = ee.ImageCollection('LANDSAT/LC08/C01/T1_SR')
  .map(renameBandsOLI);

You can then merge the collections:

var landsat = ee.ImageCollection(etm.merge(oli));