Birds improve vs. TEs, but at what price?

It's no secret that the Eagles have become notorious for a perceived inability to cover opposing tight ends. Back on November 5, Igglesblog posted an item analyzing whether the Eagles' tight end coverage was as bad as people seemed to think. In their analysis, they drew upon data from Football Outsiders to see how the Eagles' defense compared to the rest of the league.

The statistic they used to measure performance was an advanced metric called DVOA, or Defense-Adjusted Value over Average. You can read all the gory details about DVOA here. The gist of DVOA is that it takes into account a number of contextual factors traditional statistics like yards allowed do not, then controls for the strength of the opponent and gives a value compared to the league average.

That sounds confusing, so just keep this in mind: DVOA is comprehensive, and lower values are better from a defensive standpoint.

Igglesblog found that the Eagles played average tight end coverage, but their defense against wide receivers was so good that the Eagles' tight end coverage looked poor in comparison.

Well, that was Week 8. Now it's Week 17, and the Eagles have had eight more games under their belt, with Jason Witten and the Cowboys looking to thwart the Birds' attempt to secure the No. 2 playoff seed in the NFC this weekend.

So, how has the Eagles' defense changed over the past eight weeks?

The following table breaks down the Eagles' defense in terms of DVOA against specific opposing offensive positions. You will see that two main changes have taken place:

1. The Eagles are better against tight ends
2. They are worse against No. 2 wide receivers

Taken together, these two observations reveal that the Eagles may be devoting better coverage to tight ends at the expense of their coverage to secondary wideouts. If this is true, then the Eagles pass defense has not gotten better over the past eight weeks; it has only changed which positions get softer coverage.

For now, let's focus on the improvement defending tight ends.

Table 1: Eagles defense DVOA ratings against specific opposing positions

The bottom line, in terms of DVOA, is that the Eagles have gotten significantly better at covering opposing tight ends during their last eight games. In fact, they have improved their tight end coverage so much that they now rank fifth-best in the NFL over the course of the entire season.

Which raises an interesting question: Have the Eagles become that much better at tight end coverage during their past eight games, or has the caliber of opposing tight ends just gotten that much worse?

For reference, here are the tight ends the Eagles have faced during the first 15 games of the season, their performance against the Eagles in terms of yardage and their average yards per game this season. For the sake of simplicity, I am only including the top receiving tight end per team; so teams that utilize multiple tight ends will be undervalued here:

Table 2: Performance of specific Tight Ends against the Eagles

*Chicago tight end Kellen Davis scored a touchdown in this game; this is reflected in the Games 8-15 average row

**Chris Cooley was out this game, so Davis was the number one tight end for Washington

In terms of yards per game, the Eagles only allowed 3.8 fewer yards per game to tight ends during the last eight games compared to the first seven. At first glance, that seems like a trivial improvement.

However, a closer inspection reveals that the Eagles' tight end coverage improvement came during the half of their season where they essentially faced every premier tight end in the league not named Dallas Clark: Jason Witten, Antonio Gates, Tony Gonzalez and Vernon Davis. That would seem to be a tougher lineup than the likes of Zach Miller, Jeremy Shockey and Kellen Winslow, whom the Eagles faced in their first seven contests. The fact that the latter eight tight ends are averaging almost 10 more receiving yards per game this season than their counterparts in Weeks 1-8 bears this out.

In other words, the reason that DVOA finds the Eagles to have improved significantly is because they have performed slightly better against much better competition.

In any event, does it matter that the Eagles have improved in coverage against tight ends if they have gotten worse against the opponent's No. 2 receivers? That the Eagles dropped from 13th to 23rd in DVOA against opposing No. 2 WRs is a concern. Have the Eagles actually become a better pass defense, or not?

One possible explanation for the Eagles' second-half fall-off against complementary wide receivers is that the Birds have employed their best coverage to wide-receiver type tight ends, in the process giving softer coverage to the actual No. 2 wideout on those teams. This theory is supported by the fact that the Eagles overall defensive DVOA has fallen from first (-19.9%) to fourth (-12.3%) in the NFL over the past eight weeks despite their big improvements against the run. In other words, the Eagles pass defense as a whole has gotten significantly worse even though their tight end coverage has improved.

This pattern was in evidence in the first Philadelphia-Dallas matchup when the Birds held Witten to 43 yards and Pro Bowl WR Miles Austin to one reception (albeit, a big one). But the Eagles gave up a game-high 75 yards to Cowboys No. 2 WR Roy Williams and 74 yards to No. 3 WR Patrick Crayton. All of this suggests that the Eagles are excellent at taking two receivers out of the opponent's gameplan, but then fall off significantly in covering additional receivers.

So even if Philadelphia manages to handcuff Witten again on Sunday, they will only really improve their pass coverage if they can hold off more than the Cowboys' top two threats.

Ben Singer is a graduate of Brown University and an intern at Philly.com Sports. You can read his take on the "Contract year phenomenon" on footballoutsiders.com.