Q:

Why is my callback not a function?

Why is my offsetCardsCallback not a function?
var numEventCardsShowingInFeed = 0;
var cardsDisplayedList = [];

  onReady: function () {

        this._hideElements();
        this._startEventFeed();
    },

    _hideElements: function() {
        $('.observation-card').hide();
    },

    _offsetDisplayedCards: function(passedCardsDisplayList){
        //***the callback function, do stuff here ***
    },

    _addNewCard: function(i, thisCardId, offsetCardsCallback){
        var feedCardSchedule = [1000,2000,3000,4000];

        setTimeout(function(){

        // show the new card
        $(thisCardId).show();

        // add this card to the cardsDisplayedList.
        cardsDisplayedList.push(thisCardId);

        console.log("cardsDisplayedList: "+cardsDisplayedList);

        // increment event cards showed counter
        numEventCardsShowingInFeed++;

        // ***calling the callback***
        if (typeof(offsetCardsCallback) === "function"){
            console.log("callback");
            var passedCardsDisplayList = cardsDisplayedList.slice();
            offsetCardsCallback(passedCardsDisplayList);
        }

        }, feedCardSchedule[i]);

    },

    _startEventFeed: function() {

        var eventCardFeedQueue = ["#observation-card1", "#observation-card2", "#observation-card3"];

        var numEventCards = eventCardFeedQueue.length;
        console.log("num of event cards in queue: "+numEventCards);

        // loop through card queue
        for (var i = 0; i < numEventCards; i++) {

            var thisCardId = eventCardFeedQueue[i];
            console.log(thisCardId);

            //***the callback reference***
            var offsetCardsCallback = this._offsetDisplayedCards();

            console.log(typeof offsetCardsCallback);

            //***pass the callback***
            this._addNewCard(i, thisCardId, offsetCardsCallback);

            if($(thisCardId).is(":visible")){
                console.log(thisCardId + " is visible");
            }
        } // end card loop
    }, // end feed function,

A:

You need to pass as refference, not execute the function.
You have this:
 var offsetCardsCallback = this._offsetDisplayedCards();

And you need this:
 var offsetCardsCallback = this._offsetDisplayedCards;

